122 SCIENCE OF THRESHING. 



generated at high pressure, into one end of a cylinder 

 fitted with a piston. The end into which the steam is 

 admitted is open to the entrance of the steam from 

 the boiler and the other end is open to the atmosphere 

 (in the simple engine), so that the piston may move 

 freely in that direction. The pressure of the steam 

 from the boiler drives the piston to the opened end 

 of the cylinder. When the piston has reached that 

 end of the cylinder, that end is put in communication 

 with the boiler and the other end is opened to the air, 

 and the piston returns, driving the used steam out 

 before it. When the piston is at that end of the 

 stroke, the operation is repeated and the piston is thus 

 made to move back and forth indefinitely. 



The piston moves to the right or the left because 

 the work doing properties of the steam behind the 

 piston are greater than are those of the expanded or 

 partially expanded steam, or exhaust steam. This 

 means that the vibrations of the molecules of steam 

 on the boiler or pressure side of the piston are greater 

 than those on the other, or exhaust side. The 

 molecules of the former are moving faster than the 

 molecules of the latter and thus the difference between 

 the energy of the former and that of the latter is just 

 equal to doing the work of moving the piston. 



It is necessary to change the to-and-fro motion of 

 the piston to a continuous rotary motion in one direc- 

 tion. The usual form of mechanism for accom- 

 plishing this is known to engine men as the crank 

 motion. 



