34 



BULLETIN 420, U. S. DEPARTMENT OF AGEICULTUEE. 



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 18. 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 diflerent 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 mUk 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 heatuig coils ui the 

 bottom of the box, all of which gave imsatisfactory results, the ar- 

 rangement of apparatus shown ia figure 17 was used. With this 



