STATICS. Doctor. But there are many hydroftatical cafes, wherein the phenomenon depends not fo much upon the abfolute weight of the compared bodies, as upon their respective and specific gravity; on account whereof, a small pebble, for inftance, that weighs not a quarter of an ounce, will readily fink to the bottom of a river, on whofe furface a log of a hundred pound weight will float. 'Tis a rule in hydroftatics, that when two portions of water, or any other homogeneous liquor, prefs against each other; the prevalency will go, not according to the abfolute weight, but the perpendicular height of thofe portions. And, accordingly, we find, that if a flender pipe of glafs, being fill'd with water, have its lower orifice unftopp'd at the bottom of a veffel of water, which contains much more of that liquor than the pipe; yet if the water in the tube were, for inftance, two feet high, and that in the veffel but one; the water in the pipe will readily fubfide, till it comes almost to a level with the external water; tho' it cannot do fo, without raising the whole mass of water stagnant in the veffel.

Demonftrated We took an open-mouth'd glass, or jar, three inches and a half in diamebyexperiments. ter, and fomewhat lefs in depth, its cavity being cylindrical; into this, having put fome water to cover the protuberance that is left at the bottom of fuch glaffes, we took a convenient quantity of bees-wax, and having juft melted it, we pour'd it cautiously into the glafs, warm'd before-hand to prevent its cracking, till it reach'd to a convenient height. This vessel, and the contain❜d liquors, we fet afide to cool, that the wax might shrink from the glass, and, confequently, have a little interval every where between the concave fuperficies of the veffel, and convex of the hardned wax; when, carefully pouring fome water between the glafs and the wax, fo that it fill'd all the interval left between those two bodies, both at the bottom and the fides, the wax was made presently to float; being visibly rais'd up from the bottom, and its upper part appearing a little above the level of the water, as it ought, according to the true principles of hydroftatics. For water, being fomewhat heavier, in fpecie, than wax, and that which was poured into the bottom, and ftagnated there, being prefs'd by the collateral water, every way interpos'd between the concave part of the glafs, and convex of the wax; (fo that this collateral liquor anfwer'd what I call a hollow cylinder of water, in the Doctor's experiment) that part of the ftagnant water which was prefs'd upon by the wax, being lefs prefs'd than the other part of the fame ftagnant water was by the water incumbent on it; this latter muft difplace the former; which it could not do, but by raising up the wax that refted upon it; yet this collateral water was fo far from being heavier than the wax impell'd up by its preffure, that both the collateral and the ftagnant water together, being weigh'd, amounted to little above a fourth of the weight of the wax; which happen'd by reafon of the narrowness of the veffel: but if it had been wider, the experiment, I doubt not, would have fucceeded, tho' the wax had out-weigh'd the collateral water ten times more than in our experiment it did. But, that the folid body exceeded almoft four times the weight, not only of the collateral, but the stagnant li

quor too, fufficiently overthrows the Doctor's reafoning; the fallacy STATICS.whereof will further appear from hence, that tho' we gradually pour'd in water, as long as the veffel would contain any; the cylinder of wax was, indeed, lifted higher and higher from the bottom of the glafs, but did not appear rais'd more than at the firft, above the upper furface of the water; which argues, that it was not at all the quantity of the lower water that continually increased; but the preffure of the collateral water, which continued still at the fame height, with refpect to the wax, that caus'd the elevation of the body.

And, to manifeft yet more clearly the Doctor's mistake, I devised the following experiment. We took a round plate of lead, about the thicknefs of a fhilling; and having ftuck it faft to the bottom of the cylinder of wax, we fucceffively placed upon the upper part of the wax, several grain-weights, till the wax fubfided to the bottom: by this means, the glafs being, at first, almost fill'd with water, there fwam about an inch of that liquor above the upper furface of the wax. And, laftly, we took off, by degrees, the grain-weights that we had added, till we saw the wax, notwithstanding the adhering lead, rife, by degrees, to the top of the water; above which, fome part of it was visibly extant.

From this experiment I argue, that, according to the Doctor's fuppo-fition, here was incumbent on the wax, a cylinder of an inch in height, and of the fame diameter, or breadth, with the round surface of the wax; but, upon the removing part of the water that lay at the bottom, when the wax began to rife, there was incumbent no greater a weight than that of the collateral water; and as much of the upper and ftagnant, as was directly incumbent upon that collateral water. But now, according to the Doctor's reasoning, this cylinder of water incumbent on the wax, being an inch deep, and above three inches broad, it must prefs the wax with a far greater weight than that which the lateral, and hollow cylinder of this ftagnant water, could exert upon the reft of the collateral water; yet the height of this aggregate of collateral water, being the fame with that of the wax, and the water fwimming upon it; the difference of the preffure was so small, that, barely taking off a weight of four or five grains, the wax would, notwithstanding the preffure of the water incumbent on it, bet impell'd up, and made to float; and, by the like weight put on again, it would be made to fink; and, by another removal of fuch a weight, it would, tho' flowly, re-afcend. And this phenomenon depends fo much upon a mechanical balance of preffure, that even four grains would not have been neceffary to make the wax rife or fink, were it not for fome little accidental impediments, that are easily met with in fuch narrow glaffes; for, otherwife, in a larger veffel, we have made the fame lump of wax readily fink, or float, by putting on, or taking off, a single grain, or lefs.

