382 Prof. Tyndall's Remarks on the 



"As explained already, this excess is employed to lift the weight 

 of 2160 lbs. ] foot high. Dividing 2160 by 2'8, we find that 

 the quantity of heat sufficient to raise 1 lb. of water 1° Fahr. in 

 temperature is competent to raise aweightof 771*4 lbs. a foot high. 



" This method of calculating the mechanical equivalent of heat 

 was followed by Dr. Mayer, a physician in Heilbronn, Germany, 

 in the spring of 1842. 



u Mayer's first paper contains merely an indication of the way 

 in which he had fouud the equivalent, but does not contain the 

 calculation. The paper was evidently a kind of preliminary note 

 from which date might be taken. In it were enunciated the con- 

 vertibility and indestructibility of force, and its author referred 

 to the mechanical equivalent of heat merely in illustration of his 

 principles. Had this first paper stood alone, Mayer's relation 

 to the dynamical theory of heat would be very different from 

 what it now is ; but in 1845 he published an ' Essay on Organic 

 Motion/ which, though exception might be taken to it here and 

 there, is, on the whole, a production of extraordinary merit. 

 This was followed in 1848 by an ( Essay on Celestial Dynamics/ 

 in which, with remarkable boldness, sagacity, and completeness, 

 he developed the meteoric theory of the sun. Taking him all in 

 all, the right of Mayer to stand, as a man of true genius, in the 

 front rank of the founders of the dynamical theory of heat, can- 

 not be disputed." 



For the sake of completeness, I will append here the resume 

 which I have given of the determinations of Mr. Joule. 



" On August 21, 1843, Mr. Joule communicated a paper to the 

 British Association, then meeting at Cork, and in the third part 

 of this paper* he describes a series of experiments on magneto- 

 electricity, executed with a view to determine the ' mechanical 

 value of the heat/ The results of this elaborate investigation 

 gave the following weights raised one foot high as equivalent to 

 the warming of 1 lb. of water 1° Fahr. : — 



1. 896 lbs. 5. 1026 lbs. 



2. 1001 „ 6. 587 „ 



3. 1040 „ 7. 742 „ 



4. 910 „ 8. 860 „ 



" In 1844 Mr. Joule deduced from experiments on the conden- 

 sation of air, the following equivalents to 1 lb. of water heated 

 1° Fahr. :— 



823 foot-pounds. 

 795 „ 

 820 



814 „ 

 760 „ 

 * Phil. Mag. 1843, vol. xxiii. p. 435. 



