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 illustrated, 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 is called hydrostatics. 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 must be small, smooth, and globular; smooth, because there appears to be little or no friction among them; and globular, because touching each other but by a point would account for the slightness of their cohesion.

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

Mrs. B. Liquids comprehend only one class of fluids. There is another class 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 at our next meeting, 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 space than that which they naturally occupy. This is supposed to be owing to the extreme minuteness of their particles, which, rather than submit to compression, force their way through the pores of the substance which confires 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. Fluids gravitate in a more perfect manner than solid bodies; for the strong cohesive attraction of the particles of the latter in some measure counteracts the effects 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 gravity tends, that is to say, from the centre of the earth; hence the surface of all fluids must be bulging, 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. That is, because oil is a lighter liquid than water. If you were to pour water over it, the oil would rise to the surface, being forced up by the superior gravity of the water. Here is an instrument called a waterlevel, (fig. 1. plate XIII.) 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; when the tube is not perfectly horizontal the water runs to the lower end, and it is by this means that the level of any situation, to which we apply the instrument, is ascertained.

Solid bodies you may, therefore, consider as gravitating in masses, for the strong cohesion of their particles makes them weigh altogether, while every particle of a fluid may be considered as composing a separate mass, 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, they are more easily overcome.