MECHANICS AND USEFUL ARTS. 57 



is subtended at that point by the greatest double ordinate of the reflec- 

 tor, and having its principal focus coincident with that of the para- 

 boloid. This instrument should theoretically produce the most intense 

 light yet derived from any given flame, as it incloses and parallelizes 

 each ray of the whole sphere of light, so that none are lost by diver- 

 gence between the lips of the reflector, whereas at present not much 

 less than one half of the light is lost. This combination may also be 

 applied, mutatis mutandis, to the illumination of half of the horizon of 

 a fixed light, by means of a single light ; the only difference being the 

 substitution of two truncated parabolic conoids for the paraboloid, and 

 a refracting belt for the lens. 



ARTIFICIAL PRODUCTION OF ICE. 



THE Scientific American for Sept. 28 describes Dr. Gorrie's ma- 

 chine for the artificial production of ice.* He employs two force- 

 pumps, which are the principal parts of the machine. Into the pump 

 for the condensation of air, a smaller pump injects water in a fine 

 shower, while the air is condensing, which thus absorbs the heat of 

 the air that is given out in the act of compression. Between the con- 

 densing and expanding pumps there is an air-reservoir, which is of 

 considerable size, and made like a steam-boiler. This vessel is in- 

 tended to receive the condensed air and retard its passage, so as to 

 afford time for its effective cooling, and to act as a magazine of force 

 for working the expand ing-engine. The expanding force-pump is the 

 principal and most interesting feature of the whole, because it is the 

 agent in which the expansion of the air and the production of cold 

 first take place. All the other parts must be nicely adjusted in pro- 

 portion to this part, for the making of the ice economically. The ab- 

 sorption of the heat is accelerated by immersing this vessel in water, 

 and causing a jet of liquid to be thrown into its interior, as into the 

 condensing-pump. This liquid is not congealable, and is withdrawn 

 from a larger, though properly proportioned, quantity, contained in an 

 insulated cistern, into which, after performing its office of imparting 

 heat to. or, in other words, absorbing cold from, the expanding air, it 

 is returned through the eduction-valves of the engine. As the liquid 

 of this cistern has its heat diminished at every stroke of the engine, 

 by the abstraction of the jet at one temperature, and its return at a 

 lower, it is practically a reservoir of cold. Cold of an intensity of 

 even hundreds of degrees below the atmosphere may be obtained by this 

 process, but experiment shows that the temperature of the cistern most 

 favorable for the rapid production of ice is at about 10 F. The ex- 

 panded air partakes of the same temperature as the cistern, and there- 

 fore, at 10 F., leaves it charged with a high degree of cold, which 

 the economy of the scheme requires should not be wasted. Instead, 

 therefore, of being allowed to escape into the atmosphere, it is direct- 

 ed through an apparatus, made like a brewer's refrigeratory for 

 cooling worts, around which is placed the water it is intended to 



* Sae Annual of Scientific Discovery, ISoO. p. 76. 



