habitants of both those situations, at the same time. Besides, as the orbit of the moon is very nearly parallel to that of the earth, she is never vertical, but to the inhabitants of the torrid zone.

Caroline. In the torrid zone, then, I hope you will grant that the moon is immediately over, or opposite the spots where it is high water?

Mrs. B. I cannot even admit that; for the ocean naturally partaking of the earth's motion, in its rotation from west to east, the moon, in forming a tide, has to contend against the eastern motion of the waves. All matter, you know, by its inertia, makes some resistance to a change of state; the waters, therefore, do not readily yield to the attraction of the moon, and the effect of her influence is not complete, till three hours after she has passed the meridian, where it is full tide.

When a body is impelled by any force, its motion may continue, after the impelling force ceases to act: this is the case with all projectiles. A stone thrown from the hand, continues its motion for a length of time, proportioned to the force given to it: there is a perfect analogy between this effect, and the continued rise of the water, after the moon has passed the meridian at any particular place.

Emily. Pray what is the reason that the tide is three-quarters of an hour later every day?

Mrs. B. Because it is twenty-four hours and three-quarters before the same meridian, on our globe, returns beneath the moon. The earth revolves on its axis in about twenty-four hours; if the moon were stationary, therefore, the same part of our globe would, every twenty-four hours, return beneath the moon; but as during our daily revolution, the moon advances in her orbit, the earth must make more than a complete rotation, in order to bring the same meridian opposite the moon: we are three-quarters of an hour in overtaking her. The tides, therefore, are retarded, for the same reason that the moon rises later by three-quarters of an hour, every day.

We have now, I think, concluded the observations I had to make to you on the subject of astronomy; at our next interview, I shall attempt to explain to you the elements of hydrostatics.

45. Why in the open ocean, is it high water, some hours after the moon has passed the meridian? 46. Why are the tides three-quarters of an hour later every day?

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, 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 class of fluids. There

What are the two divisions of the science which treats of the mechanical properties of liquids? 2. Of what do hydrostatics and hydraulics treat? 3. What is a fluid defined to be? 4. From what is fluidity supposed to arise. 5. Into what two classes are fluids divided?

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 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 space, 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

6. What is said of the incompressibility of liquids, and what experiment is related? 7. Ought this experiment to be considered as conclusive? 8. Why do fluids appear to gravitate more freely than solids ?

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 masses, while every particle of a fluid may be considered as separate, and gravi tating 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 independent ly, 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

9. When is a fluid said to be in equilibrium? 10. What is there in the nature of a fluid, which causes it to seek this level? 11. What circumstances occasion oil to float upon water? 12. What is the nature and use of the instrument represented in fig. 1, plate 13? 13. What difference is there in the gravitation of solid masses, and of fluids?