abbot: solar constant of radiation 103 



intensity at JMountain Camp just quoted with a coefficient j) 

 which was certainly wrong. The argument on which Langley 

 acted may be stated in a plausible form as follows: If Bouguer's 

 exponential formula with the transmission coefficient obtained by 

 high and low sun observations at Lone Pine gives too low a value 

 of the intensity of homogeneous solar radiation for a station 

 within the atmosphere like Mountain Camp, as was shown by 

 actual observation, much more will it give too low a value out- 

 side the atmosphere. An equally plausible, and equally falla- 

 cious argument is the following : It is said that the density of 

 water decreases with increasing temperature at the mean rate 

 of about 0.00041 per degree from 0° to 100°, but observations 

 at 4° prove that water is actually denser at this temperature than 

 at 0°, therefore the supposed decreased density at 100° is a 

 delusion. 



Solar constant work of the Smithsonian Astrophysical Observa- 

 tory. The earlier years of the work of the Astrophysical Obser- 

 vatory were devoted to the improvement of the bolometer and 

 the use of it for the determination of the positions of lines in the 

 infra-red solar spectrum. About 1902 attention began to be de- 

 voted to measurements of the solar constant of radiation. We 

 approached these measurements with a very much better instru- 

 mental equipment than that which had been Langley's in the 

 IVIount WTiitney expedition of 1881. Soon after the Astrophysi- 

 cal Observatory was founded, about the year 1890, Langley 

 introduced the automatic registration of the galvanometer in 

 connection with the spectro-bolometer, and in the subsequent 

 years the difficulties connected with the use of the recording 

 spectro-bolometer were so far overcome that the solar spectrum 

 could be observed from the extreme ultra-violet end of the spec- 

 trum at about 0.3^ to a wave length of about Sfx in the infra-red 

 with great ease and accuracy, in an interval of seven minutes of 

 time. Drift of the galvanometer, which in Langley's expedition 

 to ]\Iount Whitney he has told me often amounted to a meter 

 a minute on the scale, was now so far reduced that a centimeter 

 an hour would be unusual. In fact the bolometer, despite its 

 great sensitiveness, is about as easy to use for this work as an 



