At a Meeting of the Council of the Royal Society, Ianuary 28. 1674/5.
THat a Discourse, made before the Royal Society, the 12th of November 1674. by Dr. John Wallis, concerning Gravity and Gravitation, grounded on Expe∣rimental Observations, be Printed by the Printer of the Royal Society.
BROUNCKER, Pres. R. S.
A DISCOURSE OF Gravity and Gravitation, GROUNDED ON EXPERIMENTAL OBSERVATIONS: Presented to the Royal Society, NOVEMBER 12. 1674.
BY JOHN WALLIS, D. D. A Member of that SOCIETY.
LONDON, Printed for John Martyn, Printer to the Royal Society, at the Bell in St. Pauls Church-yard, 1675.
A DISCOURSE OF Gravity and Gravitation, GROUNDED ON EXPERIMENTAL OBSERVATIONS, Presented to the ROYAL SOCIETY, The 12th of November, 1674.
IN Compliance with the Commands of this Royal Society, in order to the pro∣moting of Experimental Philosophy, to present you a Philosophical Dis∣course, either grounded on, or lead∣ing to, Experiments; the Subject I have chosen for this Discourse, is that of Gravity: the Being and Effects of which, is no otherwise known to us than by Experience, or Discourse grounded thereupon.
Page 2The Subject is copious, and therefore I must single out some few Experiments out of a great many: And they shall be the most simple and unperplexed in their kind, that the Inferences may be the more clear and perspicuous; and such Experiments onely, as are either commonly known, or have already been made before You, or may easily be, whensoever you please.
I will not dispute the Nature of Gravity or Gravitation, what or whence it is; whether from a Quality within, or a Pressure from above, or a Magnetick Traction from be∣low: But shall take for granted (what every days Experi∣ence testifies) that there is, (at least in this our Sublunary World) such a thing as Gravity and Gravitation; whereby those we call Heavy Bodies, have a natural Propension to move downwards (towards the Earth, or its Center) if not hindered by some more Potent, or at least an Equivalent, Strength.
This Motion downward, we call Descent; the Endeavour so to move, we call Gravitation; and the Principle from whence this Endeavor proceeds, we call Gravity. And things are said to be more or less Heavy, as they have more or less of Gravity: Which may be understood, either Ex∣tensively, according to the Quantity of it; as when we say a Pound is heavier than an Ounce, though that be Fea∣thers, and this be Lead: Or Intensively, according to the Degree; as when we say, that Lead is heavier than Cork, or Quick-silver than Water; that is, gradually heavier, pro∣portionably heavier (bulk for bulk) or (as it is now wont to be called) Specifically heavier.
I say, the Endeavour thus to move, I call Gravitation; though by reason of some Impediment, there be not any actual Descent. (And it is allowed me, by those from whom in some other things I differ, that not onely Motus, but Conatus ad motum, is properly Gravitation.) Which I thought necessary thus to define, that I be not misunderstood Page 3 in the sequel. But I add further, this Endeavour of Descent, implies an Aversness to Ascent, with equal force; and that the one and the other are equally the Effects of Gra∣vitation.
This Gravitation, or endeavour to descend, is one kind of Strength; and may be opposed not onely by a contra∣ry Gravitation, but by any opposite strength whatsoever; whether by way of Impediment onely, or of contrary Force. For, though there be divers kinds of Strength; yet they are all thus far Homogeneous, as to be compared each with o∣ther, as equal or unequal, greater or less, and that in any Proportion.
Thus the Gravitation of the Scale A (in Fig. 1.) may be opposed, or hindered of its Effect, by a contrary Gravita∣tion at B, (supposing all the tackle strong enough;) or by a Force under it, which Thrusts it up; or by a Force a∣bove it, which Pulls it up, (or doth at least endeavour so to do;) as, for Instance, that of a mans Hand. Every of which, being contrary Forces, if equal to that of Gravita∣tion at A, will stop its Descent; if less, they will Retard it; if greater, they will force it upwards: Not by making it cease to Gravitate; but by defeating the Effect of that Gravitation.
But it is opposed also by the Strength and Stiffness of the Beam; (for if that either break or bend, A descends:) And by the Strength, though not the Stiffness, of the Strings that hold it, (for if they either break or stretch, the weight will descend, at least in part;) or by the Hardness, Strength, and Solidity of the Floor or Table on which it rests; which, if strong enough, will support it; or, if the Medium be Viscous, this viscosity (which is a degree of so∣lidity) will at least retard its Descent. All which do op∣pose it, not as contrary Forces, but onely as bare Impedi∣ments: Which, if strong enough, do hinder the Descent; but, though more than so, do not Thrust it up.
Page 4But if the Medium be supposed perfectly Fluid, in eve∣ry Point, without any aversness to separation; it may Hin∣der, or Retard the Descent of A, by a contrary Force, or contrary Gravitation, (it self also endeavouring a Descent by its own Gravity, or at least to preserve its Station, a∣gainst an Ascent;) but not as a bare Impediment from its Solidity, Firmness, or aversness from Separation; which is supposed to be none. It hath a resistance to Motion, not, to Separation. And of such Heavy Fluides, I intend prin∣cipally to discourse, and of Solid consistent Bodies, onely with reference unto such.
If it be objected, That there be no such perfectly fluid Bodies; but that those which we call Fluides, are either made up of very fine, but disunited, Atoms, (each having its own shape and figure, though very small) as the Ato∣mists suppose; or at least in some degree Unctuous and Viscous: I will not dispute that point, as not now neces∣sary; but only express what it is I mean by Fluid or Li∣quid Bodies; and the nearer any thing comes to such a condition, the nearer it is to perfect Fluidity. And where such Viscosity is very little and undiscernable, we consider it as none at all. And even that which is, if it be not stronger than the incumbent weight can break, it doth not wholly hinder the Descent, but onely retard it: The weight sinks, though not so fast.
Now, there is in all Heavy Bodies (whether Firm or Fluid) and in every part of them a Prospensity, not onely to a Direct Descent, but (if that cannot be obtained) to an Oblique Descent, according to any Declivity. (For a Ri∣ver will run down-hill, and so will a Bowl also; and a sloping Pole, if not supported, will fall obliquely.) Which I the rather note, because I find some to put a great stress on the Lateral Gravitation of Fluides, as peculiar to them; without taking notice, that the same is common to Solides also: The difference being but this; in Fluides the parts Page 5 be separable, but not in Solides: But the Tendencies are in both the same.
Now, of Fluid Bodies it is that I intend principally to speak. Of which the first and great Phoenomenon is this, That they will (if undisturbed) reduce themselves, by their own weight, to a Level; that is, to an Horizontal Plain, or what as to sense is such; and will so continue, if either not pressed at all, or equally pressed on all parts.
As if the surface, by any means, be Undulous, as ABAB, (in Fig. 2.) the Prominences at A, will sink to fill up the Cavities at B, till all come to the Level of LE. And this they will do, partly by spreading abroad, and flowing into those Cavities as lower places: And partly (the whole being fluid) by pressing down what is under A, and pres∣sing up what is under B (in Fig. 3.)
For though onely the former of these would happen in case all under LE (in Fig. 4.) were a firm Solid surface, (like as when water overflows the dry ground, and fills up all the furrows;) and onely the latter, in case such Promi∣nences (whether one or more) were contained within So∣lid Pipes, (in Fig. 5.) so as that they could not flow late∣rally into the adjacent Cavities: Yet in the present case, where both occasions happen, both Causes will operate.
For Nature doth not work by Election, but ad ultimum virium, and all the ways it can, where one doth not op∣pose the other. And like as if a Vessel have two holes, the one at the side, the other at the bottom; the water will run out at both: So the Prominences at A, being not hindred of either, will partly by lateral Fluxion, partly by direct Depression, fill up the Cavities of B (in Fig. 5.)
