112 



A curious inference is pointed out, that an animal would be sen- 

 sibly less warm in going up-hill than in going down-hill, were the 

 breathing not greater in the former case than in the latter. 



The application of Cai-not's principle, and of Joule's discoveries re- 

 garding the heat of electrolysis and the calorific effects of magneto- 

 electricity, is pointed out ; according to which it appears nearly certain 

 that, when an animal works against resisting force, there is not a con- 

 version of heat into external mechanical effect, but the full tliei'mal 

 equivalent of the chemical forces is never produced ; in other words 

 that the animal body does not act as a thermo-dynamic engine ; and 

 very probable that the chemical forces produce the external mechani- 

 cal effects through electrical means. 



Certainty reoarding the means in the animal body by which ex- 

 ternal mechanical effects are produced from chemical forces acting 

 internally, cannot be arrived at without more experiment and ob- 

 servation than has yet been applied; but the relation of mechanical 

 equivalence, between the work done by the chemical forces, and the 

 final mechanical effects produced, whether solely heat, or partly 

 heat and partly resistance overcome, may be asserted with confi- 

 dence. Whatever be the nature of these means, consciousness 

 teaches every individual that they are, to some extent, subject to the 

 direction of his will. It appears, therefore, that animated creatures 

 have the power of immediately applying, to certain moving particles 

 of matter within their bodies, forces by which the motions of these 

 particles are directed to produce desired mechanical effects. 



On the Sources available to Man for the Production of 

 3Iechanical Effect. 



Men can obtain mechanical effect for their own purposes either 

 by working mechanically themselves, and directing other animals to 

 work for them, or by using natural heat, the gravitation of descend- 

 ing solid masses, the natural motions of water and air, and the heat, 

 or galvanic currents, or other mechanical effects produced by chemical 

 combination, but in no other way at present known. Hence the stores 

 from which mechanical effect may be drawn by man belong to one or 

 other of the following classes ; — 



I. The food of animals. 

 II. Natural heat. 



