A continuation of new experiments physico-mechanical, touching the spring and weight of the air and their effects. The I. part whereto is annext a short discourse of the atmospheres of consistent bodies / written by way of letter to the right honourable the Lord Clifford and Dungarvan by the honourable Robert Boyle ...

EXPERIMENT XLVII.

About an attempt made to measure the force of the Spring of included Air, and examine a Conjecture about the difference of its strength in unequally broad mouth'd Vessels.

THough several of the foregoing Tryals have sufficiently ma∣nifested that the Spring of the Air in its natural or wonted state, hath a force very considerable, and indeed much Greater than men seem to have hitherto believed; Yet I could not hope by any of these Experiments to determine by any known weight, how Great that force is, so as to conclude that it is equivalent to such a Weight, as so many Pounds, Ounces, &c. and to no more. Wherefore among the Uses I had design'd to make of our Sy∣ringe, formerly often mentioned, it was One, to try if by the help

of that Instrument, we could determine somewhat near (for no more was to be expected) how much Weight a Cylinder of un∣comprest Air included in it, and consequently of the same Diame∣ter vvith the cavity of the Barrel, would be able to sustain or also to lift up.

In order to this Tryal, 1. we provided a stable Pedestal, or Frame, wherein the Syringe might be kept firm, and erected. Next, vve also provided a Weight of Lead shap'd like our Brass-hoop, or Ring, formerly describ'd, that by the advantage of its figure it might be made to hang down by strings from the top of the Handle of the Rammer, and so press evenly enough on all sides, without making the upper part of the instrument top-hea∣vy. 3. We took care to leave, between the bottom of the Sy∣ringe (which was firmly clos'd with strong Cement) and that part of it where the Sucker was, a convenient quantity of Air, to expand its self, and lift up the Weight, when the Air external to that included Air should be pump'd out of the Receiver: And last∣ly, the Handle of the Rammer (from which the Annular weight lately spoken of depended) was so fastned to the Turning-key of the Cover of the Receiver, that the Weight might not compress the Air included in the Syringe, but leave it in its natural state or wonted Laxity, till the Air were withdrawn from the Recei∣ver.

But notwithstanding all this, when we actually tryed the Expe∣riment, That hapned which I feared. For though by this method the included Air would well enough lift up a Weight of 7 or 8 pound, yet when the Rammer came to be clogg'd with so consi∣derable a Weight, as my scope in making the Experiment requi∣red, the Instrument prov'd not so stanch, but that it was easier for some particles of Air to force themselves a passage, and get a∣way between the Sucker and the inside of the Barrel, than to heave up so great a Weight. And yet I have thought fit to relate the Experiment thus particularly, because, if an exact Syringe can be procured, (which I fear will be very difficult, but do not

think impossible, this seems to be one of the likeliest and least ex∣ceptionable wayes I know, of measuring the force of the Airs Spring.

But despairing to get such a Syringe, as I desir'd, in the place where I then was, I bethought my self of another way, by which I hop'd to be able (though not to arrive at an exact knowledge of the full force of the Airs Spring, yet) at least to approach nea∣rer it than I have been able to do by the help of the Syringe. For this purpose considering with my self, that if a convenient quanti∣ty of Air were included in a fine small Bladder, the sides of it would hinder the Air from getting away, and the limberness of them would permit the Air to accommodate it self and the Blad∣der to the Figure of a Cylindrical vessel, into which it might be put.

Wherefore with much adoe I procured to be made by a person exercised in Turning a couple of hollow Cylinders, whose sides were of a sufficient thickness, (that they might resist the pressure of the Air to be imprisoned in them,) and of such differing breadths, that the first had but one inch in Diameter, and the 2^d two; their depths being also unequal, that the one might receive a much larger Bladder than the other.

