STATICS. written of ftatics, are confefs'd to be mathematically, or mechanically demonftrated; tho' those authors do not affign the true cause of gravity, but take it for granted, as a thing univerfally acknowledg'd. And, if in each fcale, of an ordinary balance, a pound weight, for inftance, be put; he who shall say, that the fcales hang in equilibrium, because the equal weights balance one another; and, in cafe an ounce be added to one of the fcales, and not to the oppofite, he who fhall fay, that the former is deprefs'd, because urg'd by a greater weight than the other, will be thought to have given a mechanical explanation of the equilibrium of the fcales, and their lofing it; tho' he cannot give a true caufe why either of thofe fcales tends towards the center of the earth. Since, then, to affign the true caufe of gravity, is not required, even in ftatics, tho' one of the principal, and moft known parts of mechanics; why may not other propofitions, and accounts, that fuppofe gravity in the air, and prove it too, be look'd on as mechanical ?

That the upper

The Doctor, however, is pleas'd to grant me almost as much as I need defire, as to the truth of the hypothesis whereon my explanations are founded. The principal thing which I fuppofe, in my hydroftatical writings, is, that in water, tho' ftagnant, the upper parts actually gravitate upon the lower; or prefs upon them, even when they do not fenfibly deprefs them. This hypothefis, the Doctor allows, agreeable to the principles of the mechanical philofophy: and, accordingly, having fhew'd, that in a fufpended tube of water, the whole liquor gravitates upon the bottom of it; and, confequently, that all the parts thereof do fo, the upper upon the lower, "provided" fays he" "there be no immaterial principle in nature."

Now, I as freely, as the Doctor himself, affert an incorporeal Being, that made and governs the world. All that I have endeavour'd at, in explaining what happens among inanimate bodies, is to fhew, that fuppofing the world to have been at firft made, and to be continually preferv'd by God's divine power and wifdom; and fuppofing his general concourfe to the maintenance of the laws he has eftablifh'd in it; the phenomena I confider, may be folv'd mechanically; that is, by the mechanical properties of matter; without recourfe to nature's abhorrence of a vacuum, to fubftantial forms, or to other incorporeal creatures. And, therefore, if I have fhewn, that the phenomena, I attempted to folve, are explicable by the motion, magnitude, gravity, fhape, and other mechanical affections of the fmall parts of liquors; I have done what I pretended: which was not to prove, that no angel, or other immaterial creature, could interpofe in thefe cafes; for, concerning fuch agents, all that I need fay, is, that, in the cafes propofed, we want not their affiftance; and, therefore, have no occafion to fly to it, in folving our phenomena.

But the Doctor, it feems, would have the gravitation of the elements, in their parts of fluids proper places, fufpended by an incorporeal principle; and hereto he is led gravitate upon the lower. by this experiment, which, he fays, is moft manifeftly repugnant to our hypothefis. He conceives, then, that in a bucket of water, with a perfectly cylindrical cavity, whoe diameteri s of 62 parts, there is forcibly

kept

kept at the bottom, by means of a ftick, a round piece of wood, whofe STATICS. diameter amounts but to 61 of those parts; and that as foon as ever the ftick is remov'd, the piece of wood will emerge to the top, and float: "which," fays he "is impoffible, if all the parts of the water, did not only "jointly prefs the bottom of the veffel, but each prefs'd the other directly "downwards."

But, firft, fince according to his computation, the area of the intervalbetween the fides of the veffel, and the edges of the round board, is 123 of fuch parts, whereof the area of the board amounts to 3721; 'tis evident, that there must be room enough for the water to pafs between the fides of the veffel, and the edges of the board; which is fuppofed, on all hands, to be of fome wood lighter in fpecie than water, fince elfe it would not rife, upon with-drawing the ftick.

Secondly, this round board is not fuppos'd to be made exactly fit to the bottom of the veffel, and, confequently, the water may get in between them; for which reafon, 'tis neceffary to keep the piece of wood forcibly down with a ftick; which were, otherwife, needlefs; and, confequently, this interpos'd water, will communicate with the upper, along the fides of the veffel; which latter may, according to the laws of hydroftatics, by means of that interpofed, exercife its preffure upwards, against the lower furface of the wooden plate.

Thirdly, the Doctor fuppofes an imaginary plane of water to be parallel to the bottom of the veffel, and to pafs along the bottom of the board; fo that of the water that lies between this plane, and the bottom of the veffel, one part is cover'd by the piece of wood; and the other, between the edges of that, and the fides of the bucket, is cover'd with the incumbent water only.

Now, 'tis manifeft, that in water, thofe parts which are moft prefs'd, will thruft out of their place, thofe that are lefs prefs'd. 'Tis alfo evident, that the part of the imaginary plane, cover'd by the round piece of wood, must be prefs'd by a lefs weight than the other part of the fame plane; because the wood being, bulk for bulk, lighter than water, the aggregate of the wood and water, incumbent on the cover'd part of the fame plane, muft be lighter in fpecie, than the water alone, that is incumbent on the uncover'd part of the fame plane; and, confequently, this uncover'd part being more prefs'd than the other part of the plane, the heavier muft difplace the lighter; which it cannot do, but by thrufting up the board; as it does. when the external force that kept it down, is removed. And this greater preffure against the bottom, than against the top of bodies immers'd in water, fpecifically heavier than themfelves, is a true reafon of their emersion.

'Tis true, that according to the Doctor's computation, if the folid cylinder, confifting of the wooden plate, and all the water directly incumbent on it, were put into an ordinary balance; it would, there greatly outweigh the hollow cylinder of water alone, that refts upon the uncover'd part of the imaginary plane. And this is what feems to have deceiv'd the

Doctor:

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 refpective and fpecific 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 almoft to a level with the external water; tho' it cannot do fo, without raifing the whole mass of water ftagnant in the vessel.

Demonftrated by experiments.

We took an open-mouth'd glafs, or jar, three inches and a half in diameter, 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 veffel, and the contain❜d liquors, we fet afide to cool, that the wax might shrink from the glafs, 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 thofe two bodies, both at the bottom and the fides, the wax was made presently to float; being vifibly 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 hydrostatics. 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 interpofed between the concave part of the giafs, 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 raifing 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 ftagnant li

quor

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 glass, 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 ftill 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 miftake, I devised the following experiment. We took a round plate of lead, about the thicknefs of a fhilling; and having stuck it faft to the bottom of the cylinder of wax, we fucceffively placed upon the upper part of the wax, feveral grain-weights, till the wax fubfided to the bottom: by this means, the glafs being, at firft, almoft 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 faw the wax, notwithstanding the adhering lead, rife, by degrees, to the top of the water; above which, fome part of it was, vifibly, extant.

From this experiment I argue, that, according to the Doctor's fuppotion, here was incumbent on the wax, a cylinder of an inch in height, and of the fame diameter, or breadth, with the round furface 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 reafoning, 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 fo fmall, that, barely taking off a weight of four or five grains, the wax would, notwithstanding the preffure of the water incumbent on it, be 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 eafily 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 fingle 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.

And

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 plane, parallel to the bottom of the veffel, to pafs by the basis 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 shorter 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 plaufible miftake, which has long deluded both philofophers and mathematicians, who think a body does not actually gravitate, when it does not defcend; we have seen, that the immerfed 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 raife it, muft 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, becaufe 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