Composite Sa 



The first thing done was to show him that while 5.1 was the 

 correct average of the figures representing the tests of his 

 twenty-one cows, it was not a correct average test of the mixed 

 milk from all his cows, as he had not considered, in calculating 

 this average, the quantities of milk yielded by each cow; the 

 following illustration was used: 



Cow No. 1, yield 25 Ibs. of milk, test 3.6 per cent.=0.9 Ib.of butter fat. 

 Cow No. 2, yield 6 Ibs. of milk, test 5.0 per cent. =0.3 Ib. of butter fat. 



Total ....... 31 Ibs. 2)8.6 1.2 Ibs. 



4.3 per cent. 



The two cows gave 31 Ibs. of milk containing 1.2 Ibs. of fat; 

 the test of the mixed milk would therefore not be 4.3 per cent. 

 /3.65.0\ but 1.2X100 =3t g 7 per cent If the fat in th 



t per 



milk was calculated by the average figure 4.3 per cent., 1.33 Ibs. 

 of fat would be obtained, i. e., 0.13 Ib. more than the cows pro- 

 duced. 



In order to further demonstrate the actual composition of the 

 mixed milk of the twenty-one cows, the milk of each cow was 

 weighed and tested at each of the two niilkings of one day. The 

 weights and tests showed that the cows produced the following 

 total number of pounds of milk and of fat: 



Morning milking, 113.3 Ibs. of milk, containing 5.17 Ibs. of fat. 

 Night milking, 130 9 Ibs. of milk, containing 4.98 Ibs. of fat. 



The morning milk therefore contained 5 - 7X *:=:4.56 per cent. 



1 13 . 3 



of fat, and the night milk, 4 -^^=3.80 per cent, of fat. 



The sum of the morning and night milkings gave: milk, 244.2 

 Ibs., fat 10.15 Ibs. The mixed morning and night milk, there- 

 fore, contained 10 -^ 4 X * 00 = 4.1 per cent, of fat. This is the true 

 average test of the morning and night milkings of these twenty- 

 one cows, as found by weighing and testing separately the milk 

 of each cow at both milkings. 



The total milk was strained into a large can at the farm, both 

 in the morning and in the evening. A sample of the mixed milk 

 was in each case taken with a long-handled dipper as soon as 

 the milkings were finished. When the cans of milk were deliv- 



