Composite Samples of Milk. 161 



4.73, and this is the correct average test of the mixed 

 milk made up of the five different lots. 



In the second case, the variations in both the weights 

 of the different lots of milk and their tests, are compara- 

 tively small, and both methods of calculation give there- 

 fore practically the same average test; but also in this 

 case, the correct average test is found by dividing the 

 total amount of fat by the total quantity of milk, making 

 4.22 per cent., instead of 4.20 per cent., which is the 

 aritmetical mean of the five tests. The quantities of 

 milk in the various lots do not enter into the calculation 

 of the latter. 1 



196. 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 results 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 a Scovell sampling 

 tube for taking the composite samples; the size of each 

 of the samples taken would then represent an exact ali- 

 quot portion of the various lots of milk (180). 



197. A patron's dilemma. The following incident will further 

 explain the difficulties met in calculating the average tests of 

 different lots of milk. 



i In the experiment given on p. 137, the arithmetical mean of the tests 

 given is 5.15 per cent., while the true average fat content of the milk is 4.85 

 per cent. 

 11 



