CH. VII.] 



THE FRUIT AND SEED. 



a-29 



entitled the " Tank System of communicating 

 Heat:" 



It is a law of fluids, that their hottest portions 

 rise to the surface of the containing vessel, and the 

 coldest portions as invariably subside to the lowest 

 suiface ; because heat makes them expand, and con- 

 sequently diminishes their specific gravity ; and the 

 abstraction of heat makes them contract, and, as con- 

 sequently, increases that gravity. When the boiler 

 A and the tank g are filled with water, as well as 

 their connecting pipes, and a fire is lighted in B, the 

 hottest portions rise to the top, pass up c, flow along 

 the surface of g, and getting cool, sink to its bottom, 

 and passing down d, enter A again at the lower part, 

 to be once more heated and pass through the same 



