REPORT OF THE SECEETAEY. 37 



Only a part of the collection can now be shown for lack of room, and 

 it is practically impossible to maintain the birds in a healthy condi- 

 tion when kept in such unsuitable quarters. 



The collections in the park were enriched during the year by the 

 addition of a number of East African animals, including five lions, 

 two cheetahs, a leopard, a Grant's gazelle, a wart hog, and several 

 smaller mammals and birds, which were the gift of Mr. W. N. Mc- 

 Millan, of Nairobi ; also a pair each of eland and Coke's hartebeest, 

 a Grant's zebra, a water buck, and a Lophiomys, which were secured 

 in the same region. These animals were of such interest and value 

 as to render it desirable to send the assistant superintendent of the 

 park to Africa to arrange for their safe transfer to Washington. 



ASTROPHYSICAL OBSERVATORY. 



The work of the Astrophysical Observatory during the year has 

 brought two important results: 



(1) The first result is the establishment of an absolute scale of 

 pyrheliometry within three parts in one thousand as the result of a 

 long series of experiments with various pyrheliometers. The estab- 

 lishment of this scale through Mr. Abbot's standard pyrheliometer 

 has been supplemented by the distribution abroad and at home of 

 several secondary pyrheliometers constructed through a grant from 

 the Hodgkins Fund. The constancy of the scale of these secondary 

 pyrheliometers has been established and it is desirable to compare 

 this scale with those in use elsewhere. It is hoped that finally all 

 pyrheliometric observations will be made on the same scale as that 

 used here. 



(2) The second result of the year's work is the agreement within 

 1 per cent of the " solar-constant " observations obtained by Mr. 

 Abbot at the Smithsonian Mount Whitney station in California at 

 an elevation of 14,500 feet with those obtained simultaneously at the 

 Mount Wilson station in California at an elevation of only 6,000 

 feet. This determination, in combination with the above-mentioned 

 establishment of an absolute scale of pyrheliometry, gives 1.925 calo- 

 ries per square centimeter per minute as a mean value, for the period 

 1905-1909, of the rate at which the earth receives heat from the sun 

 when at its mean distance. Determinations made with various forms 

 of apparatus show no systematic difference in this value of the " solar 

 constant." In 1905 this " constant," according to various authorities, 

 was stated at values ranging between 1.75 and 4 calories. 



It is improbable that observations would have been continued since 

 1902 on " solar-constant " work but for a suspected variability of the 

 radiation sent to us from the sun. The laws governing this varia- 

 bility are of extreme importance for utilitarian purposes apart from 



