CONVERSATION X.

ON THE MECHANICAL PROPERTIES OF FLUIDS.

DEFINITION OF A FLUID. DISTINCTION BETWEEN FLUIDS AND LIQUIDS. OF NON-ELASTIC FLUIDS. SCARCELY SUSCEPTIBLE OF COMPRESSION. OF THE COHESION OF FLUIDS. OF THEIR GRAVITATION. OF THEIR EQUILIBRIUM. OF THEIR PRESSURE. OF SPECIFIC GRAVITY. OF THE SPECIFIC GRAVITY OF BODIES HEAVIER THAN WATER. OF THOSE OF THE SAME WEIGHT AS WATER. OF THOSE LIGHTER THAN WATER. OF THE SPECIFIC GRAVITY OF FLUIDS.

MRS. B.

We have hitherto confined our attention to the mechanical properties of solid bodies, which have been ill.u.s.trated, and, I hope, thoroughly impressed upon your memory, by the conversations we have subsequently had, on astronomy. It will now be necessary for me to give you some account of the mechanical properties of fluids--a science which, when applied to liquids, is divided into two parts, hydrostatics and hydraulics. Hydrostatics, treats of the weight and pressure of fluids; and hydraulics, of the motion of fluids, and the effects produced by this motion. A fluid is a substance which yields to the slightest pressure. If you dip your hand into a basin of water, you are scarcely sensible of meeting with any resistance.

_Emily._ The attraction of cohesion is then, I suppose, less powerful in fluids, than in solids?

_Mrs. B._ Yes; fluids, generally speaking, are bodies of less density than solids. From the slight cohesion, of the particles of fluids, and the facility with which they slide over each other, it is inferred, that they have but a slight attraction for each other, and that this attraction is equal, in every position of their particles, and therefore produces no resistance to a perfect freedom of motion among themselves.

_Caroline._ Pray what is the distinction between a fluid and a liquid?

_Mrs. B._ Liquids comprehend only one cla.s.s of fluids. There is another cla.s.s, distinguished by the name of elastic fluids, or gases, which comprehends the air of the atmosphere, and all the various kinds of air with which you will become acquainted, when you study chemistry. Their mechanical properties we shall examine hereafter, and confine our attention this morning, to those of liquids, or non-elastic fluids.

Water, and liquids in general, are scarcely susceptible of being compressed, or squeezed into a smaller s.p.a.ce, than that which they naturally occupy. Such, however, is the extreme minuteness of their particles, that by strong compression, they sometimes force their way through the pores of the substance which confines them. This was shown by a celebrated experiment, made at Florence many years ago. A hollow globe of gold was filled with water, and on its being submitted to great pressure, the water was seen to exude through the pores of the gold, which it covered with a fine dew. Many philosophers, however, think that this experiment is too much relied upon, as it does not appear that it has ever been repeated; it is possible, therefore, that there may have been some source of error, which was not discovered by the experimenters. Fluids, appear to gravitate more freely, than solid bodies; for the strong cohesive attraction of the particles of the latter, in some measure counteracts the effect of gravity. In this table, for instance, the cohesion of the particles of wood, enables four slender legs to support a considerable weight. Were the cohesion destroyed, or, in other words, the wood converted into a fluid, no support could be afforded by the legs, for the particles no longer cohering together, each would press separately and independently, and would be brought to a level with the surface of the earth.

_Emily._ This want of cohesion is then the reason why fluids can never be formed into figures, or maintained in heaps; for though it is true the wind raises water into waves, they are immediately afterwards destroyed by gravity, and water always finds its level.

_Mrs. B._ Do you understand what is meant by the level, or equilibrium of fluids?

_Emily._ I believe I do, though I feel rather at a loss to explain it.

Is not a fluid level when its surface is smooth and flat, as is the case with all fluids, when in a state of rest?

_Mrs. B._ Smooth, if you please, but not flat; for the definition of the equilibrium of a fluid is, that every part of the surface is equally distant from the point to which they gravitate, that is to say, from the centre of the earth; hence the surface of all fluids must be spherical, not flat, since they will partake of the spherical form of the globe.

This is very evident in large bodies of water, such as the ocean, but the sphericity of small bodies of water, is so trifling, that their surfaces appear flat.

This level, or 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 impression, will enable the particle by its weight, to penetrate the surface of the fluid, and mix with it.

_Caroline._ But I have seen a drop of oil, float on the surface of water, without mixing with it.

_Mrs. B._ They do not mix, because their particles repel each other, and the oil rises to the surface, because oil is a lighter liquid than water. If you were to pour water over it, the oil would still rise, being forced up by the superior gravity of the water. Here is an instrument called a spirit-level, (fig. 1, plate 13.) 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 a little water, or more commonly some spirits: it is so nearly filled, as to leave only a small bubble of air; when the tube is perfectly horizontal, this bubble will occupy the middle of it, but when not perfectly horizontal, the water runs to the lower, and the bubble of air or spirit rises to the upper end; by this instrument, the level of any situation, to which we apply it, may be ascertained.

