330 THE MECHANICAL EQUIVALENT OF HEAT. 



his investigation by a common quantitative standard, their ab- 

 solute weights. Let us suppose, however, that we could de- 

 termine one body, for instance water," only by weight, and 

 another, water-forming or explosive gas, only by volume, and 

 that we had agreed to choose one pound as the unit of weight, 

 and one cubic foot as the unit of volume ; we should then have 

 to ascertain how many cubic feet of explosive gas could be ob- 

 tained from one pound of water, and conversely. This num- 

 ber, without which neither the formation nor the decomposi- 

 tion of water could be made the subject of calculation, might 

 then be suitably called " the explosive-gas equivalent of wa- 

 ter." 



In this latter sense a raised weight might, in accordance 

 with the known laws of mechanics, be called the " equiva- 

 lent " of the motion resulting from its fall. Now, in order to 

 compare these two objects, the raised and the moving weight, 

 which admit of no common measure, we require that. con- 

 stant number which is generally denoted by g. This number, 

 however, and the mechanical equivalent of heat, whereby the 

 relation subsisting between heat and motion is defined, belong 

 both of them to one and the same category of ideas. 



In the paper that I have mentioned it is further shown 

 how we may arrive at such a conception of force as admits 

 of being consistently followed to its consequences and is sci- 

 entifically tenable ; and the importance of this subject induces 

 me to return to it again here. 



The word " force" (Kraft) is used in the higher or scien- 

 tific mechanics in two distinct senses. 



I. On the one hand, it denotes every push or pull, every 

 effort of an inert body to change its state of rest or of mo- 

 tion ; and this effort, when it is considered alone and apart 

 from the result produced, is called " pushing force," " pulling 

 force," or shortly " force," and also, in order to distinguish 

 between this and the following conception, " dead force" (vis 

 mortua) . 



