4 o6 THE POPULAR SCIENCE MONTHLY. 



alent of heat, in the year 1842, with the resources he had, and the 

 exactness which he attained, is one of the most marvelous exploits in 

 the whole history of science, is incomparably his greatest achievement, 

 and is sufficient alone to place him in advance of all the thinkers who 

 have devoted themselves to this great research. And we apprehend 

 that this would have long ago been conceded but for the rival claims 

 of Dr. Joule to this discovery. It is admitted on all hands, and even 

 by Mayer himself, that Joule's laboratory processes were necessary and 

 invaluable in completing the work, and placing this truth upon its firm 

 and experimental basis. With great patience and skill he worked out 

 the law of the mechanical equivalence of heat, as a demonstration that 

 all men can verify, and, by the award of the whole scientific world, 

 that law is permanently connected with his name. But Joule's results 

 were reached only in 1849, while Mayer had arrived at the same result 

 by other methods in 1842. What was it that both men were driving 

 at ? It was the working out of a great relation, or the establishment 

 of a universal truth of nature. . Mayer reached it, by using the data 

 that science had created for him. He got it first, he got it inde- 

 pendently, and he got it exactly, or within a small fraction of the 

 expression arrived at by Joule after six years of subsequent experi- 

 ment. Mayer was the pioneer, the revealer, the creator of the theory, 

 and Joule the verifier of his work. That verification was required and 

 has made the name of Joule immortal ; but who will compare 'it with 

 that master stroke of genius by which from scanty materials the great 

 truth was first independently seized and formulated? In 1849 Dr. 

 Joule fixed the exact mechanical equivalent of heat after many labo- 

 rious experiments, at 772 foot-pounds. Seven years previously Dr. 

 Mayer pursued a method which gave the mechanical equivalent of heat 

 as 771*4 foot-pounds. 



It was alleged by Thomson and Tait, as we have seen, that May- 

 er's method had been adopted by the Frenchman Seguin, three years 

 earlier, and that he anticipated the German in deducing the mechanical 

 equivalent of heat. Seguin, in 1839, published a work on the steam- 

 engine, in Paris ; and that work contains a table on the relations of 

 pressures, temperatures, and mechanical effects of steam, from which 

 it was alleged that the mechanical equivalent of heat may be inferred. 

 But the widest discrepancies existed among the interpretations of 

 these tables by different authorities. Upon a careful investigation of 

 the subject Professor Tyndall found that Seguin's and Mayer's numer- 

 ical results did not refer to the same things at all, and that Seguin's 

 tables did not attempt to give the mechanical equivalent of heat. 

 Professor Tyndall says : " It is only necessary, however, to read the 

 foregoing pages to see that Mayer and Seguin are speaking of two 

 totally different things ; that the degrees of the one are not the degrees 

 of the other; that the 'temperatures correspondantes ' of the latter, 

 which refer to his compressed steam, are not thermal units at all, and 



