Fluids.

6

water.

Fig. 8.

a piece of lead may be made to swim in water, by im- Pressure of merfing it to a proper depth, and keeping the water Fluids. from getting above it. Let CD be a glass tube, open 8 at both ends; and EFG a flat piece of lead, exactly How lead fitted to the lower end of the tube, not to go within may be it, but for it to ftand upon; with a wet leather be made to tween the lead and the tube, to make clofe work. Let fwim in this leaden bottom be half an inch thick, and held close to the tube by pulling the packthread IHL upward at L with one hand, whilt the tube is held in the other by the upper end C. In this fituation, let the tube be immerfed in water in the glafs veffel AB, to the depth of fix inches below the furface of the wa ter at K; and then, the leaden bottom EFG will be plunged to the depth of fomewhat more than eleven times its own thickness: holding the tube at that depth, you may let go the thread at L; and the lead will not fall from the tube, but will be kept to it by the upward preffure of the water below it occafioned by the height of the water at K above the level of the lead. For as lead is 11.33 times as heavy as its bulk of water, and is in this experiment immersed to a depth fomewhat more than 11.33 times its thickness, and no water getting into the tube between it and the lead, the column of water EabcG below the lead is pressed upward againft it by the water KDEGL all around the tube; which water being a little more than 11.33 times as high as the lead is thick, is fufficient to balance and fupport the lead at the depth KE. If a little water be poured into the tube upon the lead, it will increase the weight upon the column of water under the lead, and cause the lead to fall from the tube to the bottom of the glafs veffel, where it will lie in the fituation b d Or, if the tube be raised a little in the water, the lead will fall by its own weight, which will then be too great for the preflure of the water around the tube upon the column of water below it. But the following method of making an extremely heavy body float upon water is more elegant. Take a long glafs tube, open at both ends; flopping the lower end with a finger, pour in fome quickfilver at the other end, fo as to take up about half an inch in the tube below. Immerfe this tube, with the finger ftill at the bottom, in a deep glafs veffel filled with water; and when the lower end of the tube is about seven inches below the surface, take away the finger from it, and then you will fee the quickfilver not fink into the veffel, but remain fufpended upon the tube, and floating, if we may fo exprefs it, upon the water in the glass-veffel.

Preffure of fame height that it ftands in the small one, after a fufficient quantity had been poured into it: which shows, that the top Bf was preffed upward by the water under it, and before any hole was made in it, with a force equal that wherewith it is now preffed downward by the weight of all the water above it in the great tube. And therefore, the reaction of the fixed top Bf must be as great, in preffing the water down ward upon the bottom CC, as the whole preffure of the water in the great tube would have been, if the top had been taken away, and the water in that tube left to prefs directly upon the water in the cavity BD df. The hydro- Perhaps the bell machine in the world for demonftatic bel- ftrating the upward preffure of fluids, is the hydrolows, fig. 7. ftatic bellows, which confifts of two thick oval boards AB, EF, each about 16 inches broad, and 18 inches long the fides are of leather, joined very clofe to the top and bottom by ftrong nails. CD is a pipe fcrewed into a piece of brafs on the top-board at C. Let fome water be poured into the pipe at D, which will run into the bellows, and feparate the boards a little. Then lay three weights, each weighing 100 pounds, upon the upper board; and pour more water into the pipe, which will run into the bellows, and raise up the board with all the weights upon it; and if the pipe be kept full until the weights are raised as high as the leather which covers the bellows will allow them, the water will remain in the pipe, and fupport all the weights, even though it fhould weigh no more than a quarter of a pound, and they 300 pounds: nor will all their force be able to cause them to defcend and force the water out at the top of the pipe. The reafon of this will be made evident, by confidering what has been already faid of the refult of the preffure of fluids of equal heights without any regard to their quantity. For if a hole be made in the upper board, and a tube be put into it, the water will rife in the tube to the fame height that it does in the pipe; and would rife as high (by fupplying the pipe) in as many tubes as the board could contain holes. Now, fuppofe only one hole to be made in any part of the board, of an equal diameter with the bore of the pipe, and that the pipe holds juft a quarter of a pound of water; if a perfon claps his finger upon the hole, and the pipe be filled with water, he will find his finger to be preffed upward with a force equal to a quarter of a pound. And as the fame preffure is equal upon all equal parts of the board, each part, whofe area is equal to the area of the hole, will be preffed upward with a force equal to that of a quarter of a pound the fum of all which preffures against the under fide of an oval board 16 inches broad, and 18 inches long, will amount to 300lb.; and therefore fo much weight will be raised up and supported by a quarter of a pound of water in the pipe.

7

How a man

Hence, if a man ftands upon the upper board, and may raife blows into the bellows through the pipe, he will raife himfeif up himself upward upon the board: and the smaller the ward by his bore of the pipe is, the eafier he will be able to raife breath. himself. And then, by clapping his finger upon the

top of the pipe, he can fupport himself as long as he pleafes; provided the bellows be air-tight, fo as not to lose what is blown into it.

Upon this principle of the upward preffure of fluids, 5

bottom of

In the fame manner as an heavy body was made to How light fwim on water, by taking away the upward preffure ; wood may fo may a light body, like wood, be made to remain be made to funk at the bottom, by depriving it of all preffure lie at the from below: for if two equal pieces of wood be planed, water. furface to furface, fo that no water can get between them, and then one of them (cd) be cemented to the infide of the veffel's bottom; then the other being placed upon this, and, while the veffel is filling, being kept down by a ftick; when the stick is removed and the veffel full, the upper piece of wood will not rife. from the lower one, but continue funk under water, though it is actually much lighter than water; for as there is no refiitance to its under furface to drive it upward, while its upper furface is ftrongly preffed down, it muft neceffarily remain at the bottom.

