860 THE VARIABILITY OP COWS* MILK, L, 



The mean number of cows at each month of lactation up to and 

 indudmg the seventh, is 12, and the maximum deviation from this 

 mean is 2. After the seventh month, the number of cows in milk 

 shows a sudden falling off. 



The average number of calves to which the cows at each month 

 of lactation had given birth is given in the third column, to the 

 nearest whole number. This number, for the hrst seven months, 

 varies between 4 and 5. The average values of the composition 

 and properties are thus quite comparable for these months, both 

 as regards the numbers of samples examined at each month, and 

 as regards the mean age or number of calves of the cows. After 

 the seventh month, the variation of the number of calves is from 

 3 to 5, and as the numbers of results examined at each month ai'e 

 only half those examined in the earlier period, the mean values of 

 the composition and properties of the milk secreted during the 

 eighth and later months are, perhaps, not so strictly comparable 

 with those of the milk secreted during the earlier periods of lac- 

 tation. Of the total number of samples examined, 86 were of the 

 milk obtained during the fu'st seven months of lactation, 16 were 

 of the milk of the eighth to eleventh months, and 2 were of the 

 milk of later periods. 



In the accompanying diagram (Text-fig. 6) the values of the 

 properties and composition given in Table ix., have been plotted 

 as ordinates, while the corresponding periods of lactation have 

 been plotted as abscissae. Smooth curves have been drawn through 

 the points obtained in this way. 



The scales of the ordinates are not the same for each curve. In 

 the case of the curves for percentages of total solids, fat, solids 

 not fat, protein, and sugar, the distance between adjacent hori- 

 zontal lines represents a variation of 2%. In the case of curve 

 for the percentages of ash, this distance represents a variation of 

 0-2%. For the other curves, this distance corresponds to the fol- 

 lowing variations: density, 0*02; freezing-point, 0-02°C.; con- 

 ductivity, 2-0xl0~- reciprocal ohms; weight of milk, 5*0 kg., 

 weight of fat, 200 gm. The scales of the ordinates are the same 

 as in Text-figure 5. 



