C. C. Hutchins — Radiation of Atmospheric Air. 359 



for the entrance of air, and when the pipe is heated a strong 

 current sets through the tube and is discharged at the top. 



To cut off air currents from the outer surface of the pipe, 

 tin screens are made to surround it and move with it. The 

 upper opening of the pipe is furnished with a nozzle of lead, 

 presenting an opening l cm by 3 cm . By burning touch paper at 

 the bottom of the tube the lamps beneath being lighted, the 

 shape of the column of air from the nozzle can be inspected at 

 leisure by reason of the dense smoke that issues with it, and by 

 filing the throat of the nozzle it can be given such a shape 

 that the column of heated air will preserve uniform dimen- 

 sions for a considerable distance from its exit. After the 

 lamps have been lighted for twenty minutes the temperature 

 of the air becomes constant, and changes but slightly for hours 

 at a time. 



The lower end of the tube is supported upon a pin, thrust 

 across its diameter, so that while the bottom of the tube 

 remains fixed, the remainder can be moved in a vertical plane 

 by a pull upon a string, the tube falling back by its own weight 

 when the tension upon the string is relaxed. 



The thermopile points continuously to a large copper Leslie 

 cube containing water at the temperature of the room, and 

 furnished with a thermometer divided to 0'1°. When the 

 observer at the galvanometer releases the pipe it falls to such a 

 position that the hot air streams in front of the opening of the 

 thermopile at a distance of 3 cm causing a deflection of the 

 galvanometer needle after which the tube is raised by the 

 string attached, the heated air now being discharged high 

 above the opening leaving the apparatus unaffected save by 

 the radiation of the cube. 



The breadth of the column of air is sufficient to fill the 

 opening of the thermopile and leave a wide margin on either 

 side. For finding the instrumental reduction factor, a long 

 thin copper tank is provided which may contain water a few 

 degrees above the temperature of the room. This tank is 

 pushed between the stationary cube and the thermopile by an 

 assistant and withdrawn again at a signal from the observer at 

 the galvanometer. It is furnished with a delicate thermometer 

 which can be read to 0-01°. 



Heading Galvanometer. 



Both the forward and backward swings of the needle are 

 observed, and the mean of these constitutes a single observa- 

 tion, the mean of five or ten observations comprising a set. 

 The sensitiveness of the galvanometer has been frequently 