Hence it appears, that, for the regulation of hydroftatical phenomena, nature has her balance too, as well as art; and that, in the balance of nature, the ftatical laws are nicely obferv'd,

STATICS.

And ftill further to demonftrate, that in ftagnant water, the upper parts gravitate upon the lower; we took a very flender pipe of glafs, whofe cavity was narrower than that of an ordinary goofe-quill, that heterogeneous liquors might not be able to get by one another in it. This pipe, near one end, was bent upwards, like a fyphon, that it might have. a fhort leg parallel to the longer; into this crooked pipe we put a little oil, and then held it perpendicularly in a deep, wide-mouth'd glafs, fill'd with water, and a lump of wax, of the bignefs and fhape of that beforemention'd; fo that the preffure of the incumbent water upon the open orifice of the shorter leg, might impel the oil into the longer, above the furface of the water in the veffel. The pipe being thus held upright, 'twas eafy to take notice, by a mark fixed on the outfide, to what height the oil reach'd in it.

Now, if we conceive a horizontal plain, parallel to the bottom of the veffel, to pafs by the bafis of the floating wax; 'tis evident, that, of this imaginary plane, the part on which the wax refts, is as ftrongly prefs'd by the weight of the wax, as the lateral part of the fame plane is by the weight of the water incumbent on it; and, confequently, that part of this plane, which is placed directly over the orifice of the fhorter leg of the pipe, is no more prefs'd, than any equal portion of that part of the fame plane cover'd by the wax. This body, being taken out of the water, the liquor fubfided a great way in the veffel; and fo did, proportionably, the oil in the longer leg of the pipe. And laftly, having weigh'd out as much water as we found the wax to amount to: this liquor was, inftead of the wax, pour'd into that which remain'd in the glafs: whereupon, the oil in the longer leg of the pipe, was again impell'd up to the former mark, to which the wax had rais'd it. Whence we may gather, that the water newly put in, tho', in the air, it weigh'd no more than the wax did, yet prefs'd the water that lay beneath the fore-faid imaginary plane ; and, confequently, that which was directly over the fhorter leg of the pipe, as much as the wax before had done. And, fince we have already prov'd, that the wax confiderably prefs'd that plane; it cannot be deny'd, that the water did, in like manner, prefs that plane: and, confequently, that water may gravitate in water, as well as a folid body, fuch as is wax.

But, to rectify that plausible mistake, which has long deluded both philofophers and mathematicians, who think a body does not actually gravitate, when it does not defcend'; we have feen, that the immersed wax, and the brafs-grains which lie on it, actually prefs, or gravitate, upon the fubjacent water, and bottom of the veffel, on which 'tis incumbent; and, confequently, its preffure, not being furmounted by that of the collateral water, which is unable to raise it, must be as great as that of this collateral water. Therefore, when, upon the removal of a fingle grain, the wax, with its incumbent weight, is made to afcend, and that but very flowly; 'tis evident, that 'twas fo far from not gravitating before, because it did not actually defcend, that it retain'd its gravity even whilft it afcended; as may appear, not only by the flownefs of its motion upwards, proceeding

from its being in nature's fcale very little lefs heavy than it need be, to ba- STATICS. lance the preffure of the collateral water; but by this alfo, that if but a fingle grain be laid on, when it begins to rife, its afcent will be check'd, and hinder'd; which could not be done by the addition of fo inconfiderable a weight, if the wax, and the adhering metal, did not, even during their afcent, retain their former gravity; tho' that were fruftrated as to the act of defcending, or fo much as keeping their station, by the prevailing preffure of the collateral water. So that, fince the wax, and adhering metal amounted to confiderably above four thousand grains; it did, in the balance of nature, weigh, whilft afcending, not fo much as a four thoufandth part lefs than it did, whilft it was actually defcending.

Upon the whole, then, Dr. More, had he throughly confider'd the matter, needed not have concluded the account of his experiment, as he does, by saying, "this is fo evident a demonftration against the gravity of the "parts of water downwards; that unless it prove true, I fhall never af"fent to any reasoning of my own, for the future, nor to that of any other "man whomfoever." But, I hope, he will confider, as well as I, that a man may be very happy in other parts of learning, who has had the miffortune to mistake in hydroftatics; a fcience which very few scholars have been at all vers'd in.

