68 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



diameters, carried by the framework of the piston, raised this valve 

 in proportion as the plate joining the rod of the piston to the train 

 advanced. 



The vacuum was made in the tube by air-pumps, composed of 

 four pump-barrels worked by a steam-engine. The dimensions of the 

 tube and the machines were calculated so as to give a velocity of one 

 kilometre per minute, supposing a train of fifty-four tons, -with 

 an exhaustion of one-third of an atmosphere. 