It's true, That a Solid Body, having opportunity of both, because (by reason of the coherence of parts) it can move but one way, will move that way only which is most Declive: But a fluid Body, being partible in every Point, divides it self every way, as there is opportunity.
Page 6Now, such fluid Body being thus reduced to a Level, if undisturbed, it will so remain (in Fig. 6.) For there be now no Prominences, as at A, (in Fig. 2.) to sink or flow down; nor Cavities, as at B, to receive them: Nor is any part of it more pressed than other, whereby that should sink, or this rise.
But if at some part, as at D, (in Fig. 7.) by weight or other force, it be pressed, but not in others; or more at D, than at others; it will at D subside or be depressed, and rise elsewhere, (in Fig. 8.)
And what is thus shewed of the Level LE, holds equal∣ly of any other Level, as F G. within the Fluid, at what depth soever, (in Fig. 8.) If all parts of it be equally pressed, it keeps its level; but if some parts of it be more than others, those will subside, and these rise: because the weaker force must give way to the stronger.
The like happens in a Syphon inverted, (in Fig. 9.) where if the water be higher in the one leg at A, than in the other B, that will sink, and this rise, till they come to a Level at LE: And when so, it will there rest, if there be no other force to put it in motion. So in an Ewer, (Fig. 10.) or other Vessel with a nose; the water in the Vessel (if higher) will sink it self, till that in the nose be raised to the same height; if that in the nose be higher, this will sink, and that rise, till they come to a Level at LE.
The Reason of it (if we do not study to perplex the Phaenomenon) is very evident: Because, while the Fluid (supposing it uniform) stands at the Level LE, no part of the same Horizontal Plain, at what depth soever, is more pressed than other, whereby it should be inabled to thrust any other out of place. Upon the same account, that of two Scales equally charged, neither can descend, or force up the other; but do mutually sustain each other in Equi∣pois, and are at rest. For though both do Ponderate, yet Page 7 neither doth Preponderate. And no Power is able to over-bear another Power, unless stronger than it.
But in case the Fluid be higher at A than at B, the parts under A are more pressed than those under B; and there∣fore those thrust these away. On the same account, that if the Scale A be heavier charged than B, though both press downwards; yet the heavier prevails, and forceth up the lighter. For, of contrary Powers, the greater always over-powers the lesser.
It will yet be not amiss, (that I may not in the sequel be mistaken) to give notice by the way, That what I have said of this Level in Heavy Fluides, is not so to be understood, as if this Level were in all cases Mathematically exact: For, though it ought so to be, if nothing else did intervene than what we have hitherto taken into consideration; yet ma∣ny times some little accidents do disturb it: As, when a Drop of water, on a dry board, keeps a convex Figure, ei∣ther because of some little Viscosity therein, or as shunning the contact of that dry surface; and Quick-silver in a Glass-Pipe, or like Vessel, will have a visibly convex surface, as shunning the contact of the Glass; and the like would hap∣pen in water, if the glass were greasie. And contrary-wise, the surface of water in such a clean Vessel would be rather concave; and so, I suppose, would be the surface of Quick-silver if the Glass were guilded within, because of its easie application of it self to Gold. It is observable also, that water in very slender Pipes, will rise visibly higher than the surface of that in the broad Vessel; because the Air can more conveniently apply its pressure on that broader Ves∣sel, than in the slender Pipe. And Fluides will many times, upon motion, retain an Undulation, or dancing up and down, sometimes above, sometimes below, the true Level, for a considerable time before they rest: Upon a like rea∣son, that a Pendulum will swing back and forth beyond the Perpendicular on either side, not by its weight simply con∣sidered, Page 8 (which would rest precisely at the Perpendicular, without rising on the other side,) but by reason of its con∣tracted Impetus. But these and other little inequalities, which are to be accounted for from divers accidents, we here neglect; and consider onely, what would be the re∣sult of the Gravity and Fluidity, freed from such other Accidents, too copious here to be insisted on. Our mean∣ing therefore is, that (setting aside other Accidents) a Fluid Body, will, by its Gravity, reduce it self to such a Level; and being so reduced, will so by counterpoise preserve it self, if not disturbed by other Force.
But it is here objected, That water upon water doth not Gravitate, (and the like of other not-springy Fluides;) be∣cause an Element (say they) doth not gravitate in its own place. And, for instance, they tell us, That a man under water, feels not the weight of the water over him, (in Fig. 11.)
Before I directly answer this Objection, I have this to say to the Principle they alledge: That the intendment there∣of at first, was no more but this; That the tendency of a heavy Body, being to the Earths Center; when there it is, its Heaviness (if not otherwise pressed) will not endea∣vour any further motion; (for, to move further, were to move from the Center:) And accordingly, if the tendency of any other Body be to a certain place, as its term; when there it is, that Principle will not endeavour a motion from thence; (for, so to do, were to move contrary to its own nature:) And if it be carried further, it must be from some other cause, (as when a Pendulum swings beyond the per∣pendicular, it is not from weight simply considered, which would there have stayed; but from an Impetus imnpressed by a precedent motion.) And thus far that Principle is just and good. But the Objection perverts it to a sense never intended by the first Introducers.
Page 9Next, I would ask; What is meant by the Waters own Place? And particularly, Whether water in a Pond, artifici∣ally contrived on the top of a Tower, be in its own Place? If so, then, though a hole were in the bottom, it ought not to run out. If not in its own place, then the Reason fails; for even there a Diver shall no more feel the weight of the water, than if in the Thames. So that it is not its be∣ing in its own Place, but somewhat else, that makes the weight not to be felt.
To avoid this therefore, and the like Instances; they now explain their meaning to be, That it doth not Gravitate on any thing which is not specifically lighter than it self. And to this Explication it is that we are to apply our Answer.
But neither will this hold. For it is manifest (to use an ordinary Instance) that a Vessel pierced near the bottom, (in Fig. 12, 13.) will run with a fuller and stronger stream, than if at the middle, or near the top; and more when it is full, than when half out, or almost empty. Which argues a Pressure of the upper parts upon those near the vent.
And to say, they press not on the intermediate parts, but onely on the Air without; is a meer evasion. For the remoter parts of the water cannot press that Air, but by pressing that which is between; like as in a crowd, he that is at a distance cannot thrust him that is at the door, but by thrusting those that are between: And, with a Pole, we cannot thrust that at the end of it, but by thrusting the Pole: Nor, with a Rope, draw that which is fastened to it, but by drawing the Rope.
Where yet there is a signal difference between Trusion and Traction. In Trusion, it sufficeth, that the thing be conti∣guous, though there be no Connexion; but in Traction there must be a Connexion, and that strong enough; else the string will break, and the weight not follow. And though a Heap of Sand will suffice to press down the Scale; yet a Page 10 Rope of Sand will not serve to draw it up. And therefore Mr. Line's Funiculus (in his Explication of the Torricel∣lian Experiment) must have somewhat of Texture (as well as Contiguity) to give it strength; without which it will not be able to sustain the weight of the suspended Quick-silver. But certainly, if the parts of a Fluid be able to Draw one another, much more will they be able to Thrust one ano∣ther; that is, the one to Gravitate upon the other.
It is therefore much more conceivable (in the inverted Syphon) how the water at A (Fig. 9.) should thrust up that at B, than how the Air at E ascending, should draw up the water at B, and thereby draw down that at A. For, in the first case, there needs only a Contiguity; in the lat∣ter, there must be a Connexion of all the Parts . And there∣fore if we should allow, that Mr. Line's Funiculus, or Rope of Sands, if granted, would equally solve the Phae∣nomenon, by way of Traction; yet, since the Hypothesis of Trusion (as is acknowledged) will do it also; it is much rather to be chosen than that of Traction, by a Rope of (Sands, shall I call it, or a Rope of) Nothings.