With the lesser of these (which was very carefully Turned) I made a diligent Tryal; whose Circumstances I cannot now ac∣quaint Your Lordship with, the Paper, wherein they vvere am∣ply recorded, having been vvith other Notes belonging to this Continuation unluckily lost: but the most considerable things in the Event were, That twas very difficult to procure a Bladder small and fine enough for that litle Cylinder; and that one, which at length we procured, would not continue stanch for many Try∣als, but would after a vvhile part with a litle Air in the well exhau∣sted Receiver, when twas clog'd with the utmost Weight it could sustain: but whilst it continued stanch vve made one fair Tryal vvith it, from vvhence vve concluded, that a Cylinder of Air of but an inch in Diameter, and lesse than two inches in length, was

able to raise visibly (though but a litle) a Weight of above ten Pounds, (I speak of Averdupoiz vveights, vvhere a Pound con∣tains 16 ounces.) The manner of making this Experiment, and the cautions us'd in judging of it, Your Lorship may learn by the recital of the subsequent Tryal; my Notes about which were not so unfortunate as those that concern'd the former.

Into a hollow Cylinder of Wood of four inches in depth, and two in Diameter, furnished with a broad and solid bottom or Pe∣destal, to make it stand the firmer, was put a Lambs or Sheeps bladder very strongly tyed at the Neck, on vvhich vvas put a Wooden Plug, markt with Ink where the Edg of the Cylinder vvas contiguous to it; this Plug being loaded with Weights, a∣mounting to 35 pound, (the uppermost of vvhich Weights was fastned to the Turning-key, to keep it upright, and to help to raise it at first,) the Receiver vvas exhausted, till the Mark ap∣peared very manifestly above the brim of the Cylinder; and then, though the string were by turning the Key quite slackned, yet the mark on the Plug continued very visible: and vvhen so much Air was let into the Receiver, as made the Weight depress the Plug quite beneath the Mark, upon the repumping out of the Air the Weight was without the help of any Turning-key lifted up, and by degrees all the mark on the Plug was raised about 3/8 above the edge of the Cylinder.

Wherefore we substituted for a 7 pound weight one that was estimated at 14, (for then we had not a Ballance strong enough to weigh it with,) and using the same Bladder we repeated the Ex∣periment, onely having a care to support a litle the uppermost Weight by the Turning-key, till the Bladder had attained its ex∣pansion; and then the Weight being gently let go, depress'd not the Plug so low, but that we could yet see the mark on it, (which yet was all we could do,) though that part of the Plug, where the mark vvas, vvere manifestly more deprest than the other.

For the clearing up of some particulars relating to this Tryal, we will subjoyn the following Notes.

1. The Plug is to be so fitted to the Cavity of the Cy∣linder, as easily to slip up and down in it, without Grating a∣gainst the sides of it, lest it needlesly increase the resistance of the Weight to be rais'd. And this Plug ought to be of a conveni∣ent length, as about an inch and 1/2 at least, that it may be the fitter to help to reduce the Bladder by compression into a somewhat Cylindrical shape, and yet that it may not be thrust in too deep by the incumbent Weight; and that the Weight might rest more firmly upon it, there was a broad and strong Ledge made at the top of it, by which it might lean on every side upon the brim of the hollow Cylinder.

2. Before the Instrument was conveyed into the Receiver, the Bladder (which ought to be of a just size, and not full blown, and of a fine and limber contexture) was put into the Cylinder, and by divers gradual (but not immoderate) compressions was reduc'd to conform its self, as much as might be, to the Cylin∣drical shape of the containing vessel. And then the Weight be∣ing put on, and taken off again, there was a mark (in the form of an horizontally plac'd Arch) made with Ink, where the edge of the brim of the hollow Cylinder did almost touch the Plug. This we thought necessary to do, to avoid a mistake; for we must not judg, that all the Weight, that might be rais'd by our Bladder, may pass for the Weight sought after by our Experiment; since the Air in the Bladder is by reason of the incumbent weight more comprest than twas before, and consequently its being able to heave up a Great weight will not infer, that our common Air is able in its natural state (as they call it) to exert so Great a strength; that Weight being onely to be lookt on as rais'd or sustain'd by the uncomprest Air, that is rais'd or sustain'd when the Plug is lifted up to the mark, since till then the Spring of the Air does but bring it back from its new state of adventitious compression to its natural or wonted Laxity.

