ON THE MECHANICAL EQUIVALENT OF HEAT 405 



I shall simply refer to the few experiments which a priori seem to 

 possess the greatest value, and which have been made rather for the 

 determination of the quantity than for the illustration of a method, 

 and shall criticise them to the best of my ability, to find, if possible, the 

 cause of the great discrepancies. 



1. GENERAL REVIEW OF METHODS 



Whenever heat and mechanical energy are converted the one into 

 the other, we are able by measuring the amounts of each to obtain the 

 ratio. Every equation of thermodynamics proper is an equation 

 between mechanical energy and heat, and so should be able to give us 

 the mechanical equivalent. Besides this, we are able to measure a 

 certain amount of electrical energy in both mechanical and heat units, 

 and thus to also get the ratio. Chemical energy can be measured in 

 heat units, and can also be made to produce an electric current of known 

 mechanical energy. Indeed, we may sum up as follows the different 

 kinds of energy whose conversion into one another may furnish us with 

 the mechanical equivalent of heat.' And the problem in general would 

 be the ratio by which each kind of energy may be converted into each of 

 the others, or into mechanical or absolute units. 



a. Mechanical energy. 



6. Heat. 



c. Electrical energy. 



d. Magnetic energy. 



e. Gravitation energy. 



f. Radiant energy. 



g. Chemical energy. 

 h. Capillary energy. 



Of these different kinds of energy, only the first five can be measured 

 other than by their conversion into other forms of energy, although Sir 

 William Thomson, by the introduction of such terms as " cubic mile of 

 sunlight," has made some progress in the case of radiation. Hence for 

 these five only can the ratio be known. 



Mechanical energy is measured by the force multiplied by the dis- 

 tance through which the force acts, and also by the mass of a body multi- 

 plied by half the square of its velocity. Heat is usually referred to the 

 quantity required to raise a certain amount of water so many degrees, 

 though hitherto the temperature of the water and the reduction to the 

 air thermometer have been almost neglected. 