But further, it is confessed by a very learned Author, the Author of two Treatises; the one intituled, An Essay touching the Gravitation or Non-Gravitation of Fluid Bodies; the other, Observations touching the Torricellian Experiment, (who is pleased to conceal his Name) that defends the Fu∣niculus, and denies our Hypothesis; that not onely Water, but even Oyl in the Pipe A, (Fig. 5.) will force up the water at B: And if (the Pipe being empty) Oyl were pour∣ed on B, it would force up water into the Pipe A; not to a Level, but to an Equipois; that is, (as his own words are) to such a proportion of height in the Tube, as will coun∣tervail the weight of a like Cylinder of oyl; and gives the same reason for it, that we do; The disparity of pressure cau∣sing Motion or Elevation of the water, in that part nor equal∣ly pressed. So that here, a lighter Body doth Gravitate on Page 11 a Heavier; Oyl, upon Water: And that not onely ad pon∣dus, but ad motum, as Himself admits; that is, (in our Lan∣guage) it doth not onely Gravitate, but Pregravitate; not onely Weigh, but Out-weigh.
So that here, the notion of a Fluid not Gravitating on a Heavier than it self, or one as Heavy, is quite destroyed.
And it is manifest also, that not the Level, but the Equi∣pois, is that which is here attended. For the surface of the Oyl without the Pipe, because specifically Ligh∣ter, will be somewhat Higher than that of the Wa∣ter within it; and just so much as to make up the Equi∣pois. And, contrary-wise, if that in the Pipe were Oyl, and that without it were Water; that within the Pipe would be higher, and in such proportion higher.
The same would be, if that at B were stagnant Quick-silver, and that in the Pipe A were Oyl or Water, or some Lighter Fluid. A pound of Water poured into the Pipe, would it self stand higher, (because it would take up more room;) but would raise the stagnant Quick-silver just as high, as if a Pound of Quick-silver had been poured on; without any respect had to the specifick Gravity or Le∣vity
And a Ship laden, (Fig. 8.) will draw just as much wa∣ter, if laden with so many hundred weight of Timber, as with so many hundred weight of Lead; though that be Lighter, and this Heavier, than a like quantity of Wa∣ter
And a piece of Wood (Fig. 7.) though Lighter than Water, yet doth not float on the very top, but sinks so far into the Water, till it possess the place of so much Water, as is of equal Weight with it self; that is, till the Hori∣zontal Plain, passing by the bottom of the Wood, be in all places Equally pressed, partly with Wood, partly with Water.
Which being known Experiments, and confessed on all Page 12 hands, do quite destroy the notion of Non-Gravitation of Fluids on what is not specifically Lighter than themselves. And himself grants, (Essay, p. 14.) that Air in a Bladder, doth Gravitate on Water.
To avoid the Pressure of these Evidences; it is now al∣ledged, That the Oyl or Water in the Pipe A, (in Fig. 5.) though not intrinsecally Heavier, yet its Higher Position gives it an Accidental weight more than that in the Vessel; and hence it comes to pass, that That doth depress This.
But he doth not consider, that this doth destroy the whole design of his second Chapter; which is to prove, That C doth not Gravitate on D, nor D on E, in Fig. 14; that is, that the Upper parts of the Water do not Gravitate on the Neather. Whereas, if meerly a Higher Position will make it Gravitate; and that not onely ad pondus, but ad motum also; then must the Water be in perpetual Motion, (the Upper parts still pressing away the Neather, like as, on an∣other account, it happens in Boiling Water; I mean when the Fire is under it: for, if it be heated by Fire above it, the case is much alter'd;) which perpetual Motion, the said Au∣thor there urgeth as a great Absurdity. Yet I am not igno∣rant, that Mr. Boyle is indeed of opinion, That in all Fluids the minute parts are in continual motion; (making this the specifick nature of Fluidity, as contradistinct of Fixedness;) but that is on another account, and concerns not this Point at all. It is not therefore Safe for our Antagonist, to ascribe it onely to the Accidental Weight of an Higher Position. Nor is it Sound so to do. 'Tis true, that a different Position may give to the same Weight a different Ponderation:
As, for Instance; a Weight at G, (in Fig. 15.) will Pon∣derate more than at H; not, because Higher; but, because, at G, it is to descend directly; but, at H, on an Oblique Plain; which abates its Force, and doth more abate it as it is more Oblique.
Page 13And a Weight at rest in F or E, is of less Force to move the Balance, than when from A it falls to E; and less there than when it is fallen to F; and even this less, than if it had been violently thrown down: Because, in the latter cse, there is a greater contracted Impetus.
Again, at E or F, it will Ponderate more than at I or K; because those suspended at A, are at a greater distance from the Center C, than those suspended at D: The different Position, in all these, and many other the like cases, giving to the same Weight an Accidental additional Force.
But a Higher Position, meerly because Higher, gives no such advantage at all: The Weight at E being but just of the same force, as at F; and at I, as at K. For the length or shortness of the String on which it hangs, doth not at all al∣ter the Weight: As is agreed by all; and Experience testifies.
The Reason therefore of this Phaenomenon is not, be∣cause that at A in a higher Position, is of a greater Weight than a like quantity at B: But, because the parts at C, (in Fig. 5.) are more pressed than those at B; (as bearing the weight of CA, which B bears not:) whereby C is pressed down, and B thereby pressed up.
BUt, against this Explication, he brings an Experiment on wch he lays great weight. A Porringer filled with Lead, &c. which in the Air, as at A, weighed 78 ounces; weighed in the water about 68½ ounces; and the same Weight it held (with some inconsiderable difference, which he excuseth) whether at C, the depth of 40 or 25 inches; or at D, the depth of but 12, or scarce 1 inch, (Fig. 16.)
Where he attempts the account of two Phaenomena: First, Why it weighs less in Water than in Air? And Se∣condly, Why it weighs alike at several depths in Water?
Why it should weigh less in Water than in Air, he ascribes to the Resistance and Crassitude of the Water: And he tells us elsewhere, that, if we strike with our hand the surface of Page 14 Water, we shall find its Resistance not much less than if we struck a Board.
By which, if he mean the Viscosity, or Resistance to Separa∣tion, he speaks not to the present purpose: For, as to that, it is to be so far considered as a Firm Body, not a Fluid, which is that we are now speaking of.
But if he mean, a Resistance to be displaced, and thrust up∣ward, to make way for the Porringers descent; he says just the same thing with us: For such Resistance is properly Gravitation; and doth countergravitate to that of the Por∣ringer, and take off so much of its Praegravitation. Just as when the Scale B, (Fig. 16.) by its Gravitation resists the descent of A; because A cannot Descend without the Ascent of B, to which by reason of its Gravitation it is averse. And because the Porringer cannot descend but by thrusting up so much Water, the Water must needs give so much resistance to this Descent, as it gives to its own Ascent; that is, so much as the weight of the Water that must As∣cend; and hath just the same effect as if so much Water were put into the Scale B. And just so much, the Porrin∣ger weighs less in the Water than in the Air.
And as to what he says of the great resistance which the hand finds, when we strike hard on the Water; we are to consider, not onely the Weight of the Water, but the Swiftness requisite to make way for the Hand moving so Fast: Like as if a Weight of 10 pounds hang in the Air by a Thread; the least touch of the finger will move it, slowly: But, to move it 10 times so Fast, will require a force 10 times as strong: And, if you strike it hard with a swift stroke of the hand; that which made very little resistance to a gentle touch, will considerably withstand the stroke of a swift hand: Not, because the Weight is 10 times heavier than before, or doth 10 times as much resist Motion; but because it doth 10 times as much resist a Motion 10 times as Swift. Now, so much Strength as is requisite to move so Page 15 much Water with so much Swiftness as is necessary to make way for so swift a Motion of the hand; so much resistance must the Water give to such a stroke, from its own Gravi∣ty, without the assistance of the supposed Crassitude or Viscosity. But when in the present case we consider, how much the Porringer weighs in Water; we consider onely, whether it Remove so much Weight, though never so slowly; not, with what swiftness it will remove it; and, as to that, a very little weight more than what it moves will suffice.
