16 HEAT 



a small hole at a and partly up from the end C of the smaller tube. The 

 larger tube D is filled with mercury, which extends round to the lower 

 part of the bulb E, the upper part of which is filled with benzoline or 

 paraffin, or some more expansible, and, therefore, more sensitive liquid 

 than mercury. The tube A can be raised or lowered, and is so adjusted 

 that when the thermostat is at the desired temperature, the mercury 

 just reaches to the end C of the tube. If the temperature now rises, the 

 mercury seals up the end C, and the gas only passes through the small 

 sidehole a, and is just enough to keep the burner lighted. If the 

 temperature falls, the mercury allows a free passage to the gas through 

 the end C, and the supply of heat increases. 



