Chemistry and Physics. 247 



the horizontal telescope has been examined with the spectro- 

 bolometer to determine the absorption of radiation within the 

 solar envelope itself. If we grant for argument's sake that the 

 rate of solar radiation outside our atmosphere fluctuates rapidly 

 from time to time, then as you have observed, the cause of this 

 fluctuation cannot reasonably be a variability of the temperature 

 of so immense a body as the sun itself, but must rather be in a 

 change in the absorption of a more or less opaque envelope sur- 

 rounding the sun. Accordingly the two researches I have men- 

 tioned are intimately associated, for if we And a considerable 

 increase in the rate of solar radiation outside our atmosphere we 

 ought to find a corresponding decrease in the absorbing power of 

 the solar envelope. 



Such is in fact one of the most notable results of the year's 

 work. In August, September, and October, 1903, the observa- 

 tions of the 'solar constant' of radiation indicated that the rate 

 of radiation was about 10 per cent below that observed in Feb- 

 ruary, 1904. On the other hand, measurements of the absorption 

 of the solar envelope indicated considerably less absorption in 

 February, 1904, than in September, 1903. 



The third kind of evidence of change in solar radiation is 

 based on a study of the temperature of the North Temperate 

 Zone, as indicated by the Internationale Dekadenberichte pub- 

 lished by the Kaiserliche Marine Deutsche Seewarte, and received 

 at the Observatory through the kindness of the Librarian of the 

 United States Weather Bureau. This publication gives the 

 mean temperature at 8 a. m. for each ten days at each one of 

 about one hundred stations distributed over the principal land 

 areas of the North Temperate Zone, and for about ninety of these 

 stations there is also given the normal temperatures for the same 

 ten-day periods, representing the mean of many years. From 

 these data there have been computed here the temperature depar- 

 tures irom the normal since January 1, 1903, and these are com- 

 pared graphically in the accompanying chart, Plate VII,* with 

 the measures of the solar constant made in 1903. It will be seen 

 that shortly after the observed fall of solar radiation in March, 

 1903, a general fall of temperature occurred, which would be a 

 natural result of such a change. It has been shown here, in 

 accordance with the known laws of radiation, that 10 per cent 

 fall in the solar radiation could not produce more than *l°'h C. fall 

 in terrestrial temperatures, and that several causes, notably the 

 presence of the oceans, would prevent so great a change of tem- 

 perature as this resulting from a temporary diminution of solar 

 radiation of only a few months' duration. The observed fall of 

 about 2°-5 C. in the mean temperature of the land areas of the 

 North Temperature Zone during April, 1903, seems to be there- 

 fore in good accord with the observations of solar radiation. 



Owing to the uncommon cloudiness of the first six months of 



* Shown also in your article on " A Possible Variation of the Solar Radia- 

 tion," Astrophysical Journal, June, 1904. 



