380 Prof. Tyndall's Remarks on the 



Water changes to steam in the boiler ; the steam is reconverted 

 into water in the condenser; the water and the steam are differ- 

 ent forms of the same thing. The quantity of steam which con- 

 denses is, moreover, neither more nor less than what the boiler 

 yields. So also the heat of the furnace passes into the visible 

 motion of the train ; this visible motion again transforms itself 

 into heat at the wheels and axles; heat and motion, like the 

 steam and water, are merely different forms of the same thing 

 (verschiedene Erscheinungsformen eines und desselben Objects). 

 And inasmuch as we have in the condenser neither more nor 

 less of steam than the boiler yielded, so in the wheels and axles 

 we have, as heat, all the motion, no more and no less, expended 

 by the furnace to give velocity to the train. 



Had Mayer paused at this illustration and not written another 

 word, he would have proved himself, as regards the conservation 

 of energy, in advance of every living man in 1842 *. But he went 

 further. Having thus given an outline of his principles, he con- 

 cludes his paper by what he calls a " practical deduction." To 

 express numerically the relations of force and motion in the case 

 of gravity, it is necessary, he says, to determine experimentally 

 the distance through which a body falls in a given time, say in 

 the first second of its fall. So likewise as regards the relation 

 between motion and heat, we must determine the quantity of 

 heat which corresponds to a certain quantity of motion. We 

 must, for example, determine how high a definite weight must be 

 raised above the earth's surface so that its fall to the earth shall 

 be equivalent to the warming of an equal weight of water 

 1° C. That such an equation is founded in nature, Mayer 

 regards as the resume of his paper. The term ' equivalent ' is 

 here used by Mayer for the first time, and his next act is to in- 

 dicate how it may be determined numerically, and to give the 

 result of the calculation. In making this calculation he employed 

 the specific heat of air, which was at that time accepted as the 

 most correct. Subsequently the refined experiments of M. 

 Regnault proved the specific heat of air to be somewhat less 

 than what the determinations prior to 1842 had made it. Sub- 

 stituting in Mayer's formula the corrected specific heat, we 

 obtain a number which is almost identical with the mean of all 

 the best determinations of Mr. Joule. 



* The relative ripeness of the ideas of Dr. Mayer and Mr. Joule 

 twenty years ago may be inferred from a comparison of the above 

 remarks published in May 1842, with the following passage first pub- 

 lished by Mr. Joule in December 1843 : — "I shall lose no time in repeat- 

 ing and extending these experiments, being satisfied that the grand agents 

 of nature are, by the Creator's fiat, indestructible, and that wherever 

 mechanical force is expended, an exact equivalent of heat is always 

 obtained." 



