STATICS.

Another fly, that feem'd but about half fo big, as one of thofe hitherto mention'd, being well placed, with fome mercury under her, in a glafs pipe, held upright, fuftain'd a mercurial pillar of 25 inches; and tho fhe was not obferv'd to move, under fo great a weight, yet, when once that was taken off, fhe appear'd unhurt; and, probably, would have escaped under a much greater weight, if the tube, which was too large, had not, already, employ'd all the stock of mercury we then had at hand. Since, then, fo fmall an animal, as a fly, may furvive fo great a preffure, and remain able to move fuch long and flender bodies, as her legs, when prefs'd against by above 16 inches of mercury; and, confequently, by a weight, equivalent to a pillar of water, of more than 18 feet and a half; which being 590 times her own length, and many times more her height, it appears, that a diver, fix foot tall, to have as many times his height of water above him, as our fly might have had, and yet continue to have moved under it, muft dive to near an hundred fathoms; which is a depth vaftly greater than, from what I could learn by enquiry, the divers, either for coral, or pearl, defcend

to.

Thus, then, we have accounted for a phenomenon, which Dr. More thinks infoluble, without his hylarchic principle. There would, indeed, be much more weight in what he objects, if our affertion, of the gravitation of water in water, were, like the Principium hylarchicum, a mere hypothesis, advanced without any pofitive proof: but our doctrine is directly proved, by particular experiments; to elude the force whereof, fo ingenious a perfon, is obliged to call in a principle that is not phyfical.

But, whatever power he is pleas'd to fuppofe at the bottom of the fea, to fufpend the preffure of the incumbent water, I think, that fuppofition muft give place to experience; which fhews, there really is a great preffure exercis'd by the water at the bottom of the fea. A gentleman, who has been often prefident of the royal fociety, affured me, that a friend of his, having let down a pewter bottle into a deep fea, with weight enough to fink it, that he might try whether any fweet water would ftrain into it; found, when he pull'd it up again, the fides of it very much comprefs'd, and, as 'twere, fqueez'd inwards by the water. An acquaintance of mine, trying to cool his liquor, when he failed thro' the torrid zone, by letting the containing veffels to a great depth into the fea, was, at first, amaz'd to find the corks, with which the ftrong ftone bottles had been well ftopp'd before, fo forcibly, and fo far thruft in, that they could fcarce have been fo violently beaten in with a hammer. And, an ingenious perfon, who praEtifes phyfic in the Indies, told me, he try'd, in a very deep part of the fea, whether any fresh water would ftrain into ftone bottles, thro' a thick cork, ftrongly stopp'd in; and having let it down with a convenient weight to 100 fathoms, was much difappointed, when he drew it up, by finding that the preffure of the water, at fo vaft a depth, had quite thruft down the cork into the cavity of the bottle: an effect which he fcarce could have expected from the ftroke of a mallet.

And,

And, to fhew, ad oculum, that water may gradually, as it grows deeper, STATICS. prefs against the ftopple of a bottle, tho' the veffel be inverted; we took a glafs vial, furnished with a cylindrical neck, and its cavity large, in proportion; into this we put as much quick-filver as would, in the neck, make a fhort mercurial pillar, of between half an inch, and an inch; then a piece of very fine bladder, dipped in oil, was fo tyed over the orifice of the glafs, that no mercury could fall down, or get out, nor water get in at the orifice; and yet the bladder, by reafon of its great limbernefs, might be easily thruft up towards the cavity of the vial, or deprefs'd by the weight of the mercury. This little inftrument, firft furnish'd with a weight of lead, to fink it, being inverted, the mercury defcended into the neck, and clofed the orifice as exactly as a stopple; but, with its lower part, deprefs'd the bladder beneath the horizontal plane, that might be conceiv'd to pafs by the orifice: then the glafs, being, for a while, kept in the water, and, by a ftring, let further down into the fame glafs veffel, fill'd to about two feet in height; the preffure of the liquor against the orifice of the vial, by degrees drove up the bladder, and the mercurial stopple into the cavity of the neck; as was manifeft by the afcent of the quick-filver and when the inftrument was leifurely drawn up again, the weight of this mercury made it fubfide, and plump up the bladder, as before.

