INCUBATING OVENS AND THERMO-REGULATORS. 



89 



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 



FIG. 61. FIG. 62. 



the air in the bulb to expand, the mercury in the U-tube is forced up 



a, ... 



.-d 



FIG. 63. 



FIG. 64. 



and shuts off the gas flowing through the tube / at its lower ex- 

 tremity, d. A small opening, e, permits sufficient gas to pass to 



