COOLING HOT-BOTTLED PASTEURIZED MILK. 23 
was but little difference between the top and bottom layers of water 
until the 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 cooling 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 temperature 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 feet. 
Average speed of fan 1, 020 R . - P . M . . 
Average volts 98. 
Average ampere v 20. 
Total time of operation 148.5 minutes. 
