A moisture gauge of the best make will show 

 that the humidity of the egg chamber fluctuates as 

 we have stated, under the said conditions. 



Wherever there is a circulation of air in the egg 

 chamber and there must be a circulation of fresh 

 air to hatch successfully there will be some varia- 

 tion of humidity, because the humidity of the out- 

 side atmosphere changes, and it is this outside air 

 which furnishes the fresh air for the egg chamber ; 

 but with the valve closed the variation is reduced 

 to a minimum degree. 



The modern hot air incubator has no use for the 

 big trap-door valve opening from the egg chamber 

 of most hot water machines, because the heat is 

 controlled in the heater or hot air reservoir before 

 it reaches the egg chamber ; but it has a circula- 

 tion of fresh air in the egg chamber by means of 

 bottom ventilators and small outlet at top. These 

 inlets and outlet are always open and always the 

 same, so that the variation is reduced to a mini- 

 mum, being about equal to the variation in the best 

 hot-water machines with its valve closed. 



The variation of humidity in the best hot- water 

 machine with closed valve and in the best hot-air 

 machine just as it always is, is from ten to twenty 

 times less than when a large valve is being opened 

 and closed from the egg chamber every now and 

 then, as described, and the causes for the minimum 

 variation which cannot be entirely overcome are 

 two, the variation in humidity of outside atmos- 

 phere and the variation of temperature of outside 

 air. 



67 



