64 THE PRACTICAL POULTRY KEEPER. 



absolutely needed fresh air ; that, as the embryos grew, the 

 eggs themselves did far more in imparting heat to the 

 machine ; and that to be putting in cold eggs amongst 

 ethers far advanced was most injurious to the total results. 

 Hence it was found preferable to provide drawers, by which 

 these different conditions could be preserved. 



Incubators worked by hot water are still made to some 

 extent ; but simple as this system was, the provision of gallons 

 of boiling water every twelve hours was found such a tax on 

 most householders that there was a demand on all sides 

 for supplementary apparatus. The further step was soon 

 taken of carrying circulating pipes from a small boiler into 

 the tank of the machine, and this is now the usual method 

 of working even " hydro "-incubators. Instead of with- 

 drawing from three to six gallons of water, to be replaced 

 by boiling water, every twelve hours, at the same periods 

 the lamp under the boiler is lit for a short time, so as to 

 convey more heat into the tank, the water in which is 

 never renewed, beyond filling up now and then the trifling 

 loss from evaporation. 



Finally, however, manufacturers and the public have 

 returned to the old system of employing the constant heat 

 of a lamp. This is carried out on either of two systems, 

 known as the "tank" and the a atmospheric " systems 

 respectively. In the tank system the hot-air from the lamp 

 is carried constantly through flues which traverse the tank, 

 the amount of heat being controlled by some form of regu- 

 lator. In some machines very large tanks are employed, 

 which, for the reasons already given, " steady " the heat 

 sufficiently, with a variety of more or less effective regu- 

 lators. But the same result may be obtained with smaller 

 tanks by a more sensitive and perfect regulator. Of these 

 one of the best is the capsule regulator invented by Mr. 

 Hearson, which since the expiry of the patent is more 



