scale-pan, M, when the upper surface of the water is marked by a sliding bead, n. The other vessels, B and C, are successively screwed on, and filled with water up to the level, n; if any more water is poured This principle of pressure on the called, because the same pressure 76. The principle of equal pressures has been applied in the construction of a press, by means of which a single man may exert an enormous power. This press is shown in per spective in Fig. 48, and in section in Fig. 49, the letters in both figures corresponding to the same parts. What is this principle of pressure called? Why? (75.) Explain Pascal's experiment. (76.) What is the principle of the Hydraulic Press? The press consists of two cylinders, A and B, of unequal diame. ters. 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 cylinder, 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. Describe the press in detail. Explain its action. 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 tubular 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 Illustrate its power by an example. What are its uses! description. The hydraulic press was also used in launching the Great Eastern, the heaviest movable structure ever constructed by man. II. 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. ship viewed from the shore, as exhibited in Fig. 50. As the vessel recedes, 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. In defining a level surface, we said that it is everywhere perpendicular 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 What Explain the effect of the centrifugal force on the form of a level surface. is a true level? An apparent level? (79.) What are the conditions of equilibrium in communicating vessels? |