But his main Objection lyes in the other Point, That the Porringer weighs as heavy at D, the depth of 12 or but of 1 inch; as at C, the depth of 25 or 40 inches, (Fig. 16.)
And just so, say I, it ought to be. For every thing weighs in Water just so much as its Weight is heavier than so much Water. As, for Instance, if the Plain δδ, (Fig. 16.) be in all parts equally pressed; it is, confessedly, the same as if not pressed at all: (for, so long, there is no reason why one part should rise, rather than another:) And so it would be if D were just as heavy as so much Water. But if D be heavier, then is that part of it over-charged, just so much as D is heavier than so much Waters as would fill the place if this were absent: And therefore, if not relieved by so much Weight in the Scale B, it will sink. And just so much will serve at C; that is, it must weigh equally, whether at the depth of C, or D, or any other depth.
But, saith he, if the incumbent Water do Gravitate on D, it will more Gravitate on C, because at a greater depth.
True, it doth so: But, as the pressure at C is greater than at D; so is the Counter-pressure at χ more than at δ; and just so much more. So that whatever was the Pregravita∣tion at D, must be the Pregravitation at C also. (And it is the Pregravitation onely, that is Weighed.) Just as when the Scale A outweighs B by 5 ounces, and into each Scale Page 16 you put 10 Pounds; it will yet outweigh, but just 5 oun∣ces, as it did before. So that his Argument from this Ex∣periment, will not hold against us.
And the Solution he gives, will hold as little. It is (saith he) because the Porringer drives up no more Water out of its Place at the one Station, than at the other.
But this is a mistake. For while the Pillar αC, (Fig. 16.) to make room for the Porringer, drives away the Water from C to χ, that at χ thrusts up all above it as high as α, to make room for it self; as αD doth all that over δ: So that the Water displaced, is not the same in both. And therefore the Porrigner, if not assisted by the incumbent Water, would not equally weigh in different depths; con∣trary to his own Experiment. Which therefore makes a∣gainst himself.
But the great plausible Objection is, that a Man under Water feels not the weight of it. And why (saith he) but because Mans body being heavier than so much water, the water doth not Gravitate on it.
But this Reason is (as the Schools speak) non causa pro causa.
If the Question were, Why the water doth not Raise the Body, (as it would do so much Wood;) the Reason had been good; Because so much water doth not press down∣ward more than the Body doth; and therefore is not able to press it away.
But when the Question is, Why a man doth not Feel it; that is, Why he is not Hurt by it, or put to Pain; the An∣swer, Because specifically Lighter, will not serve. For,
1. A Man, by this Reason, should not feel the weight of Wood, because proportionably lighter than himself: Yet we find a Man will as much sink under a Load of Wood, as a Load of Lead, if of equal Weight. And if it be said, This is, because, though the Man be not, yet the Air about Page 17 him, is Lighter than that Wood: I say, it is so; but this should therefore cause onely a Lateral Pressure on that Air, not a Direct Pressure on the Man. And, though a Man stood up to the neck in Water, he should yet find the burden of the Wood laid on his shoulder; notwithstanding that both the Man, and all about him, be Proportionably heavier than Wood. And he shall equally feel it, as if it were an equal Weight of Lead, if both be above the Water. So that the circumjacent Air, is not that which makes the Wood weigh upon the Man.
2. Though the whole Man be Heavier than so much Water; yet many Parts of him are Lighter; and would, of themselves, swim in Water, (though, by their Connexi∣on with some Heavier, they be made to sink; like Wood tyed to a piece of Lead:) Now all these Parts, at least, ought to feel Pain, if the specifick Gravity were the onely cause of Indolency: But do not.
3. A Man immersed in Quick-silver, which is a Heavier Fluid, though he would thereby be boyed up, yet would he no more feel the incumbent Weight, than a like weight of Water. And, though the Experiment cannot so con∣veniently be made in Quick-silver as in Water; yet as to part it may be made, by thrusting the Hand into Quick-silver, which shall no more be pressed by it, than if thrust into an Equivalent depth of Water; that is, about 14 times as deep. And Flyes, or other small Animals, immersed in Quick-silver, are not thereby pressed to death, but do safe∣ly emerge to the Top. So that it is but a Fansie to think, that onely the Proportional or Specifick Lightness of the Water, is the cause of that Indolence, since Liquids Pro∣portionably heavier, if not Positively heavier, will be felt as little.
4. Let us suppose an inverted Syphon, (Fig. 17.) filled from A to B with Quick-silver; from thence to C with Water, so high as to ballance the Quick-silver at A. If Page 18 now Oyl (which is Lighter than either) be poured on A; I ask, Whether the Quick-silver at A will not be thereby depressed, and that at B and C raised? Certainly it will. But why? The Oyl cannot (by their Principles) Gravitate on AB, because this is Quick-silver: Nor yet (as they speak) mediately upon BC, for even this is Water, and therefore heavier than Oyl: No, nor on the Air above C; for the Oyl at DA is already lower than it, and therefore cannot affect to possess its place. It should therefore, by their Principles, not gravitate at all, since there is nothing be∣low it lighter than it self, on which it should gravitate: Yet gravitate (we see) it will, and thrust out of place that whole Body ABC; notwithstanding (if that be consi∣derable) the higher Position of C, and its greater Specifick Heaviness. And all this while the Animal in BC shall re∣main unhurt, notwithstanding there be not onely Gravita∣tio ad Pondus, but Gravitatio ad Motum too.
So that the notion of Non-Gravitation on a Fluid not specifically Lighter than it self, is quite out of doors.
And the truth is, supposing ABC to be in Equipoise, the superfusion of AD will equally depress A, whatever the Liquor be, if the Weight be equal. And Ounce weight, will still be an Ounce weight; and an Ounce weight will just so much depress the Quick-silver, whether it be an Ounce of Wine, Water, Oyl, or Quick-silver; (that is, just so much as to thrust half that Weight, out of the Leg AF, Fig. 17. into the Leg FC;) without any regard had to the specifick Gravity or Levity of the Liquor AD, which, as to this Point, is of no consideration at all. And if the high∣er Position of D above A be thought of moment; the high∣er Position of C above both must be so too. And there will be nothing steady to fix upon, but, that the Positive Weight of DF being (at least in Proportion to the bigness of the Pipe) more than that of FC; that will thrust this away, till they come to an Equipoise.
Page 19It's true, that, if the specifick Gravity of the Liquor AD, were greater than that of the Quick-silver in AB; there would, upon another account, have been some difference: Be∣cause then, the Heavier Liquor being upmost, it would not onely press upon, but press into, the Body of the Lighter; and they would by little and little shift places; (as when Wa∣ter is poured upon Wine, that will by little and little sink to the bottom, and this rise:) Because, by such Descent, each Particle thrusts up a Lighter Body than it self. But, if the Upper be Lighter; though it press on the Hevier, it cannot press into the Heavier, without thrusting up a Heavier Body than it self. And this, I suppose, if they will consider their own Notion, is that they mean, when they say, A Lighter Body doth not gravitate on a Heavier.
And if so much Oyl were poured on A, as to thrust the Quick-silver beyond F; some of that Oyl would pass by it, into the other Leg, as high as C.
And, in such Cases as these, the specifick Gravity or Le∣vity is considerable: But not as to the Case in hand; where an Ounce of Oyl poured on A, shall depress it just as much as an Ounce of Quick-silver would do; and thrust up C just as High.
