126 ELEMENTS OF GENERAL SCIENCE 



This system of weights and measures is so constructed that a 

 cubic centimeter of water weighs 1 gram. The pressure upon 

 any surface is therefore 1 gram per square centimeter for 

 each centimeter of depth. There are no fractions, such as are 

 encountered in calculating pressures by the English system, 

 and the operation may often be performed mentally. 



It will be well to remember the following data or to be sure 

 to know where to find them : 



1 cubic foot of water weighs 62.5 pounds. 

 1 cubic inch of water weighs 0.0362 pound. 

 1 cubic centimeter of water weighs 1 grain. 



143. Transmission of pressure in liquids. In a squirt gun 

 the pressure which is applied to the liquid by the aid of a 

 piston or a rubber bulb is transmitted to the liquid in the 

 nozzle and projects this liquid to some distance ; or the 

 pressure applied to the stopper of a bottle completely filled 

 with water may result in breaking the sides of the bottle. 

 In a city water system the pressure exerted by the pumps 

 at the pumping plant is transmitted throughout the system 

 and may be discovered in the pressure of the water at any 

 faucet. Many common appliances, such as water motors, lawn 

 sprinklers, and fire engines, depend, in their operation, upon 

 the transmission of pressure by water. 



The transmission of pressure by liquids is sometimes uti- 

 lized to secure the application of great force at a given place 

 by means of the hydraulic press. This machine consists es- 

 sentially of two communicating cylinders, each containing a 

 closely fitting piston, one of the cylinders being of larger 

 diameter than the other (fig. 67). If the diameters of the 

 cylinders are such that the area of the end of one piston is 

 ten times that of the other, it will be found that if a pressure 

 of 1 pound is applied to the water in the smaller cylinder by 

 means of the piston, the pressure exerted by the water against 

 the larger piston will be sufficient to support a weight of 



