THE STORY-BOOK OF SCIENCE 



supposes a part of the casing taken away to show 

 tin* interior. The flame of the furnace runs through 

 these pipes, themselves surrounded by water. By 

 this means the fire is made to circulate through the 

 very midst of the water, and so steam is obtained 

 very quickly. 



"Now look at the front of the locomotive. In A 

 is seen a short cylinder closed tightly, but repre- 

 sented in the picture with a part of the outside re- 



A modern Locomotive 



moved to show what is within. There are two of 

 these cylinders, one on the right, the other on the 

 left of the locomotive. Inside the cylinder is an 

 iron stopper called a piston. The steam from the 

 boiler enters the cylinder alternately in front of and 

 behind the piston. When the steam comes in front, 

 what is behind escapes freely into the air by an ori- 

 fice that opens of itself at the right moment. This 

 escaping steam ceases to press on the piston, since 

 it finds its prison open and that it can get out. We 

 do not try to force doors when other outlets are 

 open. So does steam act: the instant it can escape 

 freely, it ceases to push. The entering steam, on 

 the contrary, finds itself imprisoned. It pushes the 

 piston, therefore, with all its strength and drives it 

 to the other end of the cylinder. But then the roles 

 immediately change. The steam that hitherto has 



