PHYSICAL PARADOXES. 



frequently clashing against one another and 

 bounding against the inside of the walls of the 

 world-wide boiler which contained them. 



We are now able to understand how such a 

 light and apparently unsubstantial thing as 

 steam can exercise so heavy a pressure. It is the 

 effect of the incessant bombardment by the 

 swiftly-flying molecules. 



If a boy throws a stone against a barn-door 

 which is standing loosely open, he produces no 

 effect beyond damaging the paint. But if a 

 thousand boys each throw one stone per second, 

 and every stone hits the door on the outside, 

 this shower of stones, at the rate of a thousand 

 per second, will exercise a steady pressure on the 

 door which will easily close it. 



This is how the molecules of water in the 

 form of steam make a strong pressure upon 

 the inside of the boiler. They are individually 

 small, but they deal many million blows per 

 second upon every square inch of the boiler's 

 surface : thus the total pressure is great ; and, 

 as we know, the greater the heat that is, the 

 more violent the vibrations the swifter the 

 flight, and the harder the blows of the mole- 

 cules, the greater the pressure of the steam. 



2. Explosion or Collapse? 



A lecturer is demonstrating with a glass 

 flask steaming over a flame. Presently he pours 



'J42 



