MECHANICS OF LIQUIDS 71 



70. Hydraulic Press Machinery. 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 pres- 

 sure 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, de- 

 pending upon the pressure required. The size of the cylinder is usu- 

 ally 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 ac- 

 cordingly. 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 ito each other as the squares of the diameters.) 



