The press consists of two cylinders, A and B, of unequal diameters. In the cylinder, B, is a solid piston, C, which rises as the water is forced into B, and thus forces up a platform, K. The cylinder, A, forms the barrel of a pump by means of which water is raised from a reservoir, P, and forced into the eylinder, B. by a lever, O, attached to a solid piston, a. raised, a vacuum is formed behind it, which

This pump is worked When the piston, a, is is filled by water from

the reservoir, P, which enters by opening the valve, S. When the piston is depressed, the valve, S, closes, the valve, m, is opened, and a portion of the water is forced through the pipe, d, into the cylinder, B. By continuing to work the piston, a, up and down, additional quantities of water are forced into the large cylinder.

In consequence of the principle of equal pressures, the force applied to the piston, a, is transmitted through the tube, d, and is finally exerted upwards against the piston, C, its effect being multiplied by the number of times that the section of the piston, C, is greater than that of the piston, a. For example, if the section of C is 150 times as great as that of a, every pound of pressure on the latter will produce 150 lbs. of pressure on the former. This effect is further multiplied by means of the lever, O. The pressure exerted upon C, forces up the platform, K, with an energy that may be utilized in compressing any substance placed between it and the top of the press, MN. This upward pressure may also be used for raising heavy weights.

By varying the relative dimensions of the parts of the machine, an immense power may be exerted. In the arts, presses of this kind are constructed capable of exerting a force of more than a hundred thousand pounds.

The hydraulic press is used in compressing seeds to obtain oils, in packing hay, cotton, and other goods for shipment, in pressing books for the binder, and in a great variety of other operations. The immense tubulag bridge over the Menai Straits was raised from the level of the water to the top of the piers by means of presses of this

description. The hydraulic press was also used in launching the Great Eastern, the heaviest movable structure ever constructed by

man.

11. EQUILIBRIUM OF LIQUIDS.

Conditions of Equilibrium.

77. A solid body is in equilibrium when its centre of gravity is supported, because the particles of the body are held together by cohesion. In liquids the particles do not cohere, and unless restrained they would flow away and spread out indefinitely. A liquid can be in equilibrium only when restrained by a vessel, or something equivalent. Furthermore, each particle must be equally pressed in all directions, which requires that the free surface should be level, that is, everywhere perpendicular to the force of gravity.

In saying that the free surface must be level, we suppose that the liquid is acted upon only by the force of gravity, which is the ordinary case. If, however, it is acted upon by other forces, the free surface must, at every point, be perpendicular to the resultant of all the forces acting at that point; for if it were not so, this resultant might be resolved into two components, one perpendicular to the surface, and the other parallel to it. The former would be resisted by the reaction of the liquid, and the latter, being uncompensated, would produce motion, which is contrary to the hypothesis of equilibrium.

Level Surface.

78. The surface of a liquid is LEVEL when it is everywhere perpendicular to the direction of gravity. Small level surfaces coincide sensibly with horizontal planes. Large level surfaces are curved so as to conform to the general form of the earth's surface. That the surface of the ocean is curved is shown by the phenomena presented by a

(77.) Explain the difference between equilibrium of solids and liquids. When is a liquid in equilibrium? How is the upper surface when other forces than gravity act? Why? (78.) What is a level surface? Nature of a small level surface? Of a larger one? Illustrate.

at top, and communicating at bottom by a smaller tube. If a quantity of mercury be poured into one of the tubes, it will come to a level in both tubes, according to the principle explained in the preceding article. If a quantity of water be poured into the tube A, the level of the mercury in that tube will be depressed, whilst it will be elevated in the tube B. The difference of level, dc, can be determined by the graduated scales on the tubes. by measurement, that the column of water, ab, is as the column of mercury, dc, which it supports. It will be shown hereafter, that mercury is 13.6 times as dense as water; hence the principle is proved. Other liquids may be employed with similar results.

It will be found 13.6 times as high

Equilibrium of Heterogeneous Liquids.

81. If liquids of different densities, but which do not mix, be poured into a vessel, they will arrange themselves

(81.) What are the conditions of equilibrium of heterogeneous liquids?

in the order of their densities, the heaviest being at the

bottom, and the upper surface of each will be horizontal.

This is shown by a vial, Fig. 53, containing liquids of different densities, as mercury, water, and oil. If the vial be shaken, the liquids appear to mix, but if allowed to stand, they arrange themselves in horizontal layers, the densest liquid at the bottom.

The vial in the figure is represented as containing four liquids. It was formerly called the vial of four ele

ments.

It is in accordance with this principle that cream rises on milk, and oil on water. The principle is often employed to separate liquids of different density by the process of decantation.

Fig. 53.

III. APPLICATIONS OF THE PRINCIPLE OF EQUILIBRIUM.

The Water Level.

82. A WATER LEVEL is an instrument employed for determining the difference of level between two points. It consists of a horizontal tube of tin, 2 or 3 feet in length, into the extremities of which two glass tubes are inserted perpendicular to it. The whole rests upon a three-legged support, called a tripod, as shown in Fig. 54. A quantity of water tinged with carmine or other coloring matter is introduced into one of the glass tubes, which, flowing through the horizontal tube, rises to the same level in the other. A visual ray directed along the surfaces of the

How shown (82.) What is a Water Level? Describe it and its use.