REPORT OF THE SECRETARY. 65 



to —200° C. if that atmosphere did not possess the quality of selective absorp- 

 tion." A little later his experimental results on the temperature of the moon 

 led him to change this view, for he said : a "As between my observations and 

 my inferences, I hold to the former; and since later and long-continued ob- 

 servations * * * show that the temperature of the sunward surface of the 

 moon (which is certainly nearly airless) is almost certainly not greatly below 

 zero (centigrade), I have been led to believe myself mistaken in one of my 

 inferences drawn from former experiments." Precise knowledge of the select- 

 ive absorption of our atmosphere for earth rays is still lacking, although two 

 decades have elapsed since this was written, and contradictory views are still 

 being expressed about this very important subject by able writers. It is hoped 

 that Mr. Fowle's experiments will add much definite information, useful in the 

 study of the dependence of the earth's temperature on radiation. 



Computations of the results of Washington and Mount Wilson observations 

 have gone on steadily, but it has not been possible to keep the reductions 

 abreast with the numerous observations now being obtained. It has been con- 

 sidered desirable to make daily observations of the " solar constant of radia- 

 tion " during the observing season at Mount Wilson, and the reduction of each 

 day's observations requires several days of measurements and computations 

 at Washington. 



2. Work at Blount Wilson. 



Spectro-bolometric measurements of the " solar constant of radiation " were 

 continued by Mr. Abbot (with the assistance till September 20 of Mr. Aldrich) 

 on every favorable day until about November 20, 1908. The expedition was re- 

 newed late in the following spring by Mr. Abbot, and observations begun on 

 June 1, 1909. As in former years, evidences of a fluctuation of solar radiation 

 were found in. the results of the measurements of 1908 thus far obtained. 

 Various improvements in the modes of observing have been made, especially 

 in the bolometric measurements of the ultra-violent region of the spectrum, and 

 also in pyrheliometry. A new and improved standard pyrheliometer was tried 

 repeatedly on Mount Wilson. Its action is more satisfactory than the one used 

 in 1906, and great confidence is felt in the results obtained with it. Appar- 

 ently the results published on the provisional arbitrary scale of pyrheliometry 

 employed in Volume II of the Annals are several per cent higher than they 

 would be if expressed on the scale of the standard calory. On the other hand, 

 the results of the year indicate that a larger allowance of increase should have 

 been made for solar rays in the ultra-violent and extreme infra-red regions of 

 the spectrum not observed in 1905 and 1906 by the bolometer, and this increase 

 will probably nearly or quite compensate the change of scale in pyrheliometry, 

 leaving the mean " solar constant " value very near to 2 calories per square 

 centimeter per minute, as stated in Volume II of the Annals. Great efforts 

 have been made this past year to carry the bolometric measurements much 

 further in the ultra-violet. For this purpose a large quartz prism, a large 

 ultra-violet glass prism, and two magnalium mirrors have been procured and 

 are now in use on Mount Wilson, and daily observations are now carried as far 

 as wave-length 0.335/u. 



3. Mount Whitney Expeditions. 



In August, 1908, with Director Campbell, of the Lick Observatory, Mr. Abbot 

 spent about twenty -four hours on the summit of Mount Whitney (14,502 feet). 

 This mountain, which was the objective point of the famous expedition of Mr. 

 Langley in 1881, was recommended by him to be reserved by the Government 



a The Temperature of the Moon, p. 193. 



