8 
BULLETIN" 420, U. S. DEPARTMENT OF AGBICULTUEE. 
90° F. (32.2° C.) in 6^ hours. A corresponding drop in temperature, 
however, was obtained with moving air in 2 hours and 15 minutes. 
The relative rate of cooling between the milk contained in the two 
cans was in the ratio of about 1 to 3, or, the milk contained in the 
can which was subjected to an air blast required only about one- 
third the time to drop in temperature from 147° to 90° F. (63.9° to 
32.2° C.) as did the can placed in still air. 
Under the foregoing conditions the relative rate of cooling, then, 
seems to be about the same in vessels of equal size and shape. The 
time required, however, for cooling through a given range, other 
things being equal, depends on the size of the containing vessel. In 
ISA 
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120 
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*: 
J 
T7A/C - HOURS. 
Fig. 4.— Cooling in still air compared with forced air.— Relative drop in temperature between quart bottles 
of milk cooled in still and moving air. Velocity of moving air 1,250 feet per minute. 
fact, it is believed that in commercial practice quart bottles are about 
the limiting size of containers that should be used. Our experiments 
with forced-air cooling, therefore, deal only with pint and quart bot- 
tles of hot milk or water. 
In the foregoing experiments with moving air the air was forced 
hi a horizontal direction over the containers. The direction of the 
air flow, when coolhig milk by forced circulation, affects materially 
the rate of cooling and also the variation in temperatures of bottles 
at different points in the stack. This feature will be discussed later. 
The foregoing experiments serve principally to show that the time 
required to cool milk to any great extent by natural circulation is 
