1879] HEATING BY HOT WATER. 185 



the application of heat to cause circulation in the pipes. I venture to 

 say that it would make no difference whatever ■ the expansive pro- 

 perties of the water would remain the same. All bodies expand by 

 heit, and if it were possible to apply sufficient heat to liquefy bodies, 

 circulation would take place in the same manner. It is by motion 

 in the ultimate particles of matter in either case, whether solids or 

 liquids, by which heat is transmitted. Heat creates motion, and 

 motion creates heat. The difference is, that in solids heat is trans- 

 mitted by conduction only, but in liquids by both convection and con- 

 duction. 



Then your correspondent endeavours to show why the rise in the 

 flow hinders circulation. He says the reason for this is plain : as 

 s >on as the fire acts upon the boiler, the water in contact with its 

 inner surface bounds upward till it comes in contact with the inner 

 surface of the upper side of the pipe, where it parts with a portion of 

 its heat, and would now return, were it not still lighter than the body 

 of water at the bottom of the pipe which it has just passed unaffected; 

 therefore it has to continue its course on the top of the cold water 

 until it is reduced to the same temperature, when it returns from 

 whence it came by an opposite cold current at the bottom of the pipe 

 to the boiler, and descends through the hot water and takes its seat 

 at the bottom to be heated over again. He therefore asserts that we 

 have two currents of different degrees of temperature moving in op- 

 posite directions in the same pipe. If your correspondent's theory 

 is correct, we would have no use for a return-pipe at all — a flow 

 would be sufficient. Bat such is not the case : the water in the 

 bailer on being heated expands, consequently it then becomes lighter 

 than the water in the pipes, hence its tendency to rise. And as the 

 water from the flow which proceeds from the top of the boiler cannot 

 descend without mixing and equalising the temperature of both, 

 therefore the water from the return which is situated at the bottom 

 of the boiler rushes in and gets heated likewise, and continues to 

 expand and ascend the flow-pipe, equalising and forcing the cold 

 water before it. And that part of the structure which is situated 

 farthest from the boiler will be the hottest, the pipes being at the 

 highest elevation ; which goes far to prove that a continuous rise in 

 the pipes does not hinder circulation, but the reverse. 



C. M. 



[It is quite evident that this is a subject that requires to be discussed. 

 —Ed.] 



