THE INCUBATOR BOOM. 303 



eggs by the body or feathers of the sitting hen that 

 could not be furnished by art, it might be impossible to 

 construct the incubator of the future satisfactorily. 

 But so far as is now known, not including the purely 

 mechanical affair of change of position, the only requi- 

 sites for hatching are heat, moisture and ventilation. 



