Preservation of Animal Food. 51 



Since the volume of air gas at 32° increases by the 490th 

 part of its bulk for every rise in temperature equal to 1°, it fol- 

 lows that the volume of a cubic foot of air at 80° will be to 

 that at 32° as 490 + 48 is to 490, and that the volume of a 

 cubic foot of air at 32° will be to that at — 40° as 490 is to 

 490 — 72. Therefore a cubic foot of air at 80° will, by being 

 reduced in temperature to — 40°, occupy a volume of only 

 *776 of a foot. Assuming that the capacity of the pump in 

 the machine previously described is one cubic foot, and that 

 it makes 120 strokes in a minute (the distance that the 

 piston moves in one direction being one stroke), the quantity 

 of air compressed in one hour will be 7,200 cubic feet, but 

 the volume to which it will expand is to 7,200 as "776 is to 

 1 ; therefore the 7,200 cubic feet of compressed air will, on 

 expanding, only occupy a space of 5587'2 feet. As the 

 quantity of heat abstracted from the air after compression is 

 equal to the heat that would have to be communicated to 

 the air after expansion, to make it assume its original 

 volume, it follows that the force exerted by the 5587'2 feet 

 of air (supposing such heat to be communicated) in expand- 

 ing to its original volume, would be an exact measure in 

 force of the cold produced. This force (assuming the atmo- 

 spheric pressure to be 15 lbs. on the square inch) would be 

 equivalent to lifting 3,483,648 lbs. one foot high, equal to 

 T76 horse-power nearly, the power, less that representative 

 of friction, necessary to drive the machine for one hour. 



Assuming that 12 J cubic feet of air at 80° weigh 1 lb., the 

 total weight of the air compressed would be 576 lbs. 

 As the specific heat of air is to that of water as .24 is to 1, 

 it follows that 576 lbs. of air at — 40° would equal 138-241bs. 

 of water at the same temperature. Taking 140° as the quan- 

 tity of latent heat to be abstracted from water at 32° to 

 convert it into ice, 138 - 24 lbs. of water at — 40° equal 

 71 lbs. of ice at 32°, which would give 1704 lbs. of ice as the 

 result of 24 hours' work with 1 76 horse-power. 



In this investigation I have supposed the air employed to 

 be perfectly dry, and therefore have not considered the 

 variations in volume that would attend its use in its 

 ordinary hygrometric condition. 



Air containing moisture occupies a larger volume than 

 when dry. The quantity of moisture with which any 

 quantity of air can be saturated, the atmospheric pressure 

 and temperature, and the pressure of the vapour of water 

 being given, can be easily determined, since the pressure of a 



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