520 Dr. J. R. Mayer on the Mechanical Equivalent of Heat. 



tained that by reversing the poles of a magnetic bar a quantity 

 of heat is produced proportional to the square of the magnetic 

 tension, — a fact which was also discovered by myself, though at 

 a later date. In particular, Joule has likewise demonstrated, by 

 means of numerous experiments, that the heat evolved by friction 

 under various circumstances stands in an unvarying proportion 

 to the amount of force expended. According to his most recent 

 experiments of this kind, he has fixed the mechanical equivalent 

 of heat at 423*. 



Joule has likewise investigated experimentally, in relation to 

 this question, the thermal behaviour of elastic fluids when 

 expanded, and has thereby confirmed the earlier results of other 

 physicists. 



The new subject soon began to excite the attention of learned 

 men; but inasmuch as both at home and abroad the subject has 

 been exclusively treated as a foreign discovery, I find myself 

 compelled to make the claims to which priority entitles me ; for 

 although the few investigations which I have given to the public, 

 and which have almost disappeared in the flood of communica- 

 tions which every day sends forth without leaving a trace behind, 

 prove, by the very form of their publication, that I am not one 

 who hankers after effect, it is not therefore to be assumed that I 

 am willing to be deprived of intellectual property which docu- 

 mentary evidence proves to be mine. 



By help of the mechanical equivalent of heat many problems 

 can be solved which, without it, could not be attacked at all : 

 among them, the calculation of the thermal effect of the falling 

 together of cosmical masses may be especially mentioned. It 

 will not be out of place to indicate here briefly a few results of 

 such calculations. 



The following is one problem of this kind. It is assumed 

 that a cosmical body enters the atmosphere of our earth with a 

 velocity of four geographical miles per second, and that, in con- 

 sequence of the resistance which it here encounters, it loses so 

 much of its vis viva of motion that its remaining velocity when 

 it again quits the atmosphere amounts to 3 miles ; the question 

 now arises, How great is the thermal effect which accompanies 

 this process ? 



A simple calculation, based upon the mechanical equivalent of 

 heat, shows that the quantity of heat required is about eight 

 times as great as the heat of combustion of a mass of coal of 

 equal weight with the body in question, 1 kilogramme of coal 



* That is, 1 thermal unit =423 kilogrammetres. 



