66 CORRELATION OF PHYSICAL FORCES. 



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

 neglected ; if this compressed vapour be retained, the piston 

 cannot be depressed without an extra force capable of over- 

 coming the resistance of this, so to speak, semi-compressed 

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

 normal work of the machine ; and in whatever way the 

 residual 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 

 economised ; 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 

 theories on this subject, which are daily receiving increased 

 development, it may be well to point out how far nature 

 distances art in its present state. According to some careful 

 estimates, 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, from a given consumption of zinc in a voltaic battery, 

 a far greater mechanical effect could be produced by making 

 it act on the limbs of a recently-killed frog, notwithstanding 

 the manifold defects of such an arrangement and its inferiority 

 to the action of the living animal, than when the same battery 

 was made to produce mechanical power, by acting on an 

 electro-magnetic or other artificial motor apparatus. The 

 ratio in his experiments was nearly six to one. Thus in all 

 our artificial combinations we can but apply natural forces, 

 and with far inferior mechanism to that which is perceptible 

 in the economy of nature. 



Nature is made better by no mean ; 

 But nature makes that mean ; so o'er that art, 

 Which you say adds to nature, is an art 

 That nature makes. 



The term ' nature ' being, in fact, an abstract expression 

 for the changes and adaptations which have been brought 



