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:% ‘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 Mount 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-violet 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. 
8. 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 
@ The Temperature of the Moon, p. 193. 
