20 
BULLETIN" 420, U. S. DEPARTMENT OF AGRICULTURE. 
to vary the difference in temperature between the top and bottom 
layers ; the faster the cooling the greater the difference, and vice versa. 
In addition to the wide variation in temperature between the top 
and bottom layers of milk when cooled by cold-air blast from the 
bottom of the stack, the whole time required to cool a given quantity 
is greater than if cooled by admitting the cold air at the top. This 
point is more fully discussed elsewhere, the principal cause being that 
when a liquid is cooled in volume the cooling is carried on principally 
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Fig. 13. — Cooling by forcing air from top downward. — Relative drop in temperature of bottles of milk at 
different positions in stack. 
by the convection current set up in the liquid itself and to a lesser 
extent by conduction. 
The results obtained by reversing the direction of the air through 
the stack are now given. The curve in figure 15 shows the results 
obtained by blowing cold air downward through the stack of crates. 
The supply of cold air was at an average initial temperature of 
39.6° F. (4.2° C), and the air velocity was 2,512 feet a minute. The 
air conditions in the two experiments were practically the same, 
