STEAM. 87 



the pipe which conveys the steam to the engine. A 

 safety-valve is also represented, which will be more 

 fully explained in another paragraph. 



209. ELASTIC FORCE OF STEAM. Under 



How great is . 



the elastic ordinary circumstances, the elastic force of 

 force of steam? steam is obviously equal to the elastic force 

 or pressure of the atmosphere. A man who rises from 

 a chair with a fifty-six pound weight on his shoulder, 

 must exert an extra muscular force, equivalent to fifty- 

 six pounds, in rising ; and he must continue to exert it 

 while he stands. So every bubble of steam must have 

 an elastic force equal to that of the air which it lifts, or 

 it cannot be formed under the pressure of the atmo- 

 sphere, or continue to exist when once formed. 



210. ELASTIC FORCE, HOW INCREASED. 

 As lon S as the vessel > in which steam is 



steam 'incrcas- made, is open, the pressure is as stated in 

 the last paragraph. But if the boiler be 

 closed steam-tight, and the heat continued, more steam 

 forms, and, crowding into the same space above the 

 water increases the pressure. In other words, the space 

 becomes filled with denser steam, of greater elastic 

 force ; and the force is finally sufficient to burst the 

 boiler, unless it can find some vent. 



211. INCREASED TEMPERATURE ACCOM- 



panies in PONIES INCREASED PRESSURE. - Steam of 



creased pres- high elastic force can only be made in a 



sure of steam ? 



close vessel. But in proportion to the 

 increase of elastic force, is the increase of pressure on 

 the surface of the water. Therefore, the boiling point 

 becomes higher and higher, or, in other words, the wa- 



