-1859] WRITINGS OF JOSEPH HENRY. 137 



the deduction of effects from complex relations. The des- 

 cent of a weight is chosen, because it is perhaps the most 

 familiar, and of the easiest conception and application. The 

 value of a fall of water is always estimated by the quantit)^ 

 of liquid multiplied by the height through which it descends. 

 If we multiply these together, and divide by 772, we shall 

 have the number of degrees of heat that this will impart to 

 a pound of water; and conversely, by knowing the number 

 of degrees of heat as measured by the number of pounds of 

 water raised one degree, we shall have the number of pounds 

 of water which can be elevated to a given height by a per- 

 fect machine; and when such effects are submitted to this 

 calculation, we find that the steam engine, in its most im- 

 proved form, is far from utilizing all the heat applied to it; 

 by far the greater portion is expended in the separation of 

 the atoms of water in radiation, in overcoming friction, and 

 in the production of vibration and useless motion. 



Mr. Joule also established the relations of equivalence 

 among the energies of chemical affinities of heat, of combi- 

 nation, or of combustion, of electrical currents in the galvanic 

 battery, and in electro-magnetic machines, and of all the 

 varied and interchangeable manifestations of caloric action 

 and mechanical force which accompanies them. A series of 

 experiments has also been made on the heat of animals, 

 which is found to be the equivalent of the chemical com- 

 bination of the food and the oxygen which they inhaled. 



The influence which investigations of this kind are to have 

 on the future history of mechanical arts and the production 

 of labor-saving machines, and on the increased power of man 

 in controlling the innate forces of matter, it is impossible to 

 estimate. 



" The food of animals is either vegetable, or animals fed on 

 vegetables, or ultimately vegetable after several removes. 

 Except mushrooms and other fungi, which can grow in the 

 dark, are nourished by organic food like animals, and like 

 them absorb oxygen and exhale carbonic acid, — all known 

 vegetables get the greater part of their substance (certainly 

 all their combustible matter) from the decomposition oi 

 carbonic acid and water absorbed bv them from the air and 



