On the Inter changeability of Heat and Mechanical Action. 51 



they will be reflected at that end. The phenomena which accom- 

 pany this process in alternating discharges appear to owe their ori- 

 gin to the interference of the entering and reflected waves. 



3. An electrical discharge travels with equal rapidity in wires 

 of equal lengthy without reference to the materials of which these 

 iv ires are made. 



VIII. On the Inter changeability of Heat and Mechanical Action. 

 By the Rev. J. M. Heath*. 



THE doctrine of the equivalence of heat and mechanical 

 action, and that of the conservation of energy, are the 

 expression of one and the same thing to those who believe that 

 heat is motion, and therefore itself a form of force or energy. 

 They both alike express this — that when the action of force, 

 continued through a space, results in motion or heat, or vice 

 versa, there has been a true conversion, a change of one form 

 into another, and that when no motion results there is no con- 

 version. The selfsame expression f/efe expresses indifferently the 

 pressure accumulated at any point in a fluid mass by the action 

 of all the particles situated upon a given line upon it, or the 

 accumulation of the same force upon a single particle which 

 should move through the line for which the integral is taken. 

 But the results are not identical. The forces which accelerated 

 the moving particle have done their work and are extinguished ; 

 they now exist only in the form of the motion they have created. 

 But the corresponding fluid pressures have done no work (if the 

 creation of motion is work), and have never become any thing- 

 else than the pressures they were at first. 



If we bear this in mind, the great problem of the science of 

 thermodynamics (how much out of a given gross amount of 

 force (P) applied as a load to the piston of a gas-chamber will 

 generate its mechanical equivalent in heat or motion) becomes 

 one of extreme simplicity. The force P divides itself into two 

 parts, p = the pressure of the gas below employed in neutrali- 

 zing the resistance opposed by the gas to motion, and P— p the 

 remainder, which is wholly effective in producing motion. The 

 separate functions which these two portions of the force respec- 

 tively discharge are given by the two equations 



f(P— p)dv= \H<mv* + Q and f (p—p)dv= const., 



where v is the volume of the gas. And it appears very obvious 

 that the first of these is the answer to the question, How much 

 of the whole force P is converted into motion or heat ? 



* Communicated by the Author. 

 E2 