Meeting, cafually, with an ingenious mechanic, who devised proper accommodations, and boat, wherewith he could continue, for a great while, at a confiderable depth under water; he affured me, that when he was about four or five yards deep in the river Thames, his breaft and abdomen were fo compreffed, that, there being hardly room enough left for the free motion of his lungs, he could fcarce fetch his breath; and was obliged to be drawn quickly up: and that, to remedy this, he caufed a kind of armour for the cheft and back, to be made of copper; and tho' the metal defended him from any mifchief in thofe parts, yet, in others, where only leather was interpofed, when he came to the depth of about fix fathoms, he found a great preffure upon his legs and arms, and all the other parts, against which the water was able to thruft the leathern fuit inwards: and this preffure he found pretty equal; fo as to receive no great inconvenience from it; being able to continue under water, tho' not at any great depth, for about two hours. He farther declared, from his own experience, that the ambient water endeavour'd to prefs him, and his diving fuit, every way inwards. I have, alfo, been affured, that a profefs'd diver, when he defcended in his bell, to very great depths, has often had the blood fqueez'd out at his nofe and eyes.

Upon the whole, it appears, that water actually preffes against bodies under it, whether fpecifically lighter, or heavier than it self.

SECT.

STATICS.

Water actually

weigh'd in water, by common

Scales.

SECT. II.

O my particular method of actually weighing water in water, it has been objected, by a late writer of hydroftatics, that there is a miftake in it. I fhall here, therefore, endeavour to clear this matter, and fet my thoughts about it in a fuller light.

My opinion is, that water, as it is a heavy fluid, always retains its gravitation, and power of preffing; (by which, I mean, a tendency downwards, whatever be the cause of that gravity) whether it have a body under it, either fpecifically heavier, or lighter than it felf, or one equi-ponderant to it. For I fee not what should deftroy, or abolifh this gravity, tho' many things may hinder fome effects of it. And, therefore, I fuppofe that water retains its gravity, not only in air, but in water too, and in heavier liquors; and, confequently, by virtue of this, that liquor preffes upon them. But, if a furrounding fluid have, upon account of its fpecific gravity, an equal, or a ftronger tendency downwards, than water, it will, by virtue of that, be able to impel up this liquor, or to keep it from actually defcending; fo that a portion of water, fuppofed to be included in a veffel of the fame fpecific weight with water, will, placed in a greater quantity of the fame water, neither rife nor fall, tho' it retains its gravity there; only this gravity is kept from making it actually defcend by the contrary action of the other water, whofe fpecific gravity is fuppofed equal as when a juft balance is loaded with a pound weight, in each of the fcales, tho' neither of the weights actually defcend, being hinder'd by its counterpoife; yet each retains its whole weight, and, with it, preffes the fcale it refts on: fo that our included portion of water, really preffes the fubjacent water, tho' it does not actually deprefs it. Nor do I think that the only way of judging whether a body gravitates, is to obferve, whether it actually defcends; fince, in many cafes, its gravity may be proved, by the refiftance it makes to heavy bodies, which would otherwife raife it; as appears by equal weights in a balance. And, for want of this diftinction, I have known even learned men, treating of hydroftatics, miftake the queftion.

Now, the adverfaries I had to deal with, both in print, and in difcourfe, deny'd, that in ftanding water, the upper prefs'd, or gravitated upon the lower parts. And tho' they could not but grant, that the whole weight of the water gravitated upon the bottom of the veffel; yet they would have the parts of it to do fo actione communi, as they speak; and fancied I know not what power of nature, to keep the homogeneous portions of water, as well as other elements, from preffing one another, whilft it is in its proper place. Againft this opinion, it was alledg'd, (befides other things, which I found many, otherwife good fcholars, were not fitted to understand) that if a glafs vial, or bottle, well ftopp'd, were deeply immerfed under water, it would ftrongly tend upwards; but if it were dextroufly unftopp'd, when 'twas thus immers'd, fo that the water cou'd get in; allowing for the weight of the glass itself; 'twould, by the water that

crowds

crowds in, and thrufts out the air, be made ftrongly to tend downwards, STATICS. and continue funk. But this not fatisfying, becaufe 'twas pretended, that the reafon of the empty bottle's emerging, when ftop'd, was the pofitive levity of the air that fill'd it; and that the finking of it, when unitop'd, was from the recefs of the fame air, which, by the intruding water, was driven, with large bubbles, out of the bottle: I thought this evafion might be obviated, by contriving an experiment, wherein the water fhould be plentifully, and fuddenly admitted into the glafs, and yet no air expell'd out of it; fo that, if then the glass, which was fuftain'd before, fhould fall to the bottom, with a gravitation amounting to a confiderable weight, in refpect of its capacity, the finking of it could not be afcribed, as before, to the recefs of the air, endow'd, as they fuppofe, with pofitive levity; but to the weight of the water admitted, which, when thus weigh'd, would be environ'd with water of the fame kind: and to fhew, that this water might have a confiderable weight, notwithstanding the place it was in, I employ'd a pair of fcales, after the manner recited in the experiment, page 288, Vol. II.

