30 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
makes it necessary to maintain these roads on a better basis than 
would be required if they were intended solely as entrances to the 
Zoological Park. The heavy expense which this involves falls upon 
the appropriation for the park, a fact which, it is felt, may not have 
been fully realized by the Congress in considering the park estimates. 
Attention has heretofore been called to the importance of acquiring 
the narrow tracts of land lying between the park boundaries and the 
recently established highways on the southeast and west. The high- 
ways were located as close to the park as the topography would per- 
mit, so as to reduce these tracts to a minimum width, with the 
expectation that they would be acquired by the Government. Prop- 
erty in this vicinity is gradually increasing in value, and in the inter- 
est of economy the tracts should be secured now so that the park 
boundaries may be permanently established and guarded against 
injurious encroachment by adjacent grading. 
ASTROPHYSICAL OBSERVATORY. 
The work of the Astrophysical Observatory during the year con- 
sisted : 
(1) Of bolometric observations carried on at Washington on the 
brightness of different parts of the sun’s image; also some experi- 
mental work on the transparency of the air for long-wave rays, such 
as the earth radiates. A computation of the results of these experi- 
ments is now far enough advanced to show their satisfactory quality. 
Precise knowledge of the selective absorption of our atmosphere for 
earth rays is still lacking, and contradictory views are still being 
expressed about this important subject; hence it is hoped that these 
experiments will be useful in the study of the dependence of the 
earth’s temperature on radiation. 
(2) Spectrobolometric measurements of the solar constant of radia- 
tion have been continued at the Mount Wilson observatory in Cali- 
fornia. As in former years, evidences of a fluctuation of solar radia- 
tion were found in the results of the measurements thus far obtained. 
A new and improved standard pyrheliometer was found to be more 
satisfactory than the one used in 1906, and great confidence is felt 
in the results obtained with it. Efforts have also been made to 
carry the bolometric measurements much farther in the ultra-violet 
through the use of a large quartz prism, a large ultra-violet glass 
prism, and two magnalium mirrors. Mr. Abbot, the director of the 
Astrophysical Observatory, visited the summit of Mount Whitney 
(14,502 feet), where the institution is preparing to erect a shelter 
house for the use of observers. This is the mountain upon which 
Mr. Langley carried on his well-known observations in 1881, and it 
is believed that the location will prove to be of great value in the 
further study of the solar constant of radiation. 
