THE

PREFACE.

TATICS is that part of univerfal mechanics, which confiders the gravity of bodies, in all forts of mediums ; tho° 'tis, fometimes, taken for the fame as particular mechanics, or the doctrine of motion in bodies, which depends upon. their gravity. But, hydrostatics, in general, regards the weight of bodies in all manner of fluids; and, therefore, differs but little from ftatics, unless when bodies are weighed in vacuo; or, where there is no fenfible refiftance. There is, alfo, a more limited fignification of the word, bydroftatics; which reftrains it, as its derivation implies, to the weighing of bodies in water; and'tis hydroftatics in this fenfe, wherewith we are bere principally concern'd: a doctrine which, before Mr. Boyle undertook to improve it, confifted chiefly in theory and fpeculation; having only been treated by mere mathematicians: but, that noble philofopher foon reduced it to practice, and applied it to ufeful purposes. Some of its propofitions, formerly demonftrated in a mathematical manner, be proved by the direct, obvious way of experiment; that is, by the proper way, or bydroftatically. He, also, made many new discoveries in this part of knowledge, which are highly useful in life, and teach us to examine the goodness of drugs, of metals, minerals, and other bodies, both folid and fluid.

The first of the following pieces appears under the title of paradoxes; and, truly, the propofitions it contains, were paradoxes, even to fome mathematicians, at the time wherein they were published. Nor is this ftrange, fince, as the learned Wolfius obferves, many perfons taking gravity for a permanent power in matter, which must remain unalter'd, fo long as it continues its ftate; and fuppofing fluids, whilft confin'd, and

at reft, wholly unable to act upon bodies; no reafon appears to them, why they should, as it were, take away a part of the gravity of the bodies immerfed in them; and, much lefs, why they should, fometimes, throw them upwards with a great force. This was remarkably the cafe with that great fcholar, Dr. More; who, to folve fuch an appearance, would fain bave introduced a new, and an immaterial principle into phyfics. And this gave birth to another of the ensuing pieces; which, tho' it alfo relates to fome experiments, that appear under the head of pneumatics, we thought fit to range intire, under this of ftatics; because, in ftrictness, 'tis wholly bydroftatical; taking that term in its larger fenfe; because we would not break in upon the order of the pneumatical pieces; and, laftly, because hydroftatics ought, always, to precede pneumatics; fince the latter cannot be understood without a knowledge of the former.

Hydro

Hydrostatical Paradoxes,

Proved and Illuftrated by

EXPERIMENT S.

1.

S

POSTULATA, & LEMMATA.

Uppofe a tube, open at both ends, held, with one of them, perpendicularly, under water; the lower orifice may be conceiv'd to terminate in a plain parallel to the horizon, or the upper furface

of the fluid.

2. All affignable equal portions of this furface, will be equally preffed by the water perpendicularly incumbent thereon.

For the fluid being here fuppofed homogeneous, as to its gravity, and to ftand, at the fame height, upon all the parts of the imaginary plain; no equal part can poffibly be more preffed than another, in the fame furface.

3. If any part of this imaginary plain, be preffed with a greater weight, than another, the former will be either difplaced, or depressed.

Thus, whilft an heavy body finks in water, that part of the imaginary plain, contiguous to the lower part of the body, being preffed by a greater weight than the other portion of the fame furface, muft needs give way, fucceffively, till the heavy body arrives at the bottom.

4. On the other hand, if any part of the imaginary surface be lefs preffed upon than the remainder, it will, by the weight on the remainder, be impell'd upwards, till the preffure there be equal to that upon the other parts of the fame furface.

If this be doubted, the following experiment will prove it. If a cylindrical glafs tube, open at both ends, be fteadily held in a perpendicular pofture, with one of them immerfed, two or three inches below the furface of a proper quantity of water, in a glafs veffel; the surface of the water within-fide the tube, will be nearly level with the furface that is without-fide the fame; because the water, in both cafes, has a free communication.

If, now, a convenient quantity of oil, be gently pour'd upon the water that is external to the tube, that within the tube, which we fhall call internal, will gradually rife, and continue to do fo proportionably; becaufe the imaginary plain, cutting the immerfed orifice of the tube, is every where preffed by an additional weight, except in the orifice itfelf: which preffure must neceflarily be increas'd, as more oil is pour'd upon the external water; whilft a circle of our imaginary plain, equal to the lower annulus of the tube, is, by the fides thereof, guarded from the immediate weight of the oil; whence the external water, being more preffed than the internal, is, confequently, forced up thro' the tube, where there is the leaft refiftance, till the cylinder of water within the tube, gravitates upon the fubjacent part of the imaginary plain, equally with the fum of the water and oil, upon every other equal portion of the fame furface.

