34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



ing when I began the work at Allegheny. The light-spot should move 

 only by an impulse from the Sun, but, owing to extraneous causes, it 

 was at first frequently impossible to keep it upon the scale of the 

 galvanometer during so short a time as a single minute. The apparatus 

 now, however, operates so well that such drift and tremor is relatively 

 unknown, and the zero of the galvanometer is found almost unchanged 

 for weeks together " 



After discussing the methods of observing, the solar constant of 

 radiation, and giving a table of 25 values of it observed at Washington 

 in 1902, 1903, and 1904, Langley continues: 



" Looking at the general results, these seem then to indicate a 

 possibility that a rapid fall of solar radiation occurred about the 

 close of March," and that subsequently the radiation continued nearly 

 or quite 10 percent less than before. This, if certain, would be impor- 

 tant, and we may inquire what causes on the Sun could produce such 

 a change, and what effects might be expected to be produced on the 

 Earth if it occurred. 



" The writer showed nearly thirty years ago '" that the envelope of 

 the Sun profoundly influences by its absorption the radiation received 

 by the Earth. While the absorption in the solar envelope is not exactly 

 known, still so much is known that we may infer that if it were 

 absent for a moment the Earth would receive nearly double its present 

 amount of heat. If a variation of 10 percent in the transparency of 

 this envelope occurred, nearly 10 percent of change in the solar radia- 

 tion outside the Earth's atmosphere would follow. 



" If a fall of solar radiation did occur, there ought to have been 

 a similar change of terrestrial temperatures afterward, and we may 

 inquire how great this fall of temperature should be. 



" The Earth may be regarded as a body at a mean temperature of 

 290° absolute (i7°C.), maintained at approximately constant tem- 

 perature by a balance between solar radiation received and terrestrial 

 radiation emitted. It is here assumed that all sources of heat other 

 than the solar radiation are negligible, but if any or all of them are 

 not so, the effect of their presence will be to reduce the effect on 

 temperature of a fall in solar radiation. 



" Recent studies of German physicists have experimentally verified, 

 for the perfect radiator, Stefan's law that the emission of a heated 



' It is of interest to note that a marked increase of Sun spots occurred on 

 March 21. See Report of the Council, Monthly Notices of the Royal Astronomical 

 Society, 64, 357- 



" Comptes Rendus, 81, 436, Sept. 6, 1875. 