Beside this, (of Non-Gravitation on a Heavier Body;) the same Learned Author hath two Expedients for salving the Indolence of a Man under Water, or his not feeling Pain by the Weight of it.
The First is this: Supposing a Brick-work, as in Fig. 18. but without Mortar; if some few Bricks were taken out of the bottom, there would not hereupon sink a Pillar of that Base, but onely a Kind of Pyramid; the rest being, in manner of an Arch, mutually supported. And thence he supposeth, that those middle Bricks did not bear the Weight of a Column, but onely of a Pyramid. Which Pyramid if taken away, the rest would not gravitate upon that Ca∣vity. Page 20 And in like manner he supposeth it must be, if, for Bricks, were Grains of Wheat; yea, of Sand; and, conse∣quently, of lesser Particles; and, even those of Water; which he supposeth would thus support each other, with∣out gravitating on those under them.
But he proceeds upon several mistakes.
First, he supposeth, That, because those middle Bricks being taken away the rest do not fall; therefore, when they were there, they bore nothing of that Weight. Which is just as if he should argue, Because, when a Beam (in Fig. 19.) is supported by three Posts, if the middle Post be re∣moved, it will not fall; therefore, while it was there, it did bear nothing of the Beams Weight: Or, Because a Ta∣ble, (in Fig. 20.) supported by five or six Legs, will stand, though any one of them be taken away; therefore that Leg did bear nothing: And consequently, (because that Leg is any Leg) therefore none of them did bear any Weight: Whereas, while all were there, each did bear its part, and thereby ease the rest; which, in the absence of one, must now bear the more. And if the whole space under it were filled up with such Supports, (or, which would be Equi∣valent, an intire Body of that breadth,) each would bear (without any considerable difference) just so much as what is just over it. And such is the case of Fluids. Onely this I add, That if one of the Legs should be too weak to bear its Proportional part, yet if the rest be as much more than able to bear their; that weak one will not break, being relieved by the rest.
Secondly, Admitting that in Brik-work it would so be; yet it is onely upon this account, Because those parts of the Bricks which hang over, are coherent with the parts sup∣ported, and cannot fall without breaking the Brick: But, if they were as easily separable part from part, as Brick from Brick, (which is the case of Fluids;) those parts would fall, as well as the middle Bricks: And consequently, not a Py∣ramid, but a Column, or rather more.
Page 21Again, Thirdly, Whereas he argues, from Bricks to Grains of Wheat, and from thence, to Sands, &c; The Conse∣quence will not hold. For the shorter his Bricks are, the less will hang over in each Layer. As if now, for Instance, each Brick lye two Inches over; if the Bricks were but half so long, each would lye over but one Inch; and consequent∣ly (supposing their thickness the same,) the Pyramid on that Base would be twice as great (because twice as tall;) and still, as the over-hangings decrease, that Pyramid increas∣eth; till at length, when those over-hangings come to no∣thing (which is the case of Fluids,) the Pyramid becomes a Column, or even more than so. And if, in a Heap of Wheat, Fig. 21. (as here in a Pile of Bricks,) he remove so much of the bottom; he will find, that instead of a Pyra∣mid on that Base, there will fall down more than a Column (part of an inverted Pyramid:) And the more such Heaps approach to the nature of Fluids; the more will it be so. So that, by this Argument (if there were not another Ex∣pedient, of which I shall speak by and by,) the lower parts must bear, not less, but more, than the Column incumbent on them. And, if he found it otherwise in a Tube filled with moist Calice-Sand; this was not, because that above did not gravitate; but because it was so wedged in, that it could not fall. Which, in perfect Fluids, we are not to suppose.
Lastly, He doth, by this Explication, destroy his own Hypothesis. For he grants, in a Pail of Water (for Instance,) that all the Parts, as well upper as lower, do gravitate on the Bottom, though not each on other: Whereas, if those upper Parts be so supported (as in his Brick-Arch) as not to Gravitate on the Cavity; much less will they gravi∣tate on the Bottom, under that Cavity. And if, as he sup∣poseth, a great Heap of Wheat would not break an empty Egg-shell; it is not, because the Wheat wants Weight, or Gravitation; but because the Grains are so intangled as not Page 22 to fall right down, (like as in a heap of Bushes, one would bear up another, though all do gravitate:) But in Liquids it is otherwise; which we suppose partible in every Point.
But however, this of the Egg-shell happen to prove; it serves not his Hypothesis at all. For, the Air in the Egg-shell, being Lighter than the Wheat that lyes on it; this ought to gravitate (by his own Principles,) and to break the Egg-shell. If, by Arch-work, the Egg-shell be defen∣ded; this is not for want of Gravitation; but because that Gravitation is surmounted by a greater Strength. Like as when a great Weight hangs on a strong Tack; or a heavy Scale, supported by as great a Weight in the other, or by a support underneath; and a thousand other the like Ac∣cidents.
His other Expedient is, from the Lateral Pressure which he supposeth all Fluids to have; whereby he supposeth the Perpendicular Pressure to be abated.
But here he proceeds upon a mistake also. For, though it be very true, that Water will flow upon a Declivity; yet not as Fluid, but as Heavy. For we see a Bowl runs down a Hill, though not a Fluid, but a Solid, Body. And a broad Solid, lying on a narrow Pillar, (in Fig. 22.) hath in every part a Lateral Pressure as well as Water; and, if it be cut in the middest, will fall off on either side, as Wa∣ter would do. And, when it doth not; the reason is not a want of Propension, but because this Lateral Propension is checked or impeded by a Greater strength of Cohesion; like as its Perpendicular Propension is checked by that grea∣ter strength of the Pillar. And like as the Pillar, if too weak, will break under the Perpendicular Weight; so, if the strength of Cohesion be less than its Lateral Propension, the Solid will divide as a Fluid would do. As when a So∣lid breaks by its own Weight, (in Fig. 23.)
On the contrary; Water in a Pail (or other Vessel,) Page 23 though a Fluid, hath its Lateral Propension Restrained by the sides of the Vessel (as by a greater strength,) but doth not Lose it; and, if the sides chance not to be strong e∣nough, will break through; doth at least endeavor it, though they be strong enough.
So that, both in Solids and Fluids, each Particle hath its Lateral Propension, as well as Perpendicular; though it be sometimes restrained, or over-powered; there, by the Co∣hesion of Parts; here, by the Strength of the Sides: but (in both Cases) if those Strengths be too weak, that Propen∣sion prevails.
Now, as this Lateral Propension of Fluids, is kept in by the Sides of the Vessel, as to the utmost parts of it; so, as to the inner parts of it, they keep in each other. The La∣teral Pressure of A, (Fig. 24.) is sustained by that of B; and this by that; not as by Greater, but as by Equal Strengths. For A cannot thrust away B, without thrusting up a Body as Heavy as it self; nor B thrust away A.
So that, the Lateral Pressure of the Parts being mutually sustained each by other, and the Perpendicular Pressure by the Parts under it; hence it comes to pass, that those un∣der-parts bear onely the Pressure of a Column, and no more; (which is the Expedient that I intimated but now.) And therefore, in the Heap of Wheat, but now mentio∣ned, though, upon an aperture in the Bottom, more fall down than such a Pillar, (because, when that is gone, the Lateral Pressure of the rest doth operate,) yet, while that Pillar was there, that part of the Bottom did bear no more but it.
But if these Expedients of his do not serve; What is the Reason (you will ask) that the Man under Water, feels not the Weight of it?
I would Answer, First, That it is not agreed, that, at a great depth, a Man shall feel no Pain at all. And I hear, Page 24 that Mr. Gratrix having contrived a way of taking breath, at a great depth under Water, through long Pipes reach∣ing to the top of it; yet found his breast there so compres∣sed by the water, that he could not draw breath. But, in small depths of water, I do not deny but that a Man may remain for some time without any considerable Pain.
