COMBINATION OF PUMPS 333 



the desired result; as, for example, in pneumatic dispatch 

 tubes seen in department stores. These are employed in the 

 transportation of letters and small packages from building 

 to building or between parts of the same building. A pump 

 removes air from the part of the tube ahead of the package 

 and reduces the resistance, while a compression pump forces 

 air into the tube behind the package and drives it forward with 

 great speed. 



