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also a fluid. Sometimes we hear gases spoken of as elastic fluids, to distinguish them from liquids, which are then regarded as inelastic; but this is not quite correct, for all fluids have some elasticity. Air, however, and the other gases, are much more elastic than liquids, such as water.

When force is applied to a solid body, it is communicated from particle to particle only in one direction. But in this respect fluids are altogether different. One of their distinguishing characteristics is, that they transmit pressure equally in all directions. Every particle presses on all those around it with equal force, and with equal force is pressed upon by them. With exactly the same force does it press on any solid body which it touches, and, action and reaction being equal, the solid returns a pressure equal to that which it receives.

From this principle we have some very curious results.

FIG. 40.


Suppose a vessel A CD B, the lower part of which is filled with water or any other liquid. If a water-tight piston be pressed downwards upon the surface of the water at A, the pressure so applied will be transmitted undiminished to every part of the surface of the vessel. If the lower surface of the piston at A have an area of one square inch, the pressure it communicates to the water will produce an equal pressure on every square inch of solid surface with which the water is in contact. Suppose now that a similar piston is inserted in the larger limb of the vessel, as at B, and suppose that its under surface has an area of 10 square inches. Then the whole upward pressure which the water exerts on the surface of the piston at B will be ten times the downward pressure applied to the piston at A. If the latter be made small enough, or the former large enough, a prodigious pressure may thus be produced by a comparatively feeble force. This is the principle of



the Bramah press (so named from its inventor) by which such wonders have been wrought. The piston at A is attached by its rod to a lever, which is worked by the hand or any other moving power; the weight or resistance to be overcome is connected with the rod of the piston at B. By using a machine of this description, of little more than the size of a tea-kettle, a man is enabled to cut through a thick bar of iron as easily as he could clip a piece of pasteboard. A force of 500 or 600 tons may in this way be brought to bear on any substance which we wish to press, to tear up, to cut to pieces, or to pull asunder. It was by such a machine, though of much greater size and power, that the tubes of the Britannia railway bridge, across the Menai Straits, were raised to their present position.

A similar effect will be produced, if, instead of having a piston at A, the limb A C be continued upwards, and filled with water. The weight of the water above A will cause a pressure at that point, which will be transmitted and multiplied, as already explained. Supposing the two limbs to have the same width as before, the upward pressure on the piston at B will be ten times the weight of the water above A. In other words, the pressure on the piston at B will be equal to the weight of a quantity of water sufficient to fill the wider limb to the same height at which the water stands in the other. And this last result will be equally true, however small the limb A C, or however large the limb B D. In short, the pressure of water upon any surface does not depend at all upon the bulk of the water, but only on the area of the surface against which it presses, and its own height above that surface; so that a very small quantity of water may overcome an enormous resistance. This is one form of what has usually been called the hydrostatic paradox. If, for example, a small strong pipe be fixed in the bunghole of a barrel full of water, and water poured in till it rises in the pipe to a sufficient height, the barrel will burst, although a very small quantity of water may have been required to fill the pipe. Nor does it matter in the least whether the pipe be straight or crooked, round, square, or

irregular in shape; nor, on the other hand, whether the surface exposed to the pressure be horizontal, slanting, or vertical; the extent of that surface, and the perpendicular height to which any part of the water rises above it, are the only things to be considered in calculating the effect. Hence it is easy to conceive how a little water may often be productive of much mischief. Suppose a pool of water in the bowels of a mountain, with a very small chink or fissure extending down to it from above; if that chink were to be filled with rain to a great height, the mountain might be shaken, perhaps rent in pieces with the greatest violence. Some of the most dreadful devastations have resulted from this cause.

