GRAVITY CREAMING 151 



it is simply an ordinary four-gallon can, about 8 inches in diameter 

 and 20 inches deep. It has a glass on one side near the bottom or 

 near the top, which allows the reading of the thickness of_the 

 layer of cream. On each side of the glass is a graduated scale, 

 which gives the reading in inches. In case the cream is being 

 sold to a creamery, the hauler comes along, notes the depth of the 

 layer of cream, and records the number of inches of cream oppo- 

 site the patron's name. At the end of the month, or whenever 

 the time for payment comes, the money is apportioned according 

 to the number of inches of cream delivered by each of the patrons. 

 No test for fat is made. This is what is known as the " Cooley 

 system," and is used quite extensively in the East, especially in 

 Massachusetts. 



While cream usually arrives at the creamery in a fair condition, 

 there is the objection that the cream is always thin. It seldom 

 contains any more than 1 8 or 20 per cent of fat. 



No good explanation has yet been given why cream in a deep 

 layer of milk at 40 F. should rise more quickly and more com- 

 pletely than in a thin layer at a higher temperature. Arnold 1 

 seeks to explain it by saying: " Water is a better conductor of 

 heat than fat; hence when the temperature of the milk varies 

 either up or down, the water in the milk feels the effect of the 

 heat or cold sooner than the fat in the cream does. Therefore 

 the cream is always a little behind the water in swelling with heat 

 or shrinking with cold, thus diminishing the difference between 

 the specific gravity of the milk and cream when the temperature 

 is rising, and increasing it when the temperature is falling." 



This explanation is, according to Babcock, 2 not satisfactory. 

 He says: " Though it is true that water is a better conductor 

 of heat than fat, the small size of the fat-globules renders it 

 impossible that under any circumstances there can be more than a 

 small fraction of a degree of difference between the temperature 

 of the fat and that of the milk serum. Moreover, with the limits 

 of temperature practical for a creamery, (90 to 40 F.), the 

 coefficient of expansion of butter-fat is more than three times as 



1 American Dairying, p. 210. 



2 Wisconsin Experiment Station, Bull. 18, p. 24. 



