It has already been shown that when pressure is applied to any part of a confined liquid, the pressure is transmitted equally in all directions. This law of Pascal is utilized to increase or multiply pressures. For example: If two pistons of unequal area are pressing upon the same liquid, held in connected tubes or cylinders, and weights are placed upon the pistons to keep them from moving up or down, it will be found that the weights must be proportional to the surfaces of the water if one piston is not to force out the other. This principle is applied in the construction of the hydraulic press. The hydraulic press is a machine used in mills and in boiler- and machine-shops for punching holes through plates, for exerting enormous pressure on paper, cotton, and cloth, for testing iron and wooden beams, and so on. It operates by creating a pressure over a small distance, by means of a lever and water.

The hydraulic press consists of two pistons of unequal area working in connected cylinders which are filled with water. When the small piston is raised, water rushes into the cylinders through a valve opening upwards. As soon as this piston is lowered, the valve closes. The small piston thus acts as a pump when water is forced from the small to the large cylinder, causing the large piston to rise slowly.

Usually the small piston is 1 in. in diameter, giving an area of .7854 sq. in. The large piston, called the ram, may be any size, depending upon the pressure required. The size of the cylinder is usually from 10 to 14 in. in diameter. The pressure per square inch is the same in both cylinders. As the flow of water is slow, and the distance is short, little or no pressure is lost in transmission. As the areas of the pistons are unequal, the total pressure must differ accordingly. To illustrate: If the diameter of the large piston is 10 in. and the diameter of the small piston 1 in., then the area of the large piston is 100 times that of the smaller, or 78.4 sq. in. (The areas of two circles are to each other as the squares of the diameters.)

Therefore, a pound pressure on the piston of the small cylinder gives a total pressure of 100 lbs. on the large cylinder. While the machine develops a certain amount of friction at the stuffing box, pins, etc., of the pump or smaller cylinder, the loss is probably only 5%. Therefore, as a general rule, 95% of the pressure applied to the smaller cylinder is given to or transmitted to the water in the pump. Figure 43 illustrates a hydraulic press designed to show a pressure up to 300 lbs. to the square inch. The handle of the pump which compresses the water in the small cylinder is seen on the left.