152 



CALORIFIC POWER OF FUELS. 



and in this is inserted the thermometer. The large mass of 

 the paraffin is acted on by the mean temperature, but is unin- 

 fluenced by any slight momentary changes which may occur. 

 A self-registering thermometer is very advantageous, but 

 readings at intervals of half an hour are sufficient ordinarily. 

 Of course the opening around the tube should be packed so 

 as to prevent all possible ingress of cold external air. 



JL 



FIG. 39. FLUE THERMOMETER. 



Occasionally mercury is used instead of paraffin. This 

 renders the average of the heat more exactly, perhaps, but 

 has the disadvantage of being much heavier and much more 

 expensive. There are also many difficulties in handling it 

 which do not obtain with paraffin. The paraffin should be 

 well refined, and have a high melting-point. 



THE PNEUMATIC PYROMETER. 



Uehling's pneumatic pyrometer is based on a principle 

 analogous to that of the gas-composimeter, and is now in use 

 in many places, automatically measuring the temperatures of 

 chimneys and furnaces for all temperatures up to 3000 F., 

 and registering the same on cards. The apparatus has been 

 tested at the Stevens Institute of Technology, and the 

 indications pronounced reliable. It cannot be safely used 