However, therefore, my expreffions difagree with thofe of my adverfary, the distance of our opinions is not fo wide, as it feems at firft fight: for, he allows, as well as I, that the fuperior parts of water do, by their gravity, prefs the inferior; but this he would not have amount to fo much as to mean, that water weighs, or gravitates in water. But, if he thinks, that, in my experiment, I meant to propofe a method of making water defcend in water, and weigh it in that liquor, with a pair of fcales, juft as I would a piece of lead, or a portion of mercury; which are bodies much heavier in fpecie, than water; either he mistakes my intention, or I did not fufficiently declare it. What I defign'd to fhew, and, I think, have fhewn, was, that by the help of an ordinary balance, it may be made appear, that water, admitted into the glafs bubble, I employ'd, caus'd it to weigh much heavier than it did before that liquor enter'd into it; and that this new weight, manifefted by the balance, was not due, as my adverfary fuppofes, to a recefs of the air.

To this experiment, our author objects the following. "Take a piece "of wood, lighter in Specie than water, and add weight to it, by degrees, "till it become of the fame weight with water; faften it, with a ftring, to a balance, and weigh it in water, and you will find the whole weight fupported by the water." But this is fo far from overthrowing my opinion, that it agrees very well with it. For, fuppofe the weight, added to the light wood, be lead, it cannot be faid, that the metal lofes its native ponderofity, whilft it refts in the water; and the reafon why it defcends not, is, that it, and the wood whereto it is joined, are hinder'd by the counterpoife of the collateral water, which, by its preffure, would raife the furface of the water whereon the floating body refts, if it were not hinder'd by the weight of thefe incumbent folids: and this refiftance of theirs, to the endeavour of the water upwards, being exercifed only upon account of their gravity, fhews, that they do, in my fenfe, gravitate, tho' not pregravitate.

VOL. II.

Aaa

*Tis

i

STATICS.

The relative

'Tis obvious, that if wood, wax, or other bodies, lighter in Specie than water, be detain'd under it, they will, upon removal of the levity of bodies force, emerge to the top. And this they do fo readily, and, as it feems, under water. fpontaneously, that the generality of philofophers, both ancient and modern, afcribe it to an internal principle, which they call pofitive levity.

Shewn by expe riments, which

But this principle was not, always, fo univerfally receiv'd among philofophers, as in later ages; Democritus, and feveral of the antients, admitting no abfolute, but only a relative levity: which opinion, fome of the moderns have attempted to revive.

The experiment ufually urged, to prove the pofitive levity of wood in water, feems, to me, too flightly made, to be acquiefced in. 'Tis true, indeed, if a flat board, for inftance a trencher, have its broad furface kept by a man's hand upon the horizontal bottom of a bucket, full of water; when the hand that detain'd it, is removed, the trencher will commonly foon afcend to the furface of the water. But, I do not perceive, that a decifive experiment, of this kind, is eafy to be made with fuch materials: for, the wood, whereof both the trencher, and the bottom of the bucket confift, are fuppofed to be lighter, in fpecie, than water; and, confequently, they must be of a porous, and lax texture; whilft the more folid woods, as Lignum vita, brazil, &c. whofe texture is clofer, fink in water. If, therefore, there be not much care ufed, to bring the furfaces of the trencher, and the bottom of the bucket, to a true flatnefs, and smoothnefs, the experiment will not be accurately made; and, perhaps, tho' it be mentally, yet it is fcarce practically poffible to bring fuch porous bodies, as the lighter woods, to fuch a contact as is neceffary in this cafe. And, were that actually done, I fhould not expect, that the trencher would af cend. For, in my opinion, the cause why, in ordinary inftances, wood, wax, and other bodies, fpecifically lighter than water, being let go at the bottom of a veffel, full of that liquor, emerge to the top, is chiefly, that there is no fuch exquifite congruity, and contact, between the lower fuperficies of the wood, and the upper of the bottom of the veffel; but that the lateral parts of the water, being impell'd by the parts of the fame liquor incumbent on them, are made to infinuate, and get between the lower parts of the wood, and the bottom of the veffel, and fo lift, or thruft upwards, the wood, which, bulk for bulk, is lefs heavy than the water that extrudes it.

But, as the whole of the argument of thofe I difpute with, confifts in a fuppofition, that because the trencher is placed on the bottom of the veffel, no water can come between to buoy it up; whence they conclude, it muft afcend by an internal, and pofitive principle of levity; I made the experiment after another, and, if I mistake not, a better manner.

We took, then, two round, cylindrical plates of black marble, which verthrow the had thofe fuperficies that were clapp'd together, ground very flat, and doctrine of pofi- carefully polish'd, that they might touch in as many parts as poffible; and that the upper being taken up, the other might stick to it, and afcend with

tive levity.