EEPOET OP THE SECRETARY. 29 



own atmosphere that it can be done ahnost as well in Washington as at 

 a station more favored as regards atmospheric <^ransparency and free- 

 dom from clouds. The past year has not given evidence of very 

 marked variations either in the transparency of the sun's envelope 

 or in the supposedly dependent mean temperature of the earth, but 

 on the contrary the observations have continued most of the time near 

 the mean in both respects. Such changes as have been noted are not, 

 however, contradictory to the view that alterations of the transpar- 

 eiicy of the sun's envelope do occur, and cause changes in the amount 

 of solar radiation received by the earth, which in turn cause depar- 

 tures of the earth's temperature from its mean. 



The second line of investigation to which I have referred above is 

 the determination of the total solar radiation outside our atmosphere, 

 by observations with the bolometer and pyrheliometer at a station 

 located in a relatively clear and cloudless region and at a considerable 

 altitude above sea level. As long ago as February, 1902, at the re- 

 quest of the Hon. C. D. Walcott, and for the consideration of the 

 Carnegie Institution, I urged in a letter to him the great utility of an 

 observatory for solar research to be located at a high altitude and 

 charged with the determination of the question of the amount of solar 

 radiation and the limits of its variability. An observatory for solar 

 research has now, in fact, been established by the Carnegie Institution 

 on Mount Wilson, in southern California, after extensive tests of 

 different proposed sites. By invitation of the director. Prof. George 

 E. Hale, and in accord with the authorization of Congress for the 

 undertaking of observations at high altitudes by the Astrophysical 

 Observatory, I have sent to Mount Wilson an expedition in charge of 

 Mr. C. G. Abbot, for the purpose of determining the conditions for 

 studying the variability of the sun. 



The expedition is equipped with spectro-bolometric and pyrhelio- 

 metric apparatus of the highest qualit}^, and wholly adequate to 

 making the most accurate possible determinations of solar radiation 

 and its transmission through our atmosphere. As I have elsewhere 

 remarked, I am not convinced that it is possible to estimate exactly 

 the loss of radiation in our atmosphere by any observations whatever, 

 but it does seem that the estimates which can be made from the 

 observations of the Mount Wilson expedition wnll be so close an 

 approximation to the truth that if a notable variation of solar 

 radiation outside our atmosphere occurs the results will show it. 

 Furthermore, similar observations are being continued as usual in 

 AVashington. Mr. Abbot reports that the sky above Mount Wilson 

 is of great clearness and uniformity, and that weeks and even months 

 pass there without a cloud appearing above the horizon, so that 

 observations may be made almost every day with good prospects 



