COOLING HOT-BOTTLED PASTEURIZED MILK. 23 



was but little difference between the top and bottom layers of water 

 untn tbe temperature dropped to near the point of maximum density 

 of water. At 39.1° F. (3.9° C.) there was no difference between the 

 bottom and top layers of water; below this temperature the bottom 

 became the warmer, and the convection currents in the liquid were 

 reversed. 



The same general action took place in the quart bottle of milk; the 

 readings, however, were reversed at 60° F. (15.6° C), as indicated by 

 the vertical dotted line from the point where curve 7 crosses the line 

 of maximum density of water to the intersection of curve 5. Before 

 this point was reached the top layers of milk were the hotter; after- 

 wards the bottom ones showed the higher temperature. The maxi- 

 mum density of milk occurs just above the freezing point, which is 

 about 31° F. ( — 0.6° C); hence it might be expected that the top 

 would remain the warmer above that temperature and that there 

 would be no reversal in the readings of the thermocouples. This was 

 not the case, however, in these experiments. There are several fac- 

 tors which may have influenced and probably did affect the cooling, 

 among them being: 



1. The placing of the upper couple above the cream line. 



2. The rising of the cream to the top of the bottle. 



3. Variation in the specific heat at different temperatures. 



4. Increase in viscosity with decrease in temperature. 



5. Increase in cohesion with lowering temperature. 



6. The rapidity at which cooling took place. 



7. Convection currents in milk only and not in the cream. 



8. Separation of the constituents in the milk. 



9. Relative areas of parts of bottle exposed to the cold air. 



10. Relative positions of couples from top and bottom of bottles. 



COST OF COOLING BY MEANS OF COLD OUTSIDE AIR. 



To determine the cost of coohng by using cold outside air, the read- 

 ings which appear below are given. These data were obtained. from 

 an experiment in which air at about 40° F. (4.4° C.) was blown down 

 through the crates of bottles. The temperatm"e curves are shown in 

 figure 12. 



Dry-bulb temperature 45° F. (7.2° C). 



Wet-bulb temperature , . . 44. 6° F. (6.99° C). 



Difference 4° F. (0. 23° C). 



Barometer 29. 85. 



Weight of air per cubic foot 0.07837 pound. 



Velocity head 0.40 of an inch of water. 



Average temperature of outgoing air 58.1° F. (14.5° C). 



Average temperature of incoming air 39.6° F. (4.2° C). 



Cross-sectional area of pipe 1.07 square feot. 



Average speed of fan 1,020 R. P. M. 



Average volts 98. 



Average ampere 20. 



Total time of operation 148.5 minutes. 



