CONVERSATION X.

ON THE MECHANICAL PROPERTIES OF FLUIDS.

Definition of a Fluid; Distinction between Fluids and Liquids; Of Non-Elastick Fluids; Scarcely susceptible of Compression; Of the Cohesion of Fluids; Of their Gravitation; Of their Equilibrium; Of their Pressure; Of Specifick Gravity; Of the Specifick Gravity of Bodies heavier than Water; Of those of the same Weight as Water; Of those lighter than Water; Of the Specifick 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 hydrostaticks. 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.*

* If the particles of fluids are round, there must be vacant spaces between them, in the same manner as there are vacuities between cannon balls that are piled together; between these balls smaller

574. What is the science called that treats of the mechanical properties of fluids?-575. What is meant by a fluid?576. In which is the attraction of cohesion the most powerful, solids or fluids?-577. What is inferred from the slight cohesion of the particles of fluids, and the facility with which they slide over each other?

Caroline. 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 elastick 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-elastick 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 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. 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

shot may be placed, and between these, other still smaller, or gravel, or sand, may be diffused. In a similar manner, a certain quantity of particles of sugar can be taken up in water without increasing its bulk, and when the water has dissolved the sugar, salt may be dissolved in it, and yet the bulk remain the same and admitting that the particles of water are round, this is easily accounted for.

578. What reason is given in the note for supposing that the particles of fluids are round?- -579. What is the distinction between a liquid and a fluid ?- -580. Are water and other liquids susceptible of compression ?-581. What is the reason for supposing they are not?582. What experiment has been made to show that liquids are not compressible?-583. How do flu

ids gravitate compared with solids?584. What example is given to show that solids gravitate in a less perfect manner than liquids ?

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 inheaps; 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 superiour 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

585. Why cannot liquids be moulded into figures like solids? 586. What is meant by the level or equilibrium of fluids ?587. Of what is the level or equilibrium of fluids the result?588. Why will oil remain upon the top of water?- -589. How is a water-level constructed?

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.

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 equilibrum is restored.

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 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 it downwards; but as it must continually happen, that a particle presses between two particles beneath, (fig. 3.) these last must suffer a lateral pressure.

-591. Why -592. Why -593. Why

590. Why do solid bodies gravitate in masses? is the resistance of fluids less than that of solids? are fluids inclined to a state of rest or equilibrium?will liquids run out of an opening in the vessel containing them?

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 cocks 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 cock having a greater weight above it, the water is forced out with greater velocity, whilst the lowest cock, 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 effected 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.

* An empty bottle being corked, and, by means of a weight, let. down a certain depth into the sea, it will be broken, or the cork will be driven into it by the perpendicular pressure. But a bottle filled with water, or any other liquid, may be let down to any depth, without damage, because in this case the internal pressure is equal to the external ?

594. From what does the lateral pressure of liquids proceed? -595. How would you illustrate the lateral and downward pressure of fluids by the figures?- -596. What fact is mentioned in the note concerning the pressure of liquids ?- -597. To what is the velocity of liquids, issuing from an orifice in the side of a vessel, proportional ?