SECT. III.

SECT. III. Of the Specific Gravity of Bodies.

WHEN an unfpongy or folid body finks in a veffel of water, it removes a body of water equal to its own bulk, out of the place to which it defcends. If, for inftance, a copper ball is let drop into a glass of water, we well know, that if it finks, it will take up as much room as a globe of water equal to itfelf in fize took up before.

Let us fuppofe, that this watery globe removed by the ball were frozen into a folid fubitance, and weighed in a scale againt the copper ball: now the copper ball being more in weight than the globe, it is evident that it will fink its own fcale, and drive up the oppofite, as all heavier bodies do when weighed against lighter; if, on the contrary, the copper ball be lighter than the water globe, the ball will rife. Again, then let us fuppofe the copper ball going to be immerfed in water; and that, in order to defcend, it muft difplace a globe of water equal to itself in bulk. If the copper ball be heavier than the globe, its preffure will overcome the other's refiftance, and it will fink to the bottom; but if the watery globe be heavier, its preffure upwards will be greater than that of the ball downward, and the ball will rife or fwim. In a word, in proportion as the ball is heavier than the fimilar bulk of water, it will defcend with greater force; in pro portion as it is lighter, it will be raised more to the furface.

From all this we may deduce one general rule, which will measure the force with which any folid body tends to swim or fink in water; namely, Every body immersed in water, lofes just as much of its weight as equals the weight of an equal bulk of water. Thus, for inftance, if the body be two ounces, and an equal bulk of water be one ounce, the body when plunged, will fink towards the bottom of the water with a weight of one ounce. If, on the contrary, the folid body be but one ounce, and the weight of an equal bulk of water be two ounces; the folid, when plunged, will remove but one ounce, that is half as much water as is equal to its own bulk: fo that, confequently, it can not defcend; for to do that, it must remove a quantity of water equal to its own bulk. Again, if the folid be too ounces, and the equal bulk of water two ounces, the folid, wherever it is plunged, will neither rife nor fink, but remain fufpended at any depth. Thus we fee the reafon why fome bodies fwim in water, and others fink. Bodies of large bulk and little weight, like cork or feathers, must neceffarily fwim, becaufe an equal bulk of water is heavier than they; bodies of little bulk but great weight, like lead or gold, muft fink, because they are heavier than an equal bulk of water. The bulk and the weight of any body confidered together, is called its specific gravity; and the proportion of both in any body is eafily found by water. A body of little bulk and great weight, readily finks in water, and it is faid to have specific gravity; a body of great bulk and little weight, lofes almoft all its weight in water, and therefore is faid to have but little fpecific gravity. A woolpack has actually greater real gravity, or weighs more in air, than a cannon ball; but for all that, a cannon ball may have more specific gravity, and weigh more than the woolpack, in water.

Denfity is a general term that means the fame thing; Specific Specific gravity is only a relative term, ufed when folids Gravitice. are weighed in fluids, or fluids in fluids. As folid finks more readily in water, in proevery portion as its fpecific gravity is great, or as it contains greater weight under a fmaller bulk, it will fol low, that the fame body may very often have different specific gravities, and that it will fink at one time and fwim at another. Thus a man, when he happens to fall alive into the water, finks to the bottom; for the fpecific gravity of his body is then greater than that of water but if, by being drowned, he lies at the bottom for fome days, his body fwells by putrefaction, which difunites its parts; thus its specific gravity becomes less than that of water, and he floats upon the furface.

II

terations in

Several more important ufes are the refult of our How to dif being able exactly to determine the specific gravities cover adul of bodies. We can, by weighing metals in water, metals. difcover their adulterations or mixtures with greater exactness than by any other means whatsoever. By this means, the counterfeit coin, which may be offered us as gold, will be very eafily distinguished, and known to be a bafer metal. For instance, if we are offered a brass counter for a guinea, and we suspect it; fuppofe, to clear our fufpicions, we weigh it in the ufual manner against a real guinea in the oppofite fcale, and it is of the exact weight, yet ftill we fufpect it; What is to be done? To melt or destroy the figure of the coin would be inconvenient and improper: a much better and more accurate method remains. have only to weigh a real guinea in water, and we shall thus find that it lofes but a nineteenth part of its weight in the balance: We then weigh the brass counter in water, and we actually find it lofes an eighth part of its weight by being weighed in this manner. This at once demonftrates, that the coin is made of a bafe metal, and not gold; for as gold is the heavieft of all metals, it will lofe lefs of its weight by being weighed in water than any other.

We

This method Archimedes firft made ufe of to detect a fraud with regard to the crown of Hiero king of Syracufe. Hiero had employed a goldfmith to make him a crown, and furnished him with a certain weight of gold for that purpofe; the crown was made, the weight was the fame as before, but fill the king fufpected that there was an adulteration in the metal. Archimedes was applied to; who, as the ftory goes, was for fome time unable to detect the impofition. It happened, however, one day as the philofopher was ftepping into a bath, that he took notice the water rofe in the bath in proportion to the part of his body immerfed. From this accident he received a hint; wherewith, he was fo tranfported, that he jumped out of the bath, and ran naked about the ftreets of Syracufe, crying in a wild manner, I have found it! I have found it!-In confequence of this fpeculation, he procured a ball of gold and another of filver, exactly of the weight of the crown, confidering, that if the crown were altogether of gold, the ball of gold would be of the fame bulk as the crown, and when immersed in water, would raise the water juft as high as the crown immersed; but if it were wholly of filver, the ball of filver being immerfed, would raise the water no higher than the crown immersed; and if the crown was of

gold