Tracts written by the Honourable Robert Boyle containing New experiments, touching the relation betwixt flame and air, and about explosions, an hydrostatical discourse occasion'd by some objections of Dr. Henry More against some explications of new experiments made by the author of these tracts : to which is annex't, An hydrostatical letter, dilucidating an experiment about a way of weighing water in water, new experiments, of the positive or relative levity of bodies under water, of the air's spring on bodies under water, about the differing pressure of heavy solids and fluids.

CHAP. III.

THese things being premis'd, I thus argue: 'Tis manifested by Hydrostaticians after Archimedes, that in water, those parts that are most press'd, will thrust out of place those that are less press'd: which both a∣grees with the common apprehensi∣ons of men, and might, if it were needful, be confirm'd by Experi∣ments. 'Tis also evident, that that part of the above-mention'd imagi∣nary Plane, that is cover'd by the woodden Plate, must be pressed by a less weight than the other part of the same Plane; because the wood being bulk for bulk lighter than water, the

aggregate of the wood and water in∣cumbent on the cover'd part of the same Plane must be lighter in specie, than the water alone that is incum∣bent on the uncover'd part of the same Plane; and consequently this uncover'd part being more press'd than the other part of the Plane, the heavier must displace the lighter, which it cannot do but by thrusting up the board, as it does, when the ex∣ternal force that kept it down is re∣moved. And, to add this upon the by, this greater pressure against the bottom than against the top of bo∣dies immers'd in water specifically heavier than they, is a true reason of their emersion, as I have elsewhere shewn. So that there happens no more in this case than what usually happens in the ascension of bodies in liquors specifically heavier than themselves, on the account of the

newly mention'd difference of Pres∣sure. And 'tis with an (express or suppos'd) exception of such a diffe∣rence, which in many other cases may be safely neglected, that (which I desire you to take notice of,) in most places of this discourse I speak of the Pressure of ambient Fluids on im∣mersed Solids as uniform or every way equal.

'Tis true, that according to the Doctors supputation, if the solid Cylinder, consisting of the woodden Plate, and all the water directly in∣cumbent on it, were put into an or∣dinary ballance, it would there many times out-weigh the hollow Cylinder of water alone that leans upon the un∣cover'd part of the imaginary Plane. And that is it that seems to have de∣ceiv'd the Learned Doctor. But there are divers Hydrostatical Cases, where∣in the Phaenomenon depends not so

much upon the absolute weight of the compared Bodies, as upon their re∣spective and their specifick Gravity; on whose account it is, that a small Pible, for instance, that weighs not a quarter of an Ounce, will readily sink to the bottom of the river, on whose surface a log of wood of a hun∣dred pound in weight will float. 'Tis a Rule in Hydrostaticks, that when two portions of water or any other Homogeneous liquor press against each other, the prevalency will goe, not according to the absolute weight, but the perpendicular height of those Portions. And accordingly we find, that if a slender pipe of glass, being fill'd with water, have its lower ori∣fice unstop'd at the bottom of a ves∣sel of water, which contains much more of that liquor than the pipe; yet if this last named water were, for instance, two foot high, and that in

the Vessel but one, the water in the pipe will readily subside, till it come almost to a level with the external water, though it cannot do so with∣out raising the whole mass of water that stagnated in the vessel.

And now I shall subjoin an Experi∣ment, which, though at first it may seem slight, and was made in lesser glasses & quantitys than I would have imploy'd if I could have procur'd better Accommodations, has the ad∣vantage of requiring no curious in∣struments, and yet I hope will serve for an ocular proof of the fallacious∣ness of that reasoning the Doctor is so strangely confident of.

We took an open mouth'd glass, such as some call Jarrs, and Ladys often use to keep sweet meats in, which was three inches and a half or better in Diameter, and somewhat less in depth; and had the figure of its cavity

Cylindrical enough. Into this hav∣ing put some water to cover the pro∣tuberance, wont to be at the bottom of such glasses, we took a convenient quantity of Bees-wax, and having just melted it, we poured it cautiously into the glass, warm'd before-hand to prevent its cracking, till it reach'd to a convenient height. This vessel and the contained liquors we set aside to cool, in expectation, that when the heat, that had dilated the wax, was gone, it would shrink from the glass, and consequently leave a little inter∣val every where between the concave superficies of the vessel, and convex of the harden'd wax; which accor∣dingly came to pass, and sav'd me the labour of getting the wax shap'd for my purpose with tooles; which might have been done but not with∣out trouble and less exactness. And now 'twas easie for me to try the ex∣periment

