96 PRESERVATION OF FOOD BY REFRIGERATION ; 
Having ascertained this, the rise in the temperature of 
the cold air due to the influx of heat through the walls can 
also be ascertained. Taking the average quantity of cold 
air entering the machine per hour at 50,000 cubic feet, and 
allowing 75 per cent. for effective work, which seems to be 
the most that can be expected, according to the experiments 
of Professor Colladon, of Geneva, the net amount of air 
entering the expansion cylinder, at a temperature of, say, 
50° Fah., will be 
37,500 sleet: 
As this 37,500 feet removes 8,928 units of heat per 
hour from the chamber, each cubic foot carries away 
8,928 + 37,500 = 
238 unit. 
Turning to table 3, in column 9g, under the head of zz7ts, 
will be found the weight of water thermally equal to 1 cubic 
foot of air. At a temperature of 50° F. this amounts to 
o'0185 lb., consequently °238 + -0185 = 12°9 will be the 
number of degrees Fah. that 1 cubic foot of air will be 
heated in absorbing °238 unit of heat. As the temperature 
of the room is 14° F. when heated by the flow through the 
walls, it will be correspondingly lower on entrance, or at 
1° Fah. 
Testing it another way, and taking the air as issuing 
from the expansion cylinder at a temperature of 1° Fah., 
by table 3 the weight of air at 1° is 604 grains per cubic 
foot, as compared with 545 grains at a temperature of 50°, 
consequently, 37,500 cubic feet at 50° F. will only occupy a 
space of 33,800 cubic feet at 1° (G04 2545 gg naa 
This 33,800 cubic feet of cold air absorbs 8,928 units of 
heat in neutralising the flow of air through the walls, and is 
consequently heated in a proportionate degree, or by 
8,928 + 33,800 = °264 unit per foot. 
By column 9g, in table 3, 1 cubic foot of -airal 1. seme 
thermally equal to 0:0205 lb. of water, and therefore when 
heated by ‘264 unit, its temperature would rise -264+°0205 
=12'9 degrees Fah., thus showing that if the insulation is 
practically equal to the amount as given in Box’s tables, 
the loss of 13° at a temperature of 1° Fah. will compensate 
the flow through the walls. 
