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



changed to suit the conditions of experiment by changing the 

 position of the controlling magnet. The period of the needle 

 has always been kept less than ten seconds. 



Reduction Factor. 



The instrumental reduction factor is the amount of heat in 

 small calories per second from a surface that fills the aperture 

 of the thermopile, necessary to give a deflection of one 

 division of the galvanometer scale. 



The reduction factor is always obtained one or more times 

 each day, and is considered as applying to that day's observa- 

 tion only. It is found by observing the deflection produced by 

 the warm copper tank, coated with lampblack. The amount 

 of heat radiated per degree of difference per sq. c. m. per second 

 for the difference of temperature between the tank and cube 

 is found from the equation x = 0-000238 + 3-06Xl0- 6 *5-2-6x 



10- 8 ^ 2 .* The instrumental constant c = — where a is the 



a 



galvanometer deflection per degree of difference in tempera- 

 ture between the cube and tank. 



Radiation of Air. 



Measures for finding the absolute radiating power of a 

 column of air one centimeter thick at a temperature near 100° 

 were made upon eight days. The results of each day's work 

 were fairly satisfactory, but there was no sort of agreement 

 between those of separate days. 



A preliminary experiment upon the effect of humidity, made 

 by measuring the radiation of the air in its ordinary state and 

 then again after letting steam into the room from the heating 

 pipes, showed that the radiation increases with the amount of 

 moisture in the air, confirming the result reached by Tyndall. 



The readings of the barometer, and of the wet and dry bulb 

 thermometers were thereafter recorded. But even after mak- 

 ing a very sensitive wet and dry bulb apparatus using two 

 thermometers reading to 0*01° and swinging the whole from 

 the ceiling, like a long pendulum, to the same level as the air 

 entrance to the heated tube, I was unable to make out the 

 exact connection between the heat radiated and the amount of 

 moisture contained in the atmosphere. The utmost care was 

 used and every precaution suggested by experience observed, 

 but after more than a month no result beyond what was given 

 in the preliminary experiments was reached. Notwithstand- 

 ing. Roy. Soc, 1872, p. 93. 