From the strong cohesion of their particles, you may therefore consider solid bodies as gravitating in ma.s.ses, while every particle of a fluid may be considered as separate, and gravitating independently of each other. Hence the resistance of a fluid, is considerably less, than that of a solid body; for the resistance of the particles, acting separately, is more easily overcome.

_Emily._ A body of water, in falling, does certainly less injury than a solid body of the same weight.

_Mrs. B._ 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 of pressure, and the fluid will not rest, till its equilibrium is restored.

[Ill.u.s.tration: PLATE XIII.]

_Caroline._ The pressure downwards is very natural; it is the effect of gravity; one particle, weighing upon another, presses on it; but the pressure sideways, and particularly the pressure upwards, I cannot understand.

_Mrs. B._ If there were no lateral pressure, water would not run out of an opening on the side of a vessel. If you fill a vessel with sand, it will not continue to run out of such an opening, because there is scarcely any lateral pressure among its particles.

_Emily._ When water runs out of the side of a vessel, is it not owing to the weight of the water, above the opening?

_Mrs. B._ If the particles of fluids were arranged in regular columns, thus, (fig. 2.) there would be no lateral pressure, for when one particle is perpendicularly above the other, it can only press downwards; but as it must continually happen, that a particle presses between two particles beneath, (fig. 3.) these last, must suffer a lateral pressure.

_Emily._ The same as when a wedge is driven into a piece of wood, and separates the parts, laterally.

_Mrs. B._ Yes. The lateral pressure proceeds, therefore, entirely from 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. Here is a vessel of water (fig. 5.), with three stop c.o.c.ks at different heights; we shall open them, and you will see with what different degrees of velocity, the water issues from them. Do you understand this, Caroline?

_Caroline._ Oh yes. The water from the upper spout, receiving but a slight pressure, on account of its vicinity to the surface, flows but gently; the second c.o.c.k, having a greater weight above it, the water is forced out with greater velocity, whilst the lowest c.o.c.k, being near the bottom of the vessel, receives the pressure of almost the whole body of water, and rushes out with the greatest impetuosity.

_Mrs. B._ Very well; and you must observe, that as the lateral pressure, is entirely owing to the pressure downwards, it is not affected by the horizontal dimensions of the vessel, which contains the water, 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 water.

_Emily._ The breadth and width of the vessel then, can be of no consequence in this respect. The lateral pressure on one side, in a cubical vessel, is, I suppose, not so great as the pressure downwards upon the bottom.

_Mrs. B._ No; in a cubical vessel, the pressure downwards will be double the lateral pressure on one side; for every particle at the bottom of the vessel is pressed upon, by a column of the whole depth of the fluid, whilst the lateral pressure diminishes from the bottom upwards to the surface, where the particles have no pressure.

_Caroline._ And from whence proceeds the pressure of fluids upwards?

that seems to me the most unaccountable, as it is in direct opposition to gravity.

_Mrs. B._ And yet it is in consequence of their pressure downwards.

When, for example, you pour water into a tea-pot, the water rises in the spout, to a level with the water 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 that described in fig. 4., 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.

_Emily._ If it were not for this pressure upwards, forcing the water to rise in the spout, the equilibrium of the fluid would be destroyed.

_Caroline._ True; but then a tea-pot is wide and large, and the weight of so great a body of water as the pot will contain, may easily force up and support so small a quant.i.ty, as will fill the spout. But would the same effect be produced, if the spout and the pot, were of equal dimensions?

_Mrs. B._ Undoubtedly it would. You may even reverse the experiment, by pouring water into the spout, and you will find that the water will rise in the pot, to a level with that in the spout; for the pressure of the small quant.i.ty of water in the spout, will force up and support, the larger quant.i.ty in the pot. In the pressure upwards, as well as that laterally, you see that the force of pressure, depends entirely on the height, and is quite independent of the horizontal dimensions of the fluid.

As a tea-pot is not transparent, let us try the experiment by filling this large gla.s.s goblet, by means of this narrow tube, (fig. 6.)

_Caroline._ Look, Emily, as Mrs. B. fills it, how the water rises in the goblet, to maintain an equilibrium with that in the tube.

Now, Mrs. B., will you let me fill the tube, by pouring water into the goblet?

_Mrs. B._ That is impossible. However, you may try the experiment, and I doubt not that you will be able to account for its failure.

_Caroline._ It is very singular, that if so small a column of water as is contained in the tube, can force up and support the whole contents of the goblet; that the weight of all the water in the goblet, should not be able to force up the small quant.i.ty required to fill the tube:--oh, I see now the reason, the water in the goblet, cannot force that in the tube above its level, and as the end of the tube, is considerably higher than the goblet, it can never be filled by pouring water into the goblet.

_Mrs. B._ And if you continue to pour water into the goblet when it is full, the water will run over, instead of rising above its level in the tube.

I shall now explain to you the meaning of the _specific gravity_ of bodies.

_Caroline._ What! is there another species of gravity, with which we are not yet acquainted?

_Mrs. B._ No: 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 substances in nature. Would you call wood, and chalk, light or heavy bodies?