HEAT. 79 



ed machines upon the former principle, which are said to 

 have given good experimental results. There is, however, 

 a theoretical difficulty in all these, not affecting their capabil- 

 ity of acting, but affecting the question of economy, which it 

 does not seem easy to escape from. Whether the heated air 

 or vapour be retained, or whether it yield its heat to a metal- 

 lic or other substance, this heat must exercise its usual repul- 

 sive force, and this must re-act either against the returning 

 piston or against the incoming vapour, and require a greater 

 pressure in that to neutralise it. Yapour raising a piston and 

 producing mechanical force effects this with decreasing power 

 in proportion as the piston is moved. At a certain point the 

 piston is arrested, or the stroke, as it is termed, is completed, 

 but there is still compressed vapour in the cylinder capable 

 of doing work, but so little that it is, and must in practice 

 be neglected ; if this compressed vapour be retained, the pis- 

 ton cannot be depressed without an extra force capable of 

 over coming the resistance of this, so to speak, semi-compress- 

 ed vapour, in addition to that which is requisite to produce the 

 normal work of the machine ; and in whatever way the resi- 

 dual force be retained, it must either be antagonised at a loss 

 of power for the initial force, or at most can only yield the 

 more feeble power which it would have originally given if it 

 had been allowed to act for a longer stroke on the piston. It 

 may be that a portion of this residual force may be econo- 

 mised ; indeed, this is done when the boiler is charged with 

 warm water from the condenser, instead of with cold water ; 

 but some, indeed a notable loss, seems inevitable. 



Without farther discussing the various inventions and the- 

 ories on this subject, which are daily receiving increased de- 

 velopment, it may be well to point out how far nature dis- 

 tances art in its present state. According to some careful es- 

 timates, the most economical of our furnaces consume from 

 ten to twenty times as much fuel to produce the same quantity 

 of heat as an animal produces ; and Matteucci found that. 



