172 Testing Milk and Its Products. 



the arithmetical mean of the five tests. The quantities 

 of milk in the various lots do not enter into the calcula- 

 tion of the latter. 1 



198. The second example represents more nearly 

 than the first one the actual conditions met with at 

 creameries and cheese factories. As a rule, the mixed 

 milk from a herd of cows does not vary more in total 

 weight or tests, within a short period of time like one 

 to two weeks, than the figures given in this example. 

 On account of this fact, samples taken, for instance, 

 with a small dipper may give perfectly satisfactory re- 

 sults to all parties concerned. If the different lots of 

 milk varied in weight and test from day to day, as 

 shown in the first case, it would be necessary to use a 

 "milk thief" or one of the sampling tubes for taking 

 the composite samples; the size of each of the samples 

 taken would then represent an exact aliquot portion of 

 the various lots of milk (182). 



199. A patron's dilemma. The following incident will fur- 

 ther explain the difficulties met with in calculating the average 

 tests of different lots of milk. 



Thel weekly composite sample of the milk supplied by a cream- 

 ery patron from his herd of 21 cows tested 4.0 per cent. fat. 

 One day the farmer brought to the creamery a sample of the 

 morning's milk from each of his cows, and had thorn tested; 

 after adding the tests together and dividing the sum by 21, he 

 obtained an average figure of 5.1 per cent, of fat. From this 

 he concluded that the average test of the milk from his cows 

 ought to be 5.1, instead of 4.0, and naturally asked for an ei 

 planation. 



1 In the experiment Driven on p. 146, the arithmetical nn-jin of tin- 

 tests Is 5.15 per cent., while the true average fat content of milk is 

 4.85 per cent. 



