80 CORRELATION OF PHYSICAL FOKCE8. 



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 bat- 

 tery 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 arti- 

 ficial 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. 



A speculation has been thrown out by Mr. Thompson, 

 that, as a certain amount of heat results from mechanical ac- 

 tion, chemical action, &c., and this heat is radiated into space, 

 there must be a gradual diminution of temperature for the 

 earth, by which expenditure, however slow, being continuous, 

 it would ultimately be cooled to a degree incompatible with 

 the existence of animal and vegetable life in short, that the 

 earth and the planets of our system are parting with more 

 heat than they receive, and are therefore progressively cool- 

 ing. Geological researches support to some extent this view, 

 as they show that the climate of many portions of the terres- 

 trial surface was at remote periods hotter than at the present 

 time : the animals whose fossilised remains are found in an- 

 cient strata have their organism adapted to what we should 

 now term a hot climate. There are, however, so many cir- 

 cumstances of difficulty attending cosmical speculations, 

 that but little reliance can be placed upon the most profound. 

 "We know not the original source of terrestrial heat ; still 

 less that of the solar heat ; we know not whether or not sys- 