5. The air is a ponderous body.

This has been fhewn by feveral experiments; fome whereof, indeed, are excepted to; but the following is unexceptionable, and was often repeated.

Having obtain'd a thin, and large glafs bubble, with a flender ftem, and gradually rarified the air it contain'd, as much as I conveniently could, by the flame of the lamp, whereat the bubble was blown; the ftem, whilft the bubble remain'd exceeding hot, being nimbly put into the flame, thereby became hermetically fealed, in an inftant. This glafs wei permitted to cool leifurely, and afterwards weigh'd it in a very exact baJance; then carefully breaking off the fealed end, and preferving the fragments, we weigh'd the glafs again, and found it confiderably heavier than before; which could be owing to nothing but the natural air, that rushed into the bubble, upon breaking off its ftem, whilft it contain'd air greatly expanded. For, the noife hereof, in entring, may be plainly heard by an attentive ear; and, as the rarifaction is here great, fo the bubbles will fometimes break, by the bare force of the external air; and, laftly, if the fealed end of the glafs be broke off, under water, that fluid will, by the preffure of the atmosphere, be forced into the bubble, like an artificial fountain, till it be about three quarters full: whence the weight of the air appears to be very confiderable; fince the fourth part of what the bubble would contain, remains therein not taken notice of by the balance. In one repetition of this experiment, I found the air fresh admitted into the bubble, weighed near of a grain, and its full content of water 906 grains; whence, if the re-admitted air only filled of the bubble, the whole air it contain'd, may be reasonably fuppofed at a grain. And thus the water weigh'd about 900 times as much as the air of the bubble; which, all things confider'd, comes near enough to fome other trials, whereby we determin'd the fpecific gravity of air, to be to that of

water, as I to 1600.

PARA

PARADOX I.

STATICS.

In all fluids, the upper parts gravitate on the lower. Suppofe one end of a fmall, cylindrical, open glafs tube, plunged into oil of turpentine, and that liquor to be raised, by fuction, to a convenient height therein; when the lips being removed, and the upper orifice of the tube nimbly stopp'd with the finger, to prevent the fluid from falling back, imagine it thus placed perpendicularly in the glafs ABCD Fig. almoft fill'd with water, fo that the furface of the oil may ftand fomewhat higher than that of the water. This done, and the finger removed from the upper orifice of the tube, the oil will not fall out at the lower, but remain fufpended near its former altitude. But oil of turpentine, being an heavy fluid, has a tendency downwards; and as the lower orifice of the tube is open, it muft neceflarily fall out thereat, did not the preffure of the water underneath fuftain it. No contrariety in the nature of these two liquors, can be fuppofed the caufe of this phenomenon; for, if the finger be removed, before the pipe is fufficiently immerfed, the oil will fubfide till it becomes a balance to the water; but the reafon of it is manifeftly this. Suppofe the plain wherein the extremity Q, of the pipe PQ, refts, to be GH; if that part thereof whereon the oil refts at Q, be equally preffed by the incumbent cylinder of oil QX, with the other parts of the fame furface GH, by the water incumbent upon them; the part Q, can neither be depreffed by the cylinder of oil QX, nor raifed by an equal preffure of water upon the other parts of the plain GH. That this equilibrium, betwixt the oil and water, is juftly af fign'd for the caufe of the phenomenon, appears farther, 1. From what is already obferved, viz. that if the cylinder of oil reach much higher than the furface of the water, the oil will defcend: the reafon whereof is, that the plain GH, being more charged at Q, than in any other part, it is there unable to make refiftance; whence it muft neceffarily be thruft out of its place, by the defcending oil. 2. Becaufe the oil will continue fubfiding only till its furface becomes almoft level with that of the water; when the part Q, is preffed by the oil, equally with the other parts of the plain GH, by the water incumbent on them. 3. If, while the oil and water are balanced, the pipe be gently raifed from Q to S, the oil will preponderate, and confequently fall out in drops, which by the fuperior fpecific gravity of the water, will be buoy'd up, and fo float on the furface thereof; and as the pipe is gradually elevated towards the furface L M, more and more of the oil will drop out: but if the tube be ftop'd any where in its afcent, at S, for inftance, the oil will ceafe to flow out: and, as at the plain IK, the preffure of the water, by reafon of its diminish'd depth L I, upon the other parts of the furface, is not near fo great, as upon the plain GH; fo the remaining cylinder of oil incumbent on S, is, by reafon of its proportionable efflux, unable to prefs that part more forcibly than the others of the fame plain I K, are prefs'd by the incumbent water. And, if the lower orifice be raised almoft to V, or near the upper furface of the

water