2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. ']'] 



Professor Eddington, everywhere recognized as one of the foremost 

 astronomers of the world, prepared the article " Astronomy " for 

 the recent supplement of the Encyclopaedia Britannica. Although he 

 has perhaps made more use of our results in his researches than any- 

 body in the world except Clayton, he indicates in that article that 

 appreciable solar variability probably does not exist. Dr. Exner, a 

 leading meteorologist of central Europe, in a recent letter tells me 

 that he and his colleagues are unconvinced. Dr. Linke, in a recent 

 article, paints a vivid picture of the difficulties in measuring solar 

 radiation, and concludes that only an independent investigaton, en-« 

 tirely divorced from the Smithsonian Institution, if it should confirm 

 our results, would justify confidence in the variability of the sun. 

 The summary of astronomical progress for the year 1924, published 

 by the Royal Astronomical Society, mentions several papers adverse 

 to solar variation, and leads the reader to conclude that scientific 

 opinion generally, if not actually in opposition, is still doubtful of 

 solar variation. 



We are at the very great advantage compared to our critics that 

 we know all about the work. We are aware of a great many cir- 

 cumstances that disarm criticism, and promote belief. It will be 

 impossible to enumerate all of these here, but I hope to present so 

 strong a case as to fully justify the investigations of Mr. Clayton, 

 who has adopted solar variation as a working hypothesis and sought 

 to see what comes of it. He reports these studies in the next succeed- 

 ing paper of these Miscellaneous Collections. 



Before proceeding, let me state one illuminating consideration. 

 Some writers mention our data for the past 10 or 15 years as if all 

 were of equal value. Really, tO' speak in a figure, the Washington 

 data of 1902 to 1907 were Prehistoric. As for Mount Wilson results 

 of 1905 to 1908, inclusive, before the invention of the silver disk 

 pyrheliometer, or Fowle's method for estimating total atmospheric 

 humidity, and while we yet used a flint glass prism limiting our 

 spectrum at the H and K lines in the violet — this work is Ancient. 

 Excluding altogether July and August, 1912, the year of the eruption 

 of the Katmai volcano, all Mount Wilson work of 1909 to 1920 can 

 be classed as Medieval. We had then but one station, operating only 

 in summer. We obtained only one determination per day, subject 

 to error from changes of sky transparency and also to errors of 

 computing in the enormous multiplicity of computations used in 

 the reductions of results by Langley's fundamental method. The 

 period from January, 1919, to the present is of another order of 

 accuracy, and represents the Modern period. 