The Reason, I judge, is this; Because the man incompas∣sed by a Fluid, (whether specifically Heavier or Lighter than himself, it makes no matter,) is equally pressed on all sides; and thereby suffers no luxation of Parts; and, con∣sequently, no sense of Pain. But upon the luxation or la∣ceration of any part, especially a Nervous part, Pain ariseth. Hence it is, that our Flesh feels not the hardness of our Bones, because so fitted thereunto as to suffer no luxation or laceration by it: But, if the Bone be broken or dislo∣cated, we shall then find it to hurt us; and feel it hard and sharp.
And though the Body, by such compression, may be con∣tracted into a less room, by reason of the Air, Blood, and other springy Liquids; yet these being all uniformly pres∣sed, without any tearing of the Nervous parts, he suffers nothing of Pain from it.
And hence it is, that the Egg-shell, (but now mentioned,) though pressed by a Body specifically Heavier than it self, (by which therefore, according to their Principles, it ought to be crushed,) receives no prejudice, because equally pres∣sed on all sides: Which it doth the more easily sustain by reason of its round form, in the nature of a continued Arch. And we find, in experience, that a Round Glass, though but of equal thickness, will bear a much greater Pressure from without, than from within; and more than if it were flat-sided; and more, if the Pressure be of all sides, than if but in some onely. All which concur in the Egg-shell so situated: But if pressed onely upon one side, a less Pressure would break it.
Page 25I add also, That though in perfect Fluids there be no such Arching; yet in a Heap of Solids (as that of Wheat) something there is of that nature; and the more, as those Grains be bigger, and conveniently shaped; and may there∣fore help to bear the burden: Like as 4 or 5 Legs, in the Table we mentioned, if strong enough, will supply the de∣fect of one weak one; which therefore is not broken, though not strong enough of it self to bear its part. But the more any such Heap approacheth to the nature of a Fluid, the less is there room for such Arching; and, in perfect Fluids, none at all.
Hence it is also, that a Spunge, though Lighter than Water, and Flaccid also, will not yet (though fastened to the bottom of a Vessel) be crushed together by the Weight of the incumbent Water; Because the Water within its Pores doth bear out the sides with as great a strength, as that without would press them in. And the like we see, when the Lungs, taken out of Animals, are immersed in Water.
And the same account serves, for the pressure of Air on Animals. The Air within, pressing as strongly outward (by its Spring,) as that without, presseth inward; there is no hurt to the Animal at all. And, contrary-wise, the pressure of the Air into the Mouth and Throat, doth not break o∣pen his Brest or Belly, because ballanced with as great a Pressure without.
But if a Hand or Arm, be put into the Air-Pump, and the Air about it pumped out, that there be a failure of the outward Compression to ballance that within the Arm; the Spring of that within it, will put the Arm to a great tor∣ture, (as divers of this Society have found by experience.) And many Animals, by that means, have been killed with∣in the same Pneumatick Engine, in a much shorter time, than would have been for want of Respiration onely.
The like is seen in the breaking of Glass Bubbles herme∣tically sealed, and of Lambs Bladders, in the same Pneu∣matick Page 26 Engine, upon the Subtraction of the Ambient Air; as also the boiling of warm Water, and the strange expan∣sion of Blood into Bubbles, upon such Subtraction of Air; and many the like Experiments, made by Mr. Boyle (an Ho∣nourable Member of this Society) in that Pneumatick En∣gine of his Invention.
But while I name these, I do anticipate what I am next to handle; which is the Compression of Springy Bodies.
WE have been hitherto discoursing of such Fluids prin∣cipally as Water is supposed to be; that is, Fluids unca∣pable of Compression, because not Elastical or Springy.
But Springy Fluids, such as we suppose the Air to be, may by an incumbent Weight, not onely suffer a Trusion (as Water may) into another Place, (as from A to B and E, in an open Pipe, in Fig. 17:) but a Compression, into a less place. As for Instance, if the Pipe be close stopped at C, (or Hermetically sealed,) so as AB be Water, and BC Air, or other Springy Fluid; a superfusion or addition of the Weight AD (whether Fluid or Solid,) will raise B to E, and contract the Air BC into the space EC; that is, so much as till the Spring in CB, (which was a Strength equivalent to the Pressure of AB,) becomes (by this Contraction) equi∣valent to the Pressure of DB. And if more yet be super∣fused on D, CE will be yet more contracted, and so on∣wards; the strength of the Spring being still made equi∣valent to the pressure of the Weight. For, while the Spring CB is too weak; the Weight (being a greater strength) will thrust it closer: And, if CE be too strong, it will (as a greater strength) thrust away that Pressure: And can ne∣ver rest, but when the strength of the Spring is just equi∣valent to the Pressure.
So in Solids: If (for Instance) a Room or Vessel be fil∣led with Wool as high as BB, (Fig. 25.) and more Wool or other Weight (whether Heavier or Lighter than Wool) Page 27 be laid on, as to AA; the Wool shall be depressed to LE; and more yet, if more Weight be laid on.
And in like manner, if BCB be Air, and this pressed, either by the incumbent Air AB (supposing Air to be hea∣vy,) or by a Solid Weight or Force, so close on all sides, as that the Air cannot pass by or through it.
And, this being granted; the Torricellian Experiment (with others of the same nature) is, confessedly, solved by the Pressure of the Air; which was anciently thought to be by a Fuga vacui. For, if the Air be heavy, it must Gra∣vitate; that is, endeavour a Descent (as other Heavy Bo∣dies do,) and actually Effect it, if not opposed by at least as great a strength. And the Spring of the Air (allowing it to have a Spring) must always be of such a Texture, as is equivalent to the Weight or Force which it bears.
Now, as to the Weight of Air, or its Positive Gravity, the Peripatetick Philosophy doth not acknowledge it; but takes it to be Positively Light, and consequently to endea∣vour an Ascent. And some others say the same, not onely as to Air, but as to all Heavy Bodies. And whereas we suppose in them a Positive Gravity; and that what we call Levity is but comparatively so, being onely Gravity in a less degree; they take Levity to be Positive, and Gravity to be but a less Degree of Levity; and, consequently, those Heavy Bodies, not to Affect a Descent, but to be Thrust down by Bodies more Light, which more strongly affect a Higher Place.
But against these (the one and the other) I apprehend (as to Philosophy) these Inconveniences; which, to me, seem cogent Arguments.
If this Motion up-ward be natural; it must be either an Aversness from the Center, as the Terminus à quo; or a Propension to some other Place, as the Terminus ad quem.
If they say the Former; it is true, that then B ought to Page 28 move from C, in Perpendicular Lines, as CBA, (Fig. 26;) and the Phaenomenon doth not contradict it. But if the first intendmnent of nature be, not to be here; without any Po∣sitive tendency, where to be; it seems much more intelli∣gible, that somewhat should Thrust it thence, (by some∣what more forcibly pressing between,) than that it should Fly thence, without Affectation of any other Place.
But if they say, (as seems more rational, if Levity be the Positive Principle,) that it is an Affectation of some Higher Place, suppose A: While B is just between C and A, the motion ('tis true) would be in the Perpendicular CBA, (as the streightest way thither:) But if it were any where else, as at D; then its motion to A would not be in DCE the Perpendicular, but in DA an oblique line. Which is con∣trary to all Experience: For the same Light body, where∣ever it be, moves upward in a Perpendicular; as well as a Heavy body, in a Perpendicular downward.
And if, to avoid this, they would say, That it moves not to a certain place, as A or E, determinately; but to that place, whatever it be, that is just over it: I say, this is not properly the moving To a place, (if it be indifferent whe∣ther to A or to E;) but rather a moving From a place; that is, to be as far from C as it can: Which is the former branch of the Supposition, and against which we did before urge the former inconvenience. Which makes it not likely, that there is any such thing as Positive Lightness at all; since it will be hard to assign, what shall be the Terminus ad quem, which such a Mover aims at.
