There is a big difference, then, between the weight of water and the pressure it exerts. In Figure 38 we have an L-shaped receptacle with the lower arm of the L terminating in a chamber A. The top wall B of this chamber measures ten square inches. The tube C has a cross sectional area of one square inch. If tube C is filled to a height of twelve inches above wall B we shall have an upward pressure of 0.434 pound on every square inch of wall B, or a total of 4.34 pounds. If by means of a plunger D we add a hundred pounds of pressure to the column of water in tube C, we shall be adding a thousand pounds to the pressure on the wall B. The side walls and bottom of the chamber A will also be subjected to a pressure of 1,000 pounds per inch plus the pressure due to the depth or head of water. [Illustration: FIG. 38.--DIAGRAM ILLUSTRATING HYDROSTATIC PRESSURE] [Illustration: THE AIR-LOCK OF A PNEUMATIC CAISSON] [Illustration: SUBAQUEOUS TUNNEL, SHOWING THE SHIELD IN THE BACKGROUND] [Illustration: FIG. 39.--PRINCIPLE OF THE HYDRAULIC PRESS] Here, then, we have a convenient means of multiplying force or effort and it is a means that is used very largely in certain classes of machinery. Figure 39 is a diagrammatic representation of a hydraulic press. It consists of a cylinder A in which is fitted a ram B. An L-shaped tube C connects with the cylinder and is fitted with a plunger D. The cylinder and tube are filled with water and then when the plunger is depressed the ram B has to rise, If the area of the plunger is one square inch and that of the ram thirty square inches, a 100 pounds pressure on the plunger will exert 3,000 pounds of lift on the ram. HYDRAULIC LEVERAGE However, we must remember that in mechanics, as in all walks of life, we cannot get “something for nothing.” If we multiply the pressure or force, we must pay for it in some way, otherwise we should be getting more work out of the press than we put in it, which is what the perpetual motion crank is ever trying to do. As the cross-sectional area of the plunger D is only 1/30th of that of the ram, the plunger must descend thirty inches to raise the ram one inch. We need not consider the difference in the head of water because it would not amount to more than a few ounces at most, nor need we consider frictional losses. The case is parallel to that of the lever. In fact, we may consider the hydraulic press as a fluid lever with the water in tube C as the effort arm and that in cylinder A as the weight arm. The two arms are here so proportioned that the power arm must move thirty times as far as the weight arm. The work put into the press is exactly balanced by that we get out of it. An effort of 100 pounds exerted through a distance of thirty inches is exactly balanced by the moving of 3,000 pounds through a distance of one inch.