« AnkstesnisTęsti »
face is equally distant from the point to which gravity tends; that is to say, from the centre of the earth. Hence the surface of all fluids must partake of the spherical form of the globe, and be bulging. This is evident in large bodies of water, such as the ocean; but the sphericity of small bodies of water is so trifling as to render their surfaces apparently flat.
The equilibrium of fluids is the natural result of their particles gravitating independently of each other; for when any particle of a fluid accidentally finds itself elevated above the rest, it is attracted down to the level of the surface of the fluid, and the readiness with which fluids yield to the slightest pressure, will enable the particle by its weight to penetrate the surface of the fluid and mix with it. But this is the case only with fluids of equal density, for a light fluid will float on the surface of a heavy one, as oil on water; and air will rise to the surface of any liquid whatever, being forced up by the superior gravity of the liquid. The figure here represents instrument called a water-level, which is constructed upon the principle of the equilibrium of fluids, It consists of a short tube, A B, closed at both ends and containing water and a bubble of air; when the tube is not perfectly horizontal the water runs to the lower end, which makes the bubble of air rise to the upper end, and it remains in the centre only when the tube does not incline on either side. It is by this means that the level of any situation, to which we apply the instrument, is ascertained.
Solid bodies, therefore, gravitate in masses, the strong cohesion of their particles making them weigh altogether, while every particle of a fluid may be considered as a separate mass, gravitating independently. Hence the resistance of a fluid is considerably less than that of a solid body. The particles of fluids acting thus independently, press against each other in every direction, not only downwards but upwards, and laterally or sideways; and in consequence of this equality of pressure, every particle remains at rest in the fluid. If you
agitate the fluid, you disturb this equality, and the fluid will not rest till its equilibrium be restored.
Were there no lateral pressure, water would not flow from an opening on the side of a vessel; sand will not run out of such an opening, because there is scarcely any lateral pressure among the particles. Were the particles of fluids arranged in regular columns, there would be no lateral pressure, for when one particle is perpendicularly above the other, it
can only press it downwards; but as it must continually happen that a particle passes between two particles beneath, these last suffer a lateral pressure; just as a wedge driven into a piece of wood separates the parts laterally. The lateral pressure is the result therefore of the pressure downwards, or the weight of the liquid above; and consequently the lower the orifice is made in the vessel, the greater will be the velocity of the water rushing out of it. The annexed figure represents the different degrees of velocity with which a liquid flows from a vessel furnished with three stopcocks at different heights. Since the lateral pressure is entirely owing to the pressure downwards, it is not affected by the horizontal dimensions of the vessel, which contains the liquid, but merely by its depth; for as every particle acts independently of the rest, it is only the column of particles immediately above the orifice that can weigh upon and press out the liquid.
The pressure of fluids upwards, though it seems in direct opposition to gravity, is also a consequence of their pressure downwards. When, for example, water is poured into a tea-pot, the water rises in the spout to a level with that in the pot. The particles of water at the bottom of the pot are pressed upon by the particles above them; to this pressure they will yield, if there is any mode of making way for the superior particles, and
as they cannot descend, they will change their direction, and rise in the spout.
Suppose the tea-pot to be filled with columns of particles of water similar to those described in the figure annexed, the particle 1 at the bottom will be pressed laterally by the particle 2, and by this pressure be forced into the spout, where, meeting with
the particle 3, it presses it upwards, and this pressure will be continued from 3 to 4, from 4 to 5, and so on, till the water in the spout has risen to a level with that in the pot.
The specific gravity of a body means simply its weight compared with that of another body of the same size. When we say that substances, such as lead and stones, are heavy, and that others, such as paper and feathers, are light, we speak comparatively; that is to say, that the first are heavy, and the latter light, in comparison with the generality of the substances in nature. Mahogany is a heavy body when compared to most other kinds of wood, but light when compared to stone. Chalk is a heavy body compared to coal, but light if compared to metal. Thus our notions of light and heavy are vague and undefined, and some standard of comparison is required, to which the weight of all other bodies may be referred. The body which has been adopted as a standard of reference is distilled water. When the specific gravity of bodies is to be estimated, it is necessary simply to weigh the body under trial in water. If a piece of gold be weighed in a glass of water, the gold will displace just as much water as is equal to its own bulk; a cubic inch of water must make way for a cubic inch of gold. The bulk alone is to be considered, the weight having
nothing to do with the quantity of water displaced; for a cubic inch of gold does not occupy more space, and therefore will not displace more water, than a cubic inch of ivory, or any other substance that will sink in
The gold will weigh less in water than it did out of it, on account of the upward pressure of the particles of water, which in some measure supports the gold, and, by so doing, diminishes its weight. If the body under trial be of the same weight as the water in which it is immersed, it will be wholly supported by it; if it be heavier, the water will offer some resistance to its descent; and this resistance will in all cases be the same to bodies of equal bulk, whatever be their weight. All bodies of the same size, therefore, lose the same quantity of their weight when completely immersed in water. A body weighed in water loses as much of its weight as is equal to that of the water it displaces; so that were this water put into the scale to which the body is suspended, it would restore the balance.
When a body is weighed in water, in order to ascertain its specific gravity, it may either be suspended to a hook at the bottom of the basin of the balance, or, taking off the basin, suspended to the arm of the balance. Now, supposing that a cubic inch of gold weighed nineteen ounces out of water, and lost one ounce by being weighed in water, the cubic inch of water it displaces must weigh that one ounce; consequently gold would be nineteen times as heavy as water.
The specific gravity of a body lighter than water cannot be ascertained in the same manner. If a body were absolutely light, it would float on the surface, without displacing a drop of water; but bodies have all some weight, and will, therefore, displace some quantity of water. A body lighter than water will not sink to a
level with the surface of the water, and therefore will not displace so much water as is equal to its bulk, but a quantity equal to its weight. A ship sinks to some depth in water, and the heavier it is laden the deeper it sinks, the quantity of water it displaces being always equal to its weight. This quantity cannot, however, afford a convenient test of its specific gravity, from the difficulty of collecting the whole quantity of water displaced, and of measuring the exact bulk of the body immersed.
In order practically to obtain the specific gravity of a body which is lighter than water, a heavy one, whose specific gravity is known, must be attached to it, and they must be immersed together: the specific gravity of the lighter body may then be easily calculated.
Bodies which have exactly the same specific gravity as water, will remain at rest in whatever situation they are placed in water. If a piece of wood, by being impregnated with a little sand, be rendered precisely of the weight of an equal bulk of water, it will remain stationary in whatever part of a vessel of water it be placed. If a few drops of water be poured into the vessel (so gently as not to increase their momentum by giving them velocity,) they would mix with the water at the surface, and not sink lower.
The specific gravity of fluids is found by means of an instrument called an hydrometer. It consists of a thin glass ball, A, with a graduated tube, B, and the specific gravity of the liquid is estimated by the depth to which the instrument sinks in it; for the less the specific gravity of the fluid, the further will the instrument sink in it.There is a smaller ball, c, attached to the instrument с below, which contains a little mercury; but this is merely for the purpose of equipoising the instrument, that it may remain upright in the liquid under trial.
The weight of a substance, when not compared to that of any other, is perfectly arbitrary; and when water