produces by its pulsations the manifestations of these great natural agents to which the theory is supposed to apply. But, even supposing we may, by the advances of science, prove the existence of this hypothetical Ether, we have still to seek for a force superior to and beyond it, before it can be moved. When we come to consider the laws of these great physical agencies, it will be shown to what extent the evidences of experimental examination support the idea, that variations of motion will give different forms to these subtile elements. It is, however, most important again to impress the fact, that motion cannot produce a force, and that a force must be exerted to produce MOTION.

LAWS OF SLIGHTLY ELASTIC FLUIDS.

FLUIDS AT REST.-Hydrostatics (from idwp, water, and σTATIKη, laws of bodies at rest). Flowing over the solid surface of this planet, we have a most extraordinary liquid body. It is presented to us in the remarkable features of a vast ocean of never-resting water, which is, under the influence of solar heat, vaporizing, thus charging the atmosphere with moisture, which, passing over the land, is condensed by the hills, to flow in all the varied beauty of torrent and stream, or to gather into the no less important accumulations of the lake.

The largest portion of the surface of the earth is occupied by water. In the northern hemisphere it has been

calculated that the proportion of land to water is as 419 to 1,000, while in the southern hemisphere it is as 129 to 1,000. The greatest depth of the ocean is probably about five miles; while Laplace has calculated that its mean depth is about 3,000 feet. From the porous nature of the soil, and the circumstance that the stony strata forming the external crust of this world is cleaved, jointed, and often rent, channels are formed by which water passes to a considerable depth into the interior, there performing an important part in the great chemical changes constantly taking place in that laboratory of nature. Again, the vegetable and the animal kingdoms hold water largely combined; and the fluids, compounds of water and other bodies, circulating in the capillary vessels, and filling the cells of the plant, or coursing the arteries and veins of the animal, obey those laws which now become the subjects of our consideration.

Water remains liquid in virtue of some very remarkable disposition of its molecules, under the mysterious opposition of heat and cohesion; and the same may be said of all other liquid bodies.

The particles constituting a fluid are held together by so small a cohesive force that they are readily separated : by a slight blow we break a quantity of water into exceedingly fine drops, and when separated, from their mobility, they again freely flow together. We assume that in liquids the atoms are so far removed from each other, that the attractive and repulsive forces are in equi

librium, and hence the freedom with which they move about each other. An unconfined fluid is ready to yield to the action of any pressure in any direction.

It was long thought that liquids were absolutely incompressible; and to prove this, some scientific members of the Academia del Cimento, of Florence, filled a globe of gold with water, and subjected it to great pressure. The result of this experiment was, that the water was pressed through the gold and stood as dew upon its exterior surface. Mr. Canton has, however, shown that, though water is compressible only within small limits, and with great difficulty, yet that it has elasticity. He found that the pressure of the atmosphere compressed rain-water about one part in 22,000; olive-oil, about one in 21,000; spirits of wine, one in 15,000; and mercury, as might have been expected, only one in 33,000. Some ingenious experiments by Mr. Perkins, who sank cylinders filled with water three thousand feet deep in the ocean, by which he obtained a pressure of thirteen hundred pounds, appear to confirm the previous results.

The rebounding of a piece of slate, when thrown nearly horizontal upon the surface of a lake-as in the boys' game of ducks and drakes,—or of a musket or cannonball so fired, proves water to have a certain amount of elasticity.

Fluid bodies, equally with solids, are under the influence of gravitation, and, from the freedom of motion which a fluid particle has, they have no power of retaining

a permanent form, but always adapt themselves to the shape of the vessel containing them, or to the surfaces upon which they rest: all lakes, seas, and rivers adjust themselves to the inequalities of the land.

It has been already shown, that, relieved from the action of gravitating force, liquids will assume a spherical form, by virtue of the cohesive force in operation between their particles. If a drop of water is taken up by a glass rod, it hangs to it in a shape approaching the spherical; but here we have the attraction of the surface of the glass operating on one side, and gravitation on the other. Over a greasy glass plate, water rolls in drops of a spheroidal form: the dew on the flower, and the drop on the wing of a water-fowl, all show that fluids are under the same laws of aggregation as solids, but that the exercise of the force is much weaker.

From these conditions it follows, that fluids are so constituted that all parts, undisturbed, must settle at the same level, no part sinking below or rising above another. If we

connect together a series of vessels, no matter what may be their shapes, or how

extensive the sys

Fig. 75.

tem, so that water may rise from the main channel, a B, into them, we shall find, upon pouring water into one,