146 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
constant of radiation, and it has been an object of investigation 
for the last hundred years. 
As shown by Forbes, Radau, and notably by Langley, it is not 
possible by means of the pyrheliometer alone to estimate what the 
intensity of the solar radiation outside the atmosphere would be, 
unless the pyrheliometer itself could be raised by a bailoon or other- 
wise to the extreme limit of the atmosphere. This latter procedure 
having heretofore been impracticable,’ it was necessary to have 
recourse to measurements of the solar spectrum. The defect in 
pyrheliometer observations consists in this: That the several rays of 
the solar spectrum are unequally affected in passing through the 
earth’s atmosphere. Certain rays are almost completely removed 
in the higher levels of the atmosphere, so that we can by no means 
estimate the lossés, even upon the highest mountains, unless recourse: 
is had to determinations in the spectrum. 
About the year 1880 the late Dr. Langley invented the bolometer. 
This is an electrical thermometer of great sensitiveness. It com- 
prises two fine strips of platinum, each about one-half inch long, 
one two-hundred-and-fiftieth of an inch wide, and one two-thou- 
sandth of an inch thick. The strips are blackened on the front surface 
with smoke, or with platinum-black electrically deposited. These 
two strips, with two coils of resistance wire, form a Wheatstone’s 
bridge, so called. If one strip is warmed with respect to the other, 
and thereby its electrical resistance is increased, the effect is to cause 
a slight current of electricity to flow through a very sensitive galva- 
nometer. In ordinary practice one can detect with the bolometer 
differences of temperature of a millionth of a degree; and in the most 
refined construction, with every precaution taken to avoid disturbing 
influences, it has been possible to observe the hundred-millionth part 
of a degree change of temperature. 
With the bolometer, which in those days was an instrument of 
very uncertain behavior, and one requiring the most expert atten- 
“tion and great patience for its use, Langley observed the sun’s spec- 
trum in the famous expedition of 1881 to Mount Whitney, in southern 
California. Like early investigators who had used the pyrheliometer 
alone, he observed the increase of the intensity of the sun’s rays 
from early morning to noon, and their decrease of intensity from noon 
until late afternoon. This depends, as you will see, upon the fact 
that when the sun is low and near the horizon its rays shine obliquely 
through the atmosphere, so that their path in the air is very long, 
whereas at noon, when the sun is nearly overhead, the path in the 
air is comparatively much shorter. If one observes, ther®fore, the 
intensity of each of the spectrum rays at different altitudes of the sun, 
1 The author has recently devised apparatus which has recorded solar radiation successfully at enormous 
altitudes, The results confirm those given below. 
