276 FARM MACHINERY 



common working pressures. The water capacity of a 

 tank will not be more in any case than two-thirds the 

 total capacity of the tank. As the water continually dis- 

 solves a certain amount of the air, or, rather, carries the 

 air out with it, it is necessary to supply air to the tank 

 from time to time. Pumps 'are now arranged with an 

 auxiliary air cylinder to supply this air. 



It is not advisable to pump air to pressure because it 

 is very slow work, as each cylinderful must be compressed 

 before any is forced into the tank. 



Air-pressure tanks must be very carefully made, as air 

 is very hard to contain, much more difficult than steam. 





