548 Proceedings of Royal Society of Edinburgh . 
spectrum. Finding the thermopile, which at that time was the 
most delicate instrument available for his object, far too sluggish 
in its indications, he devised the bolometer, an instrument which, 
to this day, has no superior, and only one equal, in such work. 
By its aid he pushed the investigation of the solar spectrum into 
previously unexplored regions in the infra-red radiations. 
A natural extension of that work led to the mapping, by 
Langley and Very, of the lunar energy spectrum. This was a 
work of immensely greater difficulty, because of the slight differ- 
ence between the temperatures of the source and of the surround- 
ings of the instrument, and also because of atmospheric absorption. 
The results led to the conclusion that the temperature of the 
moon’s surface is not much above 0° C. 
A further extension was made to terrestrial sources of radiation, 
the mapping being pushed more than twice as far into the long 
wave-length region as had been found possible with solar 
radiation. In the process, the dispersive power of rock-salt was 
carefully determined. 
In 1892 Langley immensely improved his bolometer by making 
it an automatic, self-registering instrument, and the investigation 
of solar radiation was pushed as far into the infra-red region as it 
had been carried with terrestrial sources. 
Another distinctively great piece of work was that on the effect 
of the earth’s atmosphere in absorbing solar radiation, and on the 
determination of the solar constant. The value found for the 
constant must be regarded as at least a good first approximation. 
After he became Director of the Smithsonian Institution, 
Langley founded the Smithsonian Astro-physical Observatory and 
arranged its work primarily for the purpose of determining the 
natural influences having a direct bearing on climate and life. A 
part of its work, not completed at the time of his death, dealt 
with the question whether or not the solar radiation was variable 
to an extent sufficient to affect the earth’s climate, and whether or 
not the effects were predictable. The results already indicate an 
affirmative answer to the former part of the question. 
Another line of work, in which the daring nature of Langley’s 
attack on unsolved problems is well exhibited, is that on the 
problem of aerial navigation. His papers on Experiments in 
