Liv HB all 
VIII. On a Possible Variation of the Solar Radiation, andiits 
probable Effect on Terrestrial Temperatures. By 8. P. 
LANGLEY *, 
[Plates I. & IT.] 
Introduction. 
FFXHE purpose of the present communication is primarily 
to discuss the validity of a surmise we may entertain, 
founded on observations here, as to certain possible changes 
in the solar constant. With this there is discussed a possible 
falling off of solar radiation about the close of March 1903, 
in connexion with certain recent values of solar radiation 
computed from observations here, with a comparison of 
actually observed temperatures for eighty-nine stations of 
the North Temperate Zone. 
im 
Methods of Observation, and Considerations governing the 
Admission or Rejection of Experimental Data. 
The determination of the solar radiation toward the earth, 
as it might be measured outside the earth’s atmosphere, 
(called the “ solar constant’) would be a comparatively easy 
task, were it not for the almost insuperable difficulties intro- 
duced by the actual existence of such an atmosphere, above 
which we cannot rise, though we may attempt to calculate 
what would be the result if we could. The determinations 
of the so-called solar constant, then, chiefly depend on 
various methods of eliminating the effects of the terrestrial 
atmospheric absorption, since only after the actual inter- 
position of this we receive the sun’s heat. 
These determinations depend principally on two classes of 
observations :— 
First, on getting as far above this atmosphere as possible, 
by the actual ascent of a lofty mountain, and by the observa- 
tion of the solar radiation there with a subsequent com- 
parison with that nearer its base This method possesses great 
advantages over any other, but presents obvious difficulties 
in the execution fF. 
The second principal class of observations, and the. only 
* Communicated by the Author. 
Reference may be made to the writer’s paper in the Astrophysical 
Journal for March 1903, and to Mr. C. G. Abbot’s paper in “ Smith 
sonian Miscellaneous Collections”? (Quarterly Issue), vol. xlv. pp. 74-83 
1903). 
+ For an example of it the reader is referred to the account of the 
writer’s expedition to Mt. Whitney, Professional Papers, U.S, Signal 
Service, No. xv. 