It follows naturally from all that has been explained, that, if both pistons be removed from the vessel represented in fig. 40, the water will rise in both limbs to the same height. Without this there could not be equilibrium. This result will not be affected by the shape, size, or number of the limbs; the free surface of the liquid, when at rest, will be everywhere at one common level. If a pipe be laid across a valley, water running in at one end, after filling all the lower part, will rise at the other end to the same level at which it enters the pipe. This is a principle of great importance in conveying a supply of water to cities, and even to single houses. A reservoir is provided on a level a little higher than the highest point at which the water is required; and then it does not matter how far below that level any intervening portion of the pipe may have to lie, provided only it be strong enough. The water, however far it may have descended, will rise again to the level of its surface in the reservoir, and will rush out at any opening below that level. With this important application of the principles just explained, it appears probable that the ancients were unacquainted; hence those stupendous works called aqueducts, many of which still remain, more or less entire, as a monument at once of their ignorance of the laws of fluid pressure, and of their vast attainments in some other departments of mechanical science.


WHEN a solid body is plunged into a liquid, it displaces, if wholly submerged, a quantity of the liquid equal to its own bulk. Now, if the solid be of the same weight with the liquid so displaced, it is clear that it will remain at rest anywhere beneath the surface, neither rising nor falling, because the liquid itself, whose place it occupies, would thus have remained at rest. If the solid is heavier than the displaced liquid, it will sink to the bottom; but, if it is lighter, it will rise to the surface.

In order, therefore, that a solid body may float in any liquid, it is necessary that, bulk for bulk, it be not heavier than that liquid. Thus a piece of glass floats in quicksilver, but sinks in water; a piece of ebony floats in water, but sinks in alcohol. Suppose now that, by way of example, we throw a cork into a basin of water. It will be observed that it does not simply rest on the level surface, but is partially immersed, displacing a quantity of the water less than its own bulk. But why does it not sink farther? The force which supports it, and counteracts the tendency of its weight to make it descend, is the upward pressure of the water on the part immersed. That pressure must therefore be equal to the weight of the cork. Now, if the cork were removed, and the hollow it makes in the surface of the water were filled with the liquid itself, it is clear that there would still be equilibrium. The pressure, therefore, which supports the cork, is such as would exactly support the water necessary to fill the hollow made by it, and is therefore equal to the weight of that water. It follows from this, that the weight of the cork, and the weight of the water it displaces, are the same; for both are equal to the pressure by which the cork is prevented from sinking. The same thing is true of every solid body floating on the surface of a liquid; the liquid it displaces, though less in bulk, must be equal to it in weight. Hence, the heavier a body is, the greater will be the part of it immersed. A ship, for example, sinks the deeper, or draws the more water, the more heavily it is


laden. On this principle, the total weight of a floating body, such as a ship, may sometimes be easily ascertained. For, if we can find how many cubic feet of the hull are below the level of the water's surface, we have only to calculate the weight of the same number of cubic feet of water, which will also be the weight of the whole vessel, including masts, sails, cargo, and all that she carries or contains.

The body of a living man is very nearly equal in weight to its own bulk of water. Being generally, however, a little lighter, it will float safely in still water, if placed and kept in such a position as to be all submerged except the face. Unskilful persons, in attempting to do this, are apt to plunge and struggle, tossing their arms out of the water, so that less of the liquid is displaced; the head consequently sinks to restore equilibrium; they then take in water at their mouths and nostrils, which, expelling the air in their bodies, increases their weight without a corresponding increase in their bulk, till at last they become heavier than the water displaced, and inevitably go down.

The bodies of some species of animals, such as waterfowl, are much lighter than the same bulk of water. Fishes, again, have the power of changing their bulk, by distending an air-vessel or bladder contained within their bodies. This they accomplish by filling it with a kind of gas, different in different species, which they generate by means of an apparatus given them for the purpose. In this way, they can render themselves at will lighter or heavier than their own bulk of water, and so rise to the surface or sink to the bottom. Surely nothing could demonstrate more clearly the wise forethought of the Creator, than this adaptation of the inhabitants of the waters to the element in which they are to live.


It has been shown that a body floating in water is supported by a pressure equal to the weight of the water it displaces.

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