But waving this Argument from Philosophy at large; I shall argue from Experiment, (as to the Air,) thus:
Suppose we Air in the Bladder AA, (Fig. 27.) of the same Tensure with the External Air; and therefore such as will not (as they speak) Gravitate, or (as I would rather say) Praegravitate thereon; nor yet Praelevitate; (being of the same specifical Gravity or Levity with it:) If this be Page 29 compressed into a less room, as BB; it will then retain the same Quantity of Gravity or Levity as before, (since all that Air is still here, with all its positive Quality:) But (because now within less Dimensions) it will be Gradually, or (as now the Language is) Specifically, more than before, Heavier or Lighter, according as that Positive Quality was Gravity or Levity. (For, as the same quantity of Heat, in lesser space, makes the subject Intensively Hotter; so the same quantity of Heaviness, in a less Room, makes it Intensively Heavier; and, of Lightness, Lighter.) But experience testifies (as is confessed) that compressed Air is Intensively Heavier, or (as we now speak) Specifically Heavier, (and on the Bal∣lance is found so to be;) not Lighter, than before. There∣fore its Positive Quality was Heaviness, not Lightness.
The Positive Gravity of the Air being thus evinced; and, consequently, that the Air Ascends, onely because it is Thrust up by Bodies more Heavy; (like as Water riseth upon the casting in of Earth, or other Heavier Bodies;) the Torricellian Experiment, with other the like Phaenomena, are easily solved from Statical Principles, without having recourse to a Fuga vacui.
For, admitting (as before) that (in a Vessel with a Nose; or a Syphon inverted, Fig. 28, 29.) the Fluid at A, by sink∣ing it self, will raise that at B, to the Level LE; then, in case the Nose at B be not so high, the Liquor (if not other∣wise stopped) must needs run over. And, if any should say, the Reason hereof is, Because the Air at B flies away (by its Levity) and the Water follows to avoid a Vacuity; he would hardly be assented to by those, who see a visible Weight or Force at A, to over-press it, and thrust it out.
And, for the same Reason, if the Nose or Pipe, before it comes to the height of E, be recurvate, (Fig. 30, 31.) and turned down to O; that which would have run over at B; will now run out at O; being thrust up to B, by the Weight Page 30 of A, and falling down from thence, by its own Weight.
But in case A be lower than B, Fig. 32. (and the Fluid uniformly Heavy;) A will not be able to drive it up to B, much less make it there run over, or turn about to O: But, contrary-wise, if it were full to B, this would praeponde∣rate, and raise that at A.
Yet, if AC were a heavier Fluid, suppose Quick-silver; & CB a lighter, as Wine or Water; the Effect would follow as before; till the greater Height of CB, do countervail the greater Heaviness of AC.
And, contrary-wise, if AC be specifically lighter than CB, (Fig. 33.) suppose that Water, and this Quick-silver; then must that be in such proportion Higher than this, or else it will not rise to B, nor run out at O. But, if AC be higher than in such proportion; the Effect will follow, from the Praegravitation of A, without having recourse to a Fuga vacui.
And thus far the Ancients would agree with us. For they never flye to a Fuga vacui, so long as there is visible Weight or Force to Thrust up the Fluid.
BUt that which gave occasion to introduce this Notion of Fuga vacui, were but these Two Experiments, (and such as are reducible thereunto;) wherein, for want of a Force to raise Liquids by way of Trusion, they had recourse to this of Traction, ne detur vacuum.
The first that of Suction, in Pumps, Syringes, and other the like occasions. The other is that of a Syphon, whereby Li∣quors are carried over considerable heights above their Level.
For if the nose of a Syringe be immersed in Water, as at B, (Fig. 34.) and the handle or Embolus be drawn back; the Water or other Fluid will follow it, from B into D: Which being contrary to the nature of a Heavy Body, and no other Force appearing to thrust it up; it was imagined, that Nature abhorred a Vacuum, and this made the Liquor rise contrary to its particular Propension. To which Fuga Page 31 vacui (as it was wont to be called,) Linus of late (and some others after him) have given the name Funiculus. And the like is to be said of all sorts of Pumps, and other the like Engines, which draw Water by way of Suction.
And as to the Syphon; If the End C be immersed in Wa∣ter, or other Liquor, (Fig. 35.) though B, the top of the Syphon, be much higher than A, the surface of the Liquor; yet, if O be lower than A, though it will not of it self be∣gin to run; yet, if by Suction or otherwise, it be set a running, this Current will continue, till either A be sunk so low as to let in Air at C, or be lower than the outward Orifice O. The reason whereof, say they, (since there ap∣pears not any Force to thrust it up,) must needs be this; BO flowing out by its own Weight, if CB did not follow it (contrary to the Propension of its own Gravity,) a Vacuum must needs ensue; which therefore, they suppose, Nature doth abhor.
For Answer, I say, First, There being no other Founda∣tion in Nature to prove this Abhorrence, but onely these Experiments; and this not otherwise known, but being onely invented as an Expedient to serve a turn: If we can otherwise solve the Phaenomenon, and shew a Force which they did not think of; there will be no need of this Expe∣dient at all. And this Abhorrence must be either gratis dictum, without any cogent proof; or some other evidence must be shewed for it, than those who did introduce it were a∣ware of. For all the subsidiary proofs of late invented, were not the grounds of introducing the opinion. And therefore, without disputing, whether Nature can or cannot admit a Vacuum; I shall onely shew, That there is no need of that Notion as to this business.
Next; That this Fuga vacui is not the cause of Water thus rising in a Pump or Syphon, I thus argue. For, if so, it ought to hold to any height whatever. A Pump (for In∣stance) must draw Water an hundred foot high; and a Sy∣phon convey Water over the highest Hills or Towers. For, Page 32 the Argument equally holds, whether the height of B, be two Foot, or two hundred Foot; if BO flow out, and CB not follow, a Vacuum must insue equally in either case.
And the Consequence of this Argument is so clear, that, in confidence thereof, the Ancients did not doubt but that it would be so. None (that we know of,) till Galilaeo's time, having ever questioned it; or assigned any determinate height beyond which a Pump would not draw water, or over which a Syphon would not convey it. And it was a surpri∣sing discovery, and wholly unexpected, when (about the end of the last Century) it was first found out by experi∣ence, that Water could not thus be drawn higher than a∣bout 34 Foot. I say, about 34 Foot (not just so much) be∣cause that alters with the temperature of the Air. When the Air is very light, it will not much exceed 32 Foot; when very heavy, it may reach 35 Foot.
Which Experiment alone did evidently evince, that the supposed Fuga vacui, was not of an Infinite, but of a De∣terminate, strength. Which put Galilaeo upon the inquiry, Whether it were not from some other cause than Fuga va∣cui, that it would be drawn so high, but not higher. And he happily lighted on this Hypothesis, of the Counter-gravitation of the incumbent Air.
The same hath been since improved by Torricellio (and others after him,) who rationally argued, that if such Coun∣ter-gravitation of the Air, would countervail the Weight of 34 Foot of Water; it ought in lighter Liquors counter∣vail a greater height; and a less height in Heavier. And found, upon Experiment, that so it was: (If some little dif∣ference chance to be sometime discovered; it is to be ac∣counted for, from some different constitution of the Air a∣bout us, or other little accidents, too many to be here recounted:) And particularly, that, as Water would be so raised about 34 Foot; so Quicksilver, to the height of about 29 inches and no more: (I mean 28, 29, or 30, as the Airs temperature doth vary.) Which agrees with the proportion Page 33 of the specifick weight of those Liquids. (Quick-silver being near upon 14 times as heavy as Water.) And, from him, the Torricel∣lian Experiment takes its name.
