72 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1908. 



A new aud greatly improved copy of the standard water-flow pyrbeliometer 

 was constructed in Wasliington by the observatory instrument maker, Mr. A. 

 Kramer, aud is being installed on Mount Wilson. 



The frequent observation of the " solar constant " during a period of years at 

 least equal to the sun-spot cycle was regarded by the late director, Mr. Langley, 

 as a research of great importance. Having proved by the expeditions of 1905 

 and 1906 that the variation of solar radiation is highly probable, and also that 

 numerous days suitable for " solar-constant " observations were found in the 

 months from May to November on Mount Wilson, it is now proposed to erect 

 on a small, well-isolated plat of ground leased from the Carnegie Institution a 

 fireproof observing shelter to be occupied by Smithsonian observers each 

 year during the months mentioned. This improvement of equipment there is 

 made necessary by the unsightly appearance and rapid deterioration of the 

 wooden sheds now occupied by the Smithsonian expedition and also by 

 the rapid development of electrical and other installation by the Carnegie solar 

 observatory too near the present location for the long continuance of good work. 



It is strongly hoped that the way may become clear for the continuance of 

 " solar-constant " observations in the months from November to May either by 

 the Smithsonian or other observers at a station equally as favorable for those 

 months as Mount Wilson in summer. 



Respectfully submitted. 



C. G. Abbot, Director, 



Mr. Charles D. Walcott, 



Secretary of the Smithsonian Institution. 



