MECHANICAL USES OF WATER AND AIR 133 



of any size. Hydraulic presses are commonly used to operate 

 barbers' chairs and some types of elevators and in manufac- 

 turing processes in which great force must be exerted, as in 

 baling cotton. 



151. The principle of the hydraulic press. In order to 

 understand the principle of the hydraulic press or other 

 appliauces in which the transmission of pressure by water is 



FIG. 67. The hydraulic press 



The diameter of B is 10 times that of A, and its area is therefore 100 times as great. 



A pressure of 1 gram exerted by A upon the water surface beneath it would be 



transmitted undiminished to each of the 100 equal areas at B, and might support 



a weight of 100 grams at B 



important, we must attempt to discover the laws of trans- 

 mission of pressure in liquids. We may illustrate this by 

 means of an aquarium. Suppose the aquarium is filled with 

 water and covered on top and that a pipe leads up through 

 the cover (fig. 68). If, now, we pour into the pipe enough 

 water to fill it to a depth of 1 inch, the total depth of water 

 will be 13 inches and not 12. Since one cubic inch of 

 water weighs 0.0362 pounds, the pressure on a single square 

 inch of the bottom of the aquarium will be 13 x 0.0362 

 pound. Since each square inch of the bottom is equally 



