Hutchins and Pearson — Air Radiation. 281 



M'Farlane's curve is dotted. It will be seen that there is 

 nothing shown by the above results from which increased 

 absorption at small excess temperatures can be argued. Con- 

 sidering the widely different methods by which these curves 

 were derived, their agreement is quite striking. We feel sure 

 then that we shall commit no appreciable error in effecting a 

 comparison of air and lampblack radiation if we assume that 

 the latter is un absorbed by columns of air of the depth used in 

 our experiments. In fact, difference in the manner of pre- 

 paration of the lampblack surfaces is known to cause greater 

 radiation differences than are shown here. 



Absorption of Air for its own Radiation. 



The radiation of air cannot be properly considered apart from 

 the question of the rate at which its own radiation is absorbed. 

 Very* obtains a change of deflection per foot of increase in 

 radiating depth of 8'3,' 10'0, 8*2, 10-4, -1% the depths being 1, 

 2, 3, 4, and 5 feet, respectively. Disregarding the rather dis- 

 cordant character of these figures, he rejects the negative change 

 for 5 feet entirely on what seems somewhat uncertain grounds. 

 We have sought to avoid the difficulty of absorption in the radi- 

 ating column itself by measuring the depth of the absorbing 

 layer from the center of a radiating column only 10 cm deep ; 

 hence if there be any outstanding error, it must be very small 

 and we believe negligible. 



A complete example of the method of proceeding is here 

 given. The long tube being in place and carefully covered 

 with asbestos steampipe covering, the current was turned on 

 in the heating box and at the end of half an hour the following 

 deflections were obtained : 





Table III. 



Den. 





55* 





52- 



Date, June 29 



54-5 





53- 



Air column, 491 cm 



55 5 





55-5 



Hot air temp. 142 



52- 





55- 



Room temp. 20*5 



56' 





Mean 54*33 Hot air exc. 121*5 



'-Atmospheric Radiation, p. 45. 



