908 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



ject to a small correction, possibly amoimtiug to T^gtli, on account of the 

 "tbennometiic scale of error." Heat and mechanical energy being 

 mutually convertible, quantities of heat may be stated in foot-pounds 

 and quantities of work in thermal units. Taking steam, the total and 

 latent heat of evaporation in thermal units at 401° Fahr. is 1,204.2. 

 This nudtiphed by the mechanical equivalent of heat, say 774.1 gives 

 932,050 foot-pounds as expressing the total and latent heat of evapora- 

 tion at 401°. Moreover, a horse-power of 33,000 foot-pounds per minute is 

 equivalent to ^-f^ = 42.48 thermal units per minute, or 2,548 thermal 

 units per hour. In working examples it is sufificienily near the truth to 

 calculate 42.5 as the heat which disappears by work done, theoretically, 

 per indicated horse-power, or 2,550 thermal units per hour. 



The next Law of thermodynamics, the second, on which the art of 

 refrigeration is based, is that " heat cannot pass from a cold body to a hot 

 one hy a pure!y self-acting proccss.^^ Mr. Cotterill says on this point, " It 

 is easy to see what enormous consequences the denial of this principle 

 would involve in the theory of the steam-engine, for all the heat ex- 

 pended in the boiler which is not transformed into mechanical energy — 

 that is to say, at least five-sixths of the whole amount — appears in the 

 condenser being employed in heating the condensation-water, and if it 

 were possible by some self-acting contrivance to cause that heat to 

 flow from the condenser into the boiler, it is manifest that the said 

 five-sixths of the consumx)tion of heat might be saved. It is certain, 

 however, that this is impossible, but that to cause the heat to flow from 

 the condenser into the boiler we must have recourse to some artificial 

 process, which, hke working a heat-engine backward " (an ice-machine), 

 " involves in some way or other, directly or indirectly, the expenditure of 

 energy to as great or greater amount than we can recover by utilizing 

 that heat in the boiler ', and the second law of thermodynamics merely 

 amounts to a statement of this impossibility." 



After describing with some detail the principal forms of ice-machines 

 produced since Jacob Perkins's invention in 1834, and with the aid of the 

 foregoing data, I shall be in a better position to explain a new tyi)e of 

 ice-machine, the least complicated and most economical capable of con- 

 struction, and to which I have applied a name perhaps more explicit 

 than elegant in any classic sense, viz, Thermo-glacial Engine. 



Meanwhile it is necessary, for the complete understanding of the sub- 

 ject of artificial refrigeration, that some notice be taken of frigorific 

 mixtui^es and the laws which control the thermometric phenomena due 

 to the admixture of water or ice and certain salts. 



G.— ON CEYOGENS OR COLD-GENERATING SALTS. 



Prof. Frederick Guthrie, of the Science and Art Department, South 

 Kensington, London, has contributed some very valuable memoirs to the 

 Phj'sical Society on salt-solutions and attached water. Space precludes 



