ESSAYS 



323 



upper part of AB is the net available portion, and the lower is non-available. 

 Then joining OA and OB — 



y = x tan a and y' = x tan /3. 



Foods of similar character must give figures of similar shape, though not 

 necessarily of like dimensions. Conversely, those which give similar figures may 

 be regarded as alike in character, though they may differ in concentration. For 

 comparison, several foods have been plotted on the same axes in fig. 2. For clear- 

 ness the lines OA and OB have not been drawn in each case, but the two directive 

 lines corresponding to angles of 15 and 25 respectively in the upper part of the 

 diagram, and one corresponding to an angle of 15 in the lower, should be sufficient 

 for the purpose. 



The net energy of the oil-seeds was reckoned, like that of the other foods, by 

 Armsby's method ; but as it is doubtful whether that method is applicable to 



A 



B 



Fig. 1. 



products which contain 40 or 50 per cent, of fat, it was recalculated by means of 

 the usual coefficients. This gives considerably higher results, and the difference 

 is indicated by the broken lines above the firm ones. 



It will be seen that for foods rich in oil the angle /3 is less than 15°, while for 

 Italian rye grass, meat meal, and pure starch, it is greater. The precise signifi- 

 cance of the fact is not apparent. It is tana, the ratio of net available to total 

 energy, that determines the character of the food. As this cannot, unfortunately, 

 be called the nutritive ratio, it may be referred to as the productive index. Swedes 

 and potatoes give very similar triangles, in which tan a is practically equal to that 

 of decorticated cotton cake, greater than that of pure starch or oil, but less than 

 that of molasses, maize meal, earthnut cake, or linseed cake. Roots are not in- 

 frequently described as coarse fodder, apparently for no other reason than because 

 they are cheap, whereas, judged by the method here described, they are among 

 the finest. Hay, if made without loss, should be very similar in character to the 

 fresh material from which it is made. In the diagram, fresh red clover and very 

 good red clover hay have the same productive index. Italian rye grass, timothy 



