immerse them, successively, in water, or any other liquid, and see how much of their weight, in each case, is lost. If, on the contrary, we would find the relative weights of two or more fluids, we take the same solid, and immerse it, successively, in each of those fluids, and the weight lost, in these several cases, will show the relative or specific gravities of the fluids, which will be in the direct ratio of their densities.

We owe the invention of the method of ascertaining specific gravities to Archimedes. He had been required, by Hiero, King of Syracuse, to determine whether a gold crown, which had been made for him, was adulterated or not. While studying this problem, he happened to bathe, and observed that the water in the bath rose, as his body was immersed. He then inferred, that the rise would be always proportioned to the bulk of the body immersed, and that, if two bodies, of equal weight but unequal bulk, were plunged in the fluid, the rise, being directly as the bulk, would be inversely as the density or specific gravity. This sug

gested to him a ready mode of testing the purity of the gold crown; and generally, we may remark, that the purity of any substance, such as drugs, chemical preparations, coins, liquids, &c., may be readily ascertained by the method of specific gravity. Different instruments have been constructed, for detecting adulterations in various substances, such as the oleometer, for oil, the lactometer, for milk, &c.

mixed with it. He cannot tell by weighing the crown against the gold, in air, since they may have equal weights, and yet not be of the same degree of purity. But if, on being weighed in water, each loses the same proportion of its weight, this is evidence that they are of equal bulk, and therefore of equal density, or specific gravity.

CHAPTER V

MECHANICAL AGENTS.-(GRAVITY CONTINUED.)

(c.) THE gravity of AERIFORM BODIES, such as the atmosphere, gases, &c., acts (in connexion with their inertia and elasticity) as a moving force, in the case of pumps, barometers, windmills, sailing-vessels, fire-engines, &c. That air has gravity or weight, may be proved by weighing a flask, from which the air has been withdrawn. It will be found, when filled with air, some grains heavier than when emptied. Now, it is found, by experiment, that the pressure of the atmosphere on a square inch is equal to about fifteen pounds; that is, that a column of this fluid, whose base is one inch square and whose height is that of the atmosphere, weighs fifteen pounds. Consequently, it follows, that a horizontal surface sustains a weight or pressure amounting to fifteen times as many pounds as there are square inches in its extent. If, then, we have a solid substance, with an horizontal surface; for example, a piston, placed in a vertical cylinder, and there is no resistance below it, it will be forced down, by a mechanical pressure of fifteen times as many pounds as there are square inches in its end; and in this way a mechanical agent, of power limited only by the magnitude of the piston, will be obtained. Before this force can be exerted, however, a vacuum must be formed; that is, the air must be withdrawn from one side of the piston; and, if this be done by mechanical means, as is the case in the pump, it is obvious, that it must require just as much force to do it, as will be subsequently gained by the pressure of the atmosphere on the other side.

Again: the air may exert its moving force, by being applied to water. This is the case in the common pump. To understand the principle of this instrument, let us suppose a bent tube, Fig. 13, contain

DI

ing water in the portion A B C, and open at both ends. The water will stand at the same height, B, in both branches. If, now, the air were withdrawn Fig. 18. from the branch CD, the pressure of the air on the water at A, in the other branch, would force it downwards towards B, and thus cause a rise of the water, in the other branch, B D. It would continue to rise, until the weight of water, in the branch B D, was sufficient to balance the pressure of the atmosphere, added to the weight of the water A B; and before that equilibrium can take place, might discharge itself at D. Here, the branch BD may represent the barrel of the pump, A B the water in the well.

Some notion of the common or suction pump may be gathered from the annexed diagram, (Fig. 14.) in which CL is the barrel of the pump, B a box fixed in the inside of the cylinder, just above the surface of the water in the well, and D another box, or piston, attached to the rod c d, and moved by a power applied at a. Both B and D

Fig. 14.