As for the last experiment in my hydrostatical paradoxes, the Doctor Water made seems to suspect the matter of fact; or, fuppofing it true, accounts for it to fupport a from his hylarchic principle, and the interpofition of the valve of the inftru- body of a much greater fpecific ment; but what confiderable intereft, the imaginary retraction of the valve, gravity than or the air itself, can have in this phenomenon, I confefs I do not difcern: itself. for, I think, the experiment would fucceed, when try'd in vacuo, tho' all the atmospherical air were annihilated.

However, to avoid all mistakes and difputes, that may arife upon account of the valve employ'd in our experiment; a quantity of quick-filver, being by fuction rais'd into a very flender glafs-pipe, whofe upper pris fice was stopp'd with the finger, to keep the mercury from falling out; we thrust the open end of the pipe, with the mercury in it, into a deep glafs of water, till the little cylinder of mercury had, beneath the furface of the water, attain'd to a depth, at least fourteen times as great as the height of the mercurial cylinder; when, the finger being removed from the upper orifice, the glass-pipe will be open at both ends, and nothing can hinder the quick-filver from falling to the bottom, but the refiftance of the cylinder of water that is under it; which cylinder can refift but by virtue of the weight, or preffure of the ftagnant water above it, tho' but collaterally placed: yet this water being, by the pipe, whofe upper part is higher than the furface of that, and acceffible only to the air, kept from preffing against the mercury any where but at the bottom of the pipe; and being about a fourteenth part of the weight of an equal bulk of mercury; it is able, at that depth, to make the fubjacent water prefs upward against the mercury, which is but a fourteenth part as high as the water is deep, with a force equal to that of the gravity, wherewith the mercury tends VOL. II.

Z z

down

STATICS. downwards. And, to manifeft that this phenomenon depends merely upon the equilibrium of the two liquors; if you gently raise the lower end of the pipe towards the furface of the water, this liquor, being not then able to exercife fuch a preffure as it could at a greater depth, the mercury preponderating, will fall out to the bottom of the tube. But if, when the quick-filver is at the first depth, inftead of raising the pipe, you thrust it down farther under the water; the preffure of that liquor against the mercury, increafing with its depth, will not only fuftain the mercury, but impel it up in the pipe to a confiderable diftance from the lower orifice, and keep it about the fame diftance from the furface of the water, which is laterally above it. And this experiment may not only ferve for the purpofe for which I here alledge it; but, alfo, if duly confider'd, and apply'd, very much illuftrate and confirm the explanation, formerly given, of the feemingly fpontaneous afcent of the clogg'd fucker in our exhaufted air-pump.

The cafe of di- The laft argument which the Doctor urges against the gravitation of vers, with re-water, in its proper place, as they speak, is deduced from what happens to gard to the divers, who, in the midst of the sea, tho' falt-water be much heavier than preffure of the water they fu- fresh, do not find themselves compreffed by the vaft load of the incumbent Stain, at great water. depths.

But if obfervations about diving were made by philofophers, and mathematicians, or, at leaft, by intelligent men, we fhould, I do not doubt, have accounts of it, very different from the current reports. A learned phyfician, of my acquaintance, upon diving leifurely, perceived a conftriction of his thorax, by the action of the furrounding fea-water.

A Spanish prelate, who liv'd long in America, fpeaking of the Indians, employ'd by their inhuman mafters in fifhing for pearls, tells us," it is "impoffible that men fhould live for any long feafon under the water,

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without taking breath; fo that they die commonly thro' vomiting blood, "and of the bloody flux." And a general of the English in the East-Indies, sent on an embassy to the emperor of Japan, speaks thus of fome femaledivers he met with in his voyage. All along the coast, and soup to Ozaca,” fays he, "we found women-divers, who lived with their houfhold and fa"mily, in boats upon the water. Thefe women," he fays, "would "he " catch fish by diving, which they missed by net and line, and that in eight "fathoms depth. Their eyes, by continual diving, grow as red as blood; whereby a diving woman is diftinguished from all others." However, the reason why fo little damage is fuftain'd by diving, feems, in my opinion, owing to the uniform preffure of the furrounding fluid, and the robuft texture of a human body*.

Tis remarked by Sir Ifaac Newton, that fince fluids, in preffing bodies immerfed in them, do not change the external figures thereof, they will not change the fituation of the internal parts among one

The

another; and, therefore, that if fenfation proceeds from the motion of the parts of animals; fluids will not damage the animals immersed therein, nor excite any fenfation in them, farther than as their