I design'd; for, pouring in warily some water between the glass and the wax, so that it fill'd all the in∣terval, left between those two bodys both at the bottom and the sides, the wax was made presently to float, be∣ing visibly lifted up from the bottom, and its upper part appearing a little above the level of the water, which was no more than I did, and had rea∣son to expect, according to the true Principles of Hydrostaticks. For water being somewhat, though but lit∣tle, heavier, in specie, than wax, and that which was poured into the bottom and stagnated there, being press'd by the collateral water, every way interpos'd between the concave part of the Glass and the convex of the Wax (so that this col∣lateral liquor answer'd what I lately called a hollow Cylinder of water in the Doctors Experiment) that part

of the stagnant water, that was lean'd upon by the wax, being less press'd than the other part of the same stag∣nant water was by the water incum∣bent on it; this latter must displace the former, which it could not doe but by raising up the wax that lean'd upon it. And yet this collateral wa∣ter was so far from being heavier than the wax its pressure impell'd up, that both the collateral, and the stagnant water all together, being weigh'd in good scales, amounted to little above a quarter of the weight of the wax, which happen'd by rea∣son of the narrowness of the Vessel, which, if it had been wide enough, I doubt not but the experiment would have succeeded, though the wax had outweigh'd the collateral water ten times more than in our experiment it did. But that the solid body excee∣ded almost four times the weight,

not onely of the collateral but the stagnant liquor too, does sufficient∣ly overthrow the Doctors ratiocina∣tion. Whose fallaciousness will yet further appear by two other improve∣ments, among others, which I made of one Experiment.

For I. though we pour'd in more and more water, as long as the Ves∣sel would contain any, the Cylinder of wax was but lifted higher and higher from the bottom of the glass, but did not appeare rais'd more than at the first, above the upper surface of the water; which argues, that 'twas not at all the Quantity of the inferior water, which was continually increas'd, but the pressure of the col∣lateral water, which continued still at the same height in reference to that wax, that caus'd the elevation of the body.

And II. to manifest yet more

clearly the Doctors mistake, I devi∣sed the following tryal. We took a round plate of Lead about the thickness of a shilling, and having made it stick fast to the bottom of the Cylinder of wax, to make this body sink the more direct∣ly, we placed one after another upon the upper part of the wax divers grain weights (first wetted to keep them from floating) till we had put on enough to make the wax subside to the bottom: For the facilitating whereof we had par'd off its edges; by this means, the glass having been at first almost fill'd with water, there swam about an inch or better of that liquor above the upper surface of the wax. And lastly, we took off by de∣grees the grain weights that we had put on, till we saw the wax, not∣withstanding the adhering Lead, rise, by degrees, to the top of the water,

above which some part of it was visi∣bly extant.

From this experiment I thus ar∣gue: 'Tis manifest, that, according to the Doctors supposition, here was in∣cumbent upon the wax a Cylinder of an inch in height and of the same Di∣ameter or breadth with the round sur∣face of the wax, whereas upon the removing part of the water, that lay at the bottom when the wax began to rise, there was incumbent no grea∣ter weight than that of the collateral water, and as much of the superior and stagnant, as was directly imcum∣bent upon that collateral water (and would have deserv'd the same name, if we had suppos'd the convex sur∣face of the wax to have been con∣tinued upwards as high as the glass reach'd.) But now, whereas, according to the Doctors ratiocination, this Cy∣linder of water incumbent on the

wax, being an inch deep, and a good deal above three inches broad, must press the wax with a greater weight by several times, than that which the lateral and hollow Cylinder of this stagnant water could have upon the rest of the collateral water; yet the height of this aggregate of collateral waters being the same with that of the wax and the water swimming up∣on it, the difference of the pressure was so small, that barely taking off a weight of four or five grains, the wax would, notwithstanding the pres∣sure of the water incumbent on it, be impell'd up and made to float: And by the like weight, put again upon it, it would be made to sink, and by ano∣ther removal of such a weight, (for I purposely reiterated the tryal more than once,) it would, though slowly, reascend. And these Phaenomena do so much depend upon a Mechanical

aequipollence of pressure, that even four grains would not have been ne∣cessary to make the wax rise or sink, if it had not been for some little ac∣cidental impediments, that are easily met with in such narrow glasses; for otherwise in a larger Vessel we have made the same Lump of Wax readi∣ly enough sink or float, by the put∣ting in or taking off a single grain or perhaps less.

By this you may see, that for the Regulation of Hydrostatical things, Nature has her ballance too as well as Art, and that in the ballance of Na∣ture the Statical Laws are nicely e∣nough observ'd.

You may also take notice, upon the by, how little the weight of the Cylinder of water upon a body im∣mers'd in stagnant water is conside∣rable, whilst there is a pressure of collateral water to counterballance

it; since in this last tryal, though the Cylinder of incumbent water did con∣tinually increase or decrease in length, whilst the lump of Wax was sinking or emerging; yet the same despicable weight of a grain or less, that was just able to depress it beneath the upper surface of the water, did by its pressure or removal procure its sinking to the very bottom, or rising again to the top, and on both occasi∣ons with an equal slowness, bating that little acceleration of motion, that ought to happen upon another ac∣count, and which therefore is to be observ'd in the wax, during its ri∣sing as well as during its sinking.