3. When, after the operation was ended, we took the Blad∣der out of the vessel, it had obtain'd a form Cylindrical enough,

and though it could be but 2 inches in Diameter, yet it was so litle as to be but half an inch more long than broad.

4. The reason why I chose to have the two Cylinders made of the unequal Diameters above mentioned, was to examine, as far as by this way I could, a conjecture I had, that the force of the Spring of differing Cylinders of Air to lift up solid Weights, would, at the very first raising of the Weights, be in duplicate pro∣portion to the Diameters of their Cylinders, (those Diameters being proportionable to the Areas of the plain Superficies, against which the Air does immediately press,) without very much consi∣dering the inequality that may be between the quantity of the se∣veral parcels of Air, whose pressures are compared. But tis to be remembred, that I said at the very first raising of the weights, because presently after That, the quantity of the parcels of Air may be very considerable: for, as I have shewn in another Trea∣tise, two very unequal quantities of Air being made by their Ex∣pansion to possess two equal spaces, the lesser quantity of Air must be much more rarified in proportion than the greater; and con∣sequently, (to bring this home to our present Argument) though both be lifted up ¼ or ½ of an inch, the Spring of a very litle Air must be much more weakned than that of a very considerable Quantity, and so it cannot continue to lift up its weight, as the a∣bove mentioned proportion would (if it were not for this Adver∣tisement) seem to require.

Taking then our conjecture in the sense now declared, the suc∣cess of our Tryals is agreeable to it, inviting us to conclude, that the Air in the Bladder, which was but two inches in Diameter, was able by its Pressure to countervaile the weight of 42 pound, which is about four times the weight that we lately observ'd the Spring of a Cylinder of Air of one inch in Diameter to be able to lift up. For though, according to what we have formerly said of a duplicate proportion, 42 pound seems to be somewhat more than ought to have been lifted up in the Cylinder of two inches bore, when that of one inch lifted up not much above 10 pound;

yet this disagrees not with the Hypothesis, if we consider that the substance of the Bladder straitens the cavity of the smaller Cylin∣der in a Greater proportion than that of the bigger.

5. Though we have thus (as far as the Instruments we were a∣ble to procure would assist us) measured the Pressure of included Air, yet I must not forbear to advertise Your Lordship, that con∣sidering what I formerly observ'd to You about the weight of an Atmospherical Pillar of an inch in Diameter, I cannot but think, that if a Cylinder, or other convenient instrument, exactly Tite, can be procured, the Spring of an Aerial Cylinder will appear to be Greater than we found it by the foregoing Tryals; in which I consider that, not to mention the resistance of the Bladder its self, the membraneous substance that lin'd the Cylinders (though twere very thin and fine) could not but somewhat straiten their Cavities, and consequently somewhat (though not much) lessen the Diameters of the included Aerial Cylinders.

6. To all these Notes I must adde this Advertisement, That it may be therefore the more difficult in such Tryals as ours to ascertain the force of the Airs Spring, because, that Air its self when tis included, being shut up with the Pressure of the Atmo∣sphere upon it, tis probable, that since that Pressure (as we have shewn) is not at all times the same, the Spring of the included Air will accordingly be varied. And, if my memory fail me not, when the lately recited Experiments were made, our Barometer declared the Atmosphere to be somewhat light.

From what has been hitherto delivered, this may result; that tis likely, that the Spring of an Aerial Cylinder an inch broad, may be able to sustain, if not raise, a pretty deal more than ten pound Weight; and that the past Tryals, without determining that the Air can raise no more than in them it did, do, at least, prove that it can raise up as much Weight as we have related, since we actually found it to do so.

Notes

• Expe. the Vth.
• See plate the Figure the