C

L

B

are so formed, as to prevent the passing of any air between them and the sides, and have valves, b and d, opening upwards, similar to the valves in a common bellows. If, now, the piston D be drawn upwards, it will evidently carry the air before it, and leave a vacuum between itself and B. But as the air presses on the water in the well, outside of the barrel, it will force it up, to supply this vacuum; and, after having passed above D, the valve b will be shut, by its downward pressure. At the returning stroke of D, the water will pass through the valve in D; and, on raising D again, will be driven out of the spout at e. Here we see, at once, that the moving power is applied at a, and that the pressure of the air is used merely as an intermediate agent, to effect, with greater expedition and convenience, what might

have been effected directly, as in a common well, by

the same power.

In the barometer, the mercury in the tube is sustained by the pressure of the air. If a vacuum be created

in a glass tube, of sufficient length and closed at one end, and if the open end be inserted in a basin of mercury, the fluid will rise to the height of twenty-nine or thirty inches, because a column of mercury of this height weighs just as much as a column of the atmosphere of the same base. This instrument, (Fig. 15,) is very useful, in showing those changes in the pressure of the atmosphere which usually precede storms.

Fig. 15.

131

30

29

If the column of the atmosphere which sustains the mercury becomes lighter, the mercury will of course fall, and vice versa. The instrument has also another important use. If we are ascending mountains, the column of atmosphere above us must be constantly growing shorter, and of course lighter; and hence, if we carry a barometer, the fall of the mercury will indicate the various heights to which we attain. It may be proper to add, that the weight of the atmosphere was not discovered till the sixteenth century; and that to this discovery we owe the barometer, as well as many improvements in the construction of pumps. Previous to the time of Galileo, philosophers explained the ascent of fluids in a vacuum, by saying that Nature abhorred a vacuum: and when it was subsequently found that water would never ascend above thirty-two feet, (the point at which its weight, together with the elasticity of the residual air, just balanced the pressure of the atmosphere,) they explained it, by saying, that Nature's horror of a vacuum did not extend beyond that distance !*

In windmills, where the machinery is turned by the

*See further, on this subject, Vol. i, p. 19, &c., of a work entitled, Pursuit of Knowledge under Difficulties,' forming the fourteenth volume of THE SCHOOL LIBRARY.'

wind striking (as in undershot water-wheels) against the vanes of a wheel, and so, also, in vessels propelled by wind striking against the sails, air acts not so much by gravity as by inertia. To adjust these vanes, so that the wind will strike upon them with the greatest effect,. and will act only while it contributes to impel the machinery, is a problem which has exercised the most accurate experimentalists and the most profound mathematicians. It is a beautiful proof of the truly mathematical principles on which the works of creation are formed, that the method of arranging the sails, ultimately adopted in the windmill, bears a striking resemblance to the arrangement of the feathers and wings of birds. These feathers are so adjusted, that, when the wing descends and strikes against the air, it will present the greatest possible surface; whereas, when it is raised, to renew the stroke, it presents the least possible surface. So in the windmill, the position of the sail varies, on opposite sides of the wheel, that, in the one case it may receive the full force of the wind, and in the other case may suffer it to pass by.

II. Elasticity.-In fire-engines, airguns, airpumps, &c., a new property of air is brought into view, which we call elasticity. It acts, in the common fire-engine, in conjunction with the gravity of the air, and in the airgun, in conjunction with that equality of pressure, which is a property of aeriform bodies as well as of liquids. In virtue of its elasticity, the air tends to expand itself; and if it be condensed, or compressed into a space smaller than that which it naturally occupies, it will, if suffered to expand freely, exert a force just equal to that which has been employed in compressing it. Thus, in the fireengine, water is forced into an airtight vessel, which had been previously filled with air: as the water enters, it crowds the air into a smaller and smaller space, by which means, the elasticity of the latter is so much increased, that it reacts upon the surface of the water, and drives it out through the spout of the engine. Here, the water is thrown out by the air, with no greater force than has