VARIOUS INCUBATORS. 67 



vided round the egg-chamber ; the main supply of air 

 enters through the aperture D, passing through a coarse 

 fabric kept moist by dipping into the tray of water, c c. 



It is impossible to describe the many variations in pattern 

 of the " tank " type of machine. The majority have rather 

 larger tanks than the above machine, which so far is at 



Fig. 18. "Forester" Incubator. 



least on the side of safety. The steadiness given by a water- 

 tank enables a variety of regulators to be used besides the 

 capsule form. Some work by the expansion of mercury, 

 others by that of a large bubble of air, or a few drops of 

 ether, imprisoned behind mercury, in a glass tube of V-form. 

 Christy's thermostat consists of a bar of two metals wound 

 into a spiral, which untwists more or less with changes of 

 temperature ; even a plain metal tube bent into a curve 



