■REPORT OF THE SECRETARY. 23 



alteration of the transparenc}' of the air has not, however, l)een con- 

 fined to the region of Washington. 



Another interesting observation is that determinations of the i-ate of 

 solar radiation outside the earth's atmosphere might appear to indicate 

 thatther(> has been adeerease of the solar radiation itself since March ^6, 

 1003; Imt I refer to this with hesitation, as I have elsewhere observed 

 that it is scarcely possible to be ciu-tain of the accuracy of results of 

 this sort when based oii observations near sea level. The value of a 

 solar observatory at a high altitude, to which 1 referred last year, can 

 hardl}' ))e overestimated. 



A new determination of the temperature of the sun, based on the 

 distribution of the solar radiation in the spectrum, has yielded a result 

 of 5,J*2() of the centigrade scah^ al)ove absolute zero. 



For the purpose of the special study of the nature of sun spots, the 

 absorption of the solar gaseous envelope, and for other observations 

 requiring a large solar image an equipment including a horizontal 

 reflecting telescope of 1-tO-foot focus and 20-inch aperature and a 

 coelostat of improved construction to furnish at all times a 20-inch 

 horizontal northerly directed solar beam has been provided. Tlie 

 form of coelostat employed seems so well suited to solar work that 

 this large instrument will be exhibited by the Observatory at the 

 Louisiana Purchase Exposition in l!>0-f. Provision has been made in 

 connection with the long-focus telescope to churn the air traversed b}^ 

 the beam from the cotlostat to the focal image after the manner 

 described in my last year's report. It is hoped that this installation 

 will have 3'ielded results of interest before another year. 



On the whole the work of the Astrophysical Observatory during the 

 past 3'ear has l)een quite as pi-oductive of results of interest as during 

 any former year of its existence, especially in showing a notal)le varia- 

 tion of atmospheric transparenc}" which is likel}' to have aft'ected 

 climate and the growth of vegetation over a considera1)le part of the 

 earth's surface, and in the studies of atmospheric absorption and those 

 relating to the solar constant, to which 1 have referred, there seems 

 renewed promise of progress toward the goal ''foretelling l)v such 

 means those remoter changes of weather which aflect harvests,'' which is 

 one of the great aims had in y'ww in the foundation of the Observatory. 



Respectfully su))mitted. 



S. P. Lancley, 

 Secretary of (he Sunt It-son Ian Institution. 



