CARE OF THE EGGS. 287 



This systematic changing of trays and of the eggs 

 within the trays is absolutely necessary to secure the 

 best results both as regards the vigor of the chicks and 

 the per cent hatched. For, be it remembered, there is a 

 liability, and a very great liability as incubators go, of 

 decided differences in the temperature of the various 

 sides of the egg chamber compared with each other, 

 greater differences when they are compared with the 

 center, and still greater differences when the center is 

 compared with the corners, these last being the coolest 

 part of the machine. To hold the heat steady at the place 

 the bulb of the thermometer occupies, is a different thing 

 from holding it the same at all parts of the egg chamber. 

 The cracks at the door, if there has been shrinkage, 

 which is likely, considering the severe ordeal an in- 

 incubator door has to undergo, and the necessary open- 

 ings for ventilation, tend to make the air vary at differ- 

 ent trays and different parts of the same tray. But if 

 the maximum variation is no greater than between the 

 center of a sitting hen's nest under normal conditions ; 

 if the operator shifts the eggs as faithfully as the hen 

 does ; if the average temperature for twenty-one days 

 is the same in both cases, and if the eggs at the center of 

 machine, or at the warmest point, wherever that is, 

 never get too hot, then the incubator is all right so far 

 as heat is concerned. It may be run thus accurately, but 

 the chances are against it, and besides, in getting the 

 heat right, which is only one of the requisites, the mat- 

 ter of moisture is liable to be made all wrong, as will 

 appear when we treat the question of evaporation fur- 

 ther on. 



MOISTURE. 



An egg is composed largely of water, the white alone 

 being 78 per cent of water, and the whole egg originally 

 about 74 per cent, a considerable part of which evapo- 



