34 
BULLETIN 420, U. S. DEPARTMENT OF AGRICULTURE. 
to 15° F. (5.6° to 8.3° C.) between the top and bottom portions of 
milk contained in the same bottle. This is shown graphically in 
figure 19, the curves in which were plotted from data obtained in the 
experiment shown in figure IS. The distances of the curves above 
the zero line are considered as positive, and those below as negative. 
The points where the curves cross the zero line indicate equal temper- 
atures at top and bottom of the bottle; hence the convective currents 
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Fig. 18. — Heating and cooling by forced air.— Variation in temperature of bottles at different positions in 
stack when heating and cooling. Hot air in at bottom when heating and cold air in at top when cooling. 
cease at these points. Above the zero line the topmost layers were the 
hottest, and below it the bottom ones were the hottest. It is of course 
very important that the milk be heated uniformly throughout the 
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Fig. 19.— Heating by forced air.— Variation in temperature between top and bottom layers of milk in same 
bottle. Same experiment as in fig. 18. 
stack as well as in individual bottles; therefore it is believed that this 
method of heating will not give the desired results. 
RESULTS OF HEATING FROM TOP DOWNWARD. 
After making several experiments with the heating coils in the 
bottom of the box, all of which gave unsatisfactory results, the ar- 
rangement of apparatus shown in figure 17 was used. With this 