The Experiment is thus administred, (in Fig. 36.) A Glass-Pipe closed at the bottom, being filled with Quick-silver, and then in∣verted; the orifice being stopped (with the finger or otherwise) till it be immersed in a Vessel of stagnant Quick-silver, and then open∣ed; If the height of the Pipe (above the stagnant Quick-silver) be not more than 29 inches, or thereabouts, it will remain full.
The cause hereof, say they, is, Ne detur Vacuum: For if the Quick-silver should sink, there being no way for the Air to enter, there would ensue a Vacuum, which nature abhors.
The cause, say we, is, Because the weight of the incumbent Air on A, (which we have already proved to be Heavy,) is equi∣valent to the weight of 29 inches of Quick-silver: Which there∣fore, being defended by the closed Glass (which we suppose other∣wise to be held firm,) from any other Pressure than its own Weight, is by that Counter-pressure sustained.
But further: If the height of the Pipe above the stagnant Quick-silver be more than about 29 inches; that in the Pipe will sink to that height, as at E, leaving space above it in the Glass, void of Quick-silver: (But, whether filled by any other imperceptible Fluid, we dispute not.)
The Reason why it so sinks, our Ancestors have not assigned; Because they were not at all aware of this Phaenomenon; but thought, that (ne detur vacuum) it would remain full, whatever the height of the Tube were.
Some Moderns (with Des Cartes) that they might avoid a Va∣cuum, do imagine, that a Materia subtilis (of which no Sense can make any dicovery) piercing the Pores of the Glass, supplies that place. But, if it will so supply the place above E, and give the Quick-silver leave to sink so low; why it might not as well come-in to relieve the rest, and so give it leave to sink to A, I do not find.
Others (with Linus) imagine, that the weight of 29 inches of Quick-silver doth stretch some part of its upper surface into a subtile matter, very thin, yet so as to fill that seemingly void space; but, because a less weight will not serve so to stretch it, it falls no low∣er; Page 34 which stretched matter, like a Funiculus, holds up the rest, ne de∣tur vacuum. But, why this Weight should stretch some very small part of it, so prodigiously thin, and not stretch the rest at all, rather than give some moderate Tensure to the whole; they do not (that I remember) assign any Reason.
Others, suppose this Funiculus to be made, not by stretching the upper part of the Quick-silver; but by squeezing out the more subtil parts from the whole body of it, which like a vapour fills that seemingly void space; but that less than such a weight would not so squeeze it, and therefore it falls no lower. But, why it should so fall out, that all Liquors whatever, of never so different Texture, should by the same weight be thus dissolvable; and not rather some require a greater, some a lesser weight thus to resolve or squeeze them; they assign no Reason: Yet we find so it is, since that the lighter the Liquor is, the greater height must be allowed, and in such proportion greater, to make up an equivalent weight.
But the cause is, say we, (and it seems the most simple and un∣forced account,) because the Counter-pressure of the Air, being equivalent to that of about 29 inches, so much it is able to sustain but no more; and just so much weight it will sustain whatever the Liquor be, whether specifically Lighter or Heavier, and whether of a more Firm or a Looser texture; and therefore to such a height it sinks, but no lower.
And had the Ancients been aware of what we find; That the Air hath a positive Gravity; and, consequently, though it be but small in proportion to that of other bodies, yet a great height of Air may countervail a lesser height of a Heavier Liquor; (like as we see that a greater height of Water will countervail a lesser height of Quick-silver:) They would not, I presume, have trou∣bled themselves with a Fuga vacui; but said roundly, That the weight of the Air at its full height, is equivalent to that of Wa∣ter at the height of about 34 foot, and of Quick-silver, at about, 29 inches, and proportionably of other Fluids. And consequent∣ly, when (in the Pump or Syringe) D by the Embolus or Sucker is defended from the Airs Pressure, but A exposed to it (in Fig. 34.) this Pressure on A, will raise, over B, so much weight of Water, Quick-silver, or other Fluid, as is equivalent to that Page 35 Pressure. In the same manner as (if A and E were equally ex∣posed to the Airs Pressure) a quantity of Oyl, poured on A, would have raised a weight of Water or Quick-silver equivalent thereunto.
The like account we give of the Syphon. The Pressure on A, (in Fig. 35.) will raise the Fluid to the height of B, if not greater than what is before described; and from thence to O, it falls by its own Weight: Yet so, that if O were higher than A, the Airs Pressure at O, would thrust up O to B (supposing the Pipe not so big, as that the Air could conveniently pass by the Liquor in∣to the Pipe,) and it would fall down to A by its own Weight. For now BO would less gravitate than BA; while yet the Airs Pressure would be much the same on both.
There is yet a considerable Objection to be removed, viz. That Air in a closed Vessel, though of no great height, pressing on A the surface of the stagnant Quick-silver, (Fig. 37.) will sustain as high a Pillar thereof in a closed Tube, suppose AE, as if A were exposed to the open Air: Whereas yet the Weight of AD within the Vessel, (defended by the Vessel from the Pressure of the in∣cumbent Air,) cannot be of equal Weight as if it had the whole height of the Atmosphere.
But the Reason of this is, from the Airs Spring; which is al∣ways equivalent to the Pressure lying upon it: And consequently, the Spring of the Air in its ordinary constitution with us, must be equivalent to the Weight of the incumbent Air. (For, if it were less, the Air incumbent would yet press it closer; if it were more, the Spring would relax it self, by thrusting away what presseth it.) Which being so; the Air included with such a Spring, must therefore press with as great a strength as is equivalent to such a Weight. Like as, in other Springs, if ACB (in Fig. 38.) be pressed by the Weight D to such a Tensure as to bear it; and then, this Spring so remaining, the Weight were taken away, and our hand put in the place of it; it would press as hard a∣gainst the hand, as before it did to sustain the Weight; that is, with a Force equal to that of the Weight it sustained: And if, thus bowed, it were put in a Vessel, (in Fig. 39.) it would, with Page 36 just the same Force, press against the sides of it. And just so it is in the present Case; where the air so included doth press by its Spring, just with the same Force as was that of the incumbent Air which gave it this Tensure.
It is yet the more evident, because if (by the Air-Pump) part of this Air be pumped out, and thereby the rest less compressed; the Quick-silver in the Tube, (in Fig. 37.) will sink from E to a lower Station, as to F or G; and so lower and lower, as more and more Air is pumped out, and the Spring thereby relaxed: That is, as the Spring grows weaker, so it is less able to support the Weight.
And this quite destroys the Evasions but now mentioned; That the seeming void space is filled by a thin Substance, which can by the Weight of 29 inches of Quick-silver, or 34 foot of Water, but not by less, be stretched to that fineness; and that therefore it will sink to that height, but not lower.
For, by this last Experiment, when the Air is included with its ordinary Tensure, it sustains the Quick-silver at the height of 29 inches; as if less than that Weight were too little to stretch the Quick-silver into that supposed fine substance: But, when that Air, by pumping, is weakned; it will sink to 20, 10, 5, yea less than 1 inch of height; as if now less than the Weight of 1 inch were enough so to stretch it, as less than 29 inches would not do before. Yet is no alteration, all this while, made in the Tex∣ture of the Quick-silver; but in the Tensure of the Air onely. 'Tis therefore from this different Tensure or Spring of the Air, not from any difference in the Quick-silver, that it stands some∣time at a higher, sometime at a lower station.
And what hath been thus said of this Torricellian Experiment, is easily applicable to others of like nature. And it is confes∣sed, that, as the notion of Fuga vacui, or that of the Airs pres∣sure, doth stand or fall as to this Experiment; so must it do as to the others also. I content my self therefore, to have shewed it in this; without expatiating to other particulars.