ON ARTIFICIAL REFRIGERATION. 951 



crating agent here, by abstraction of heat from the body to be cooled, 

 reassumes a gaseous or vaporous form. 



IV. A compressor, or jmmp, wliich (b-aws off from the refrigerator tlie 

 vapor or gas as it forms, and returns it to the boiler or heater. 1 prefer 

 to use for this purpose a double pump, analogous to the double engine, 

 and driven by the engine. The vapor is returned to the heater or boiler 

 against the pressure of the gas therein, and it is for this purpose that I 

 prefer the double pump, in which the compression is effected by steps 

 or successive stages, and not all at once. The vapor is compressed to a 

 certain extent in the first cylinder; it thence passes to the second cylin- 

 der, where it is still further compressed and forced back into the boiler. 

 The pump is provided with the usual pui^iiet or check valve, for prevent- 

 ing back -pressure; said valves opening only at the concluding part of 

 each revolution or stroke of their respective pistons. 



V. Between the pumps and the boiler, heat may be rejected in a sur- 

 face condenser in which water is the circulating medium, and the lique- 

 fied ammonia at the temperature of the surrounding water may then 

 be forced by an injector or by a special force-pump into the ammonia 

 heater. 



VI. By the use of a steam-engine aiding the compression and practi- 

 cally lifting the ammonia directly from the refrigerator to the ammonia 

 heater, no direct heat need be applied to the latter, and the amount 

 equivalent to the energy developed by the steam-engine takes its place. 



Subject strictly to thermodynamic law, a maxiuuim economy can by 

 these means be obtained for the abstraction of heat from surrounding 

 objects. 



It is evident, inasmuch as the heat abstracted from the agent to be 

 cooled is availed of, comparatively little additional heat is required to 

 bring the vapor or gas to the condition for imparting motion to the en- 

 gine piston. 



I propose to distinguish the apparatus in which this cycle of opera- 

 tions can be conducted by the name of " Thermo-glacial Engine." 



T.— DEY COLD AIE AS A PRESERVATIVE. 



We learn, from Turner's Embassy to Thibet,* that the flesh of ani- 

 mals is preserved frost-dried — not frozen — and it keeps without salt. 

 He says: "I had supplies of this prepared meat during all the time I 

 remained at Teshoo Loomboo, which had been cured in the preceding 

 winter. It was perfectly sweet, and I was accustomed to eat heartily of 

 it, without any further dressing, and at length grew fond of it. It had 

 not the appearance of being raw, but resembled in color that which has 

 been well boiled. It had been deprived of all ruddiness by the intense 

 cold." 



Frozen meat, unless losing actively in weight by evaporation owing 



* 4°, London, 1806, p. 301. Quoted by Samuel Parker iu liis Chemical Essays. Bohu, 

 1841. London. 



