INCUBATING OVENS AND THERMO-REGULATORS. 91 
bath, and when the proper temperature is reached is closed so as to 
confine the air in the bulb. An increase of temperature now causes 
Fre. 61. : Fig, 62. 
Fig. 63. Fig. 64, 
and shuts off the gas flowing through the tube f at its lower ex- 
tremity, d. A small opening, e, permits sufficient gas to pass to 
