ship sailing from the shore, as exhibited in Fig. 50. As the vessel sails away, we first lose sight of her hull, then her lower sails disappear, then her higher sails, until at last the entire vessel is lost to view.

pendicular to the direction of gravity; more strictly speaking, it is perpendicular to the resultant of gravity and the centrifugal force due to the earth's rotation on its axis. Were it not for the centrifugal force, the surface of the ocean would be perfectly spherical, but in consequence of that force, it is ellipsoidal; that is, the oceans are elevated about the equator and depressed about the poles.

The general level of the ocean is called the true level; a horizontal plane at any point is called the apparent level.

Equilibrium of Liquids in Communicating Vessels.

79. When a liquid is contained in vessels which communicate with each other, it will be in equilibrium if its

Explain the effect of the centrifugal force on the form of a level surface. What is a true level? An apparent level? (79.) What are the conditions of equilibrium in communicating vessels ?

upper surface in all of the vessels is in the same horizontal plane.

This principle is demonstrated by means of the apparatus represented in Fig. 51. This apparatus consists of a system of glass vessels of different shapes and capacities, all of which communicate by a tube, ac. If any amount of water or other liquid be poured

that its upper surface in all of the vessels is in the same horizontal plane. The reason of this is, obviously, a necessary consequence of the principle of equal pressures.

Case of Vessels containing Liquids of different Densities.

80. When liquids of different densities are contained in communicating vessels, they will be in equilibrium when the heights of the columns are inversely as their densities.

This principle is demonstrated by means of an apparatus shown in Fig. 52. The apparatus consists of two glass tubes, A and B, open

How is this demonstrated? (80.) What are the conditions of equilibrium in the case of liquids of different densities? How is this demonstrated?

83. The SPIRIT LEVEL consists of a tube of glass nearly filled with alcohol, and closed at its two extremities. The tube is slightly curved, and when placed horizontally, the

bubble of air which it contains rises to the middle of the upper side of the tube. If either end be depressed, the

bubble runs towards the other end. When used it is ordinarily mounted in a wooden case.

This form of level is much used by masons, carpenters, and other artisans. To ascertain whether a surface is level, the instrument is laid upon it, and the position of the bubble noticed. If the bubble is in the middle of the tube, the surface is level.

This form of level is also attached to many kinds of surveying and astronomical instruments.

84. It is the principle of equal pressures that causes water to rise in springs and fountains. The water which feeds them is contained in natural or artificial reservoirs higher than the spring or fountain. These reservoirs communicate with the springs or fountains by natural or artificial channels, and the pressure of the water in them

What are its uses! Applications. (84.) What is a Spring? Fountain?

causes that in the spring or fountain to boil up, or sometimes to shoot up in a jet.

Fig. 56 represents a fountain called a jet d'eau.

The reservoir is on the hill to the left, and the water reaches the bottom of the basin by a pipe represented by dotted lines.

The water of the jet tends to rise to the level of that in the reservoir, and would do so were it not for the resistance of the air, the friction of the water against the pipe, and the resistance offered by the falling particles, all of which combine to render the jet lower than the fountain-head.

The same principle determines the flow of streams from the higher to the lower grounds. The water of lakes, seas, and oceans is continually evaporating to form vapors and clouds. These are condensed in the form of rain, and the particles of water, urged by their own weight, seek a lower level. The rivulets gather to form brooks, and these unite to form rivers, by which the water is once more returned to the oceans and lakes. All of the water does not flow back to the ocean along the surface, but a portion percolates through the porous soils and accumulates in cavities to feed our springs and wells.

Artesian Wells.

85. ARTESIAN WELLS are deep wells, formed by boring through rocks and strata of various kinds of earth to reach a supply of water. These wells are named from the province of Artois, in France, where they were first used.

Fig. 57 illustrates the principle of these wells. H is the natural surface of the earth. AB and CD are curved strata of clay or rock which do not allow of the percolation of water. KK is an intermediate stratum of sand or gravel, which permits water to penetrate it. When a hole, I, is bored down to strike the waterbearing stratum, KK, the pressure of the water in the stratum forces it up in a jet. The well of Grenelle, in Paris, is nearly 1800 feet

• Explain the jet d'eau? What causes the flow of streams? (85.) What are Artesian Wells? Explain their action? Paris?

How are they fed? How deep is that at