SOLAR CONSTANT OP RADIATION ABBOT. 327 



average value of the " solar constant," but to observe its possible 

 variability. " In fact," said he, with a twinkle in his eye, " I may tell 

 you that I consider that value of the solar constant as best which 

 nearest approaches 3 calories." You will perhaps infer from this 

 that I had expressed to him the views regarding his Mount Whitney 

 result which I have given here this evening. He replied that the 

 Mount Whitney work was done in the prime of his life, and now 

 that he was old and had laid the subject aside for so long he did not 

 feel that he could reason upon it as acutely as he could have done 

 at that time, and therefore he would let his former value stand. 



In 1905, 1906, 1908, 1909, and now in 1910 I shall have occupied for 

 six months each year the Mount Wilson Smithsonian station, which 

 has now become the permanent cement structure which many of you 

 have visited. It is a pleasure to acknowledge the aid, inspiration, 

 and friendly companionship which I have had from Mr. Hale and 

 his staff. In this interval, partly with the aid of Messrs. Ingersoll 

 and Aldrich in different seasons, but much of the time alone (so much 

 are instrumental conditions superior now to those of Langley's early 

 work), I have made series of spectro-bolometric and pyrheliometric 

 observations for the determination of the " solar constant " on about 

 400 different days. During 1905 and 1906 Mr. Fowle was carrying 

 or nearly simultaneous, similar observations in Washington when- 

 ever conditions permitted. Although the direct readings at the two 

 stations differed by about 20 per cent on account of the relative con- 

 ditions at sea level and 1,800 meters elevation, yet our " solar con- 

 stant " results agreed within the experimental error at Washington ; 

 that is, within 3 per cent on the average. 



In 1908 I went to the summit of Mount Whitney with Prof. Camp- 

 bell, and we united to recommend the erection there of a permanent 

 shelter by the Smithsonian Institution. This was approved by Secre- 

 tary Walcott, and a three-room stone and steel house was built in 

 August, 1909, by aid of a Hodgkins grant, and is now available to 

 all students of science who shall receive permission of the secretary 

 to use it. In 1909 I returned to Mount Whitney with a spectro- 

 bolometric outfit, and made a complete determination of the " solar 

 constant " on September 3, simultaneously with a complete determina- 

 tion by Mr. Ingersoll here on Mount Wilson. My result differed from 

 his by less than 1 per cent, although my direct readings were, of 

 course, fully 15 per cent higher than his, on account of the higher 

 elevation of Mount Whitney, 4,120 meters. 



I have just returned from a third trip to Mount Whitney, on which 

 I had, fortunately, excellent weather during all the time when I 

 desired it. During my stay I made complete " solar constant " obser- 

 vations on four days simultaneous with those of Mr. Fowle here on 

 Mount Wilson; I observed with Prof. Kapteyn's photometer on two 



