INCUBATING OVENS AND THERMO-REGULATORS, 89 
the lower part of the instrument, and this, when immersed in water, 
acts as a thermometer bulb. This space contains mercury below 
and air or the vapor of ether above. When the air is expanded by 
heat the mercury is forced up the tube g until it meets the end of 
the inlet tube for gas at h, and by shuttihg off the flow of gas pre- 
vents the temperature from going any higher. A small opening in 
the inlet tube at e permits a small amount of gas to flow, so that the 
flame under the brood oven (Fig. 58, f) may not be entirely extin- 
guished. The lower end of the bent tube a is bevelled, so that a tri- 
angular opening is formed, which is closed gradually by the rising 
Fia. 58. 
mercury, instead of abruptly as would be the case if the lower end 
of the tube a were cut off square. To adjust the temperature in 
the air space of the incubating oven when the thermo-regulator is in 
position, a full flow of gas is admitted to the burner until the ther- 
mometer (Fig. 58, b) shows the desired temperature ; then the bent 
tube @ is pushed down through the cork until its lower extremity 
meets the mercury and the flame f is somewhat reduced. The ap- 
paratus is then left for a time, to see whether the flame runs too high 
or too low, and a further adjustment is made. When the changes 
in the exterior temperature are slight and the gas pressure regular 
the temperature in the air chamber is controlled with great precision. 
But this is not the case under the reverse conditions. Changes in 
