their spectra of the more refrangible
rays becomes greater. The lowest stage of incandescence is the familiar
one of _red_ heat. As it gains intensity, the quicker vibrations come
in, and an optical balance of sensation is established at _white_ heat.
The final term of _blue_ heat, as we now know, is attained by the
photosphere. On this ground alone, then, of the large original
preponderance of blue light, we must raise our estimate of solar heat;
and actual measurements show the same upward tendency. Until quite
lately, Pouillet's figure of 1.7 calories per minute per square
centimetre of terrestrial surface, was the received value for the "solar
constant." Forbes had, it is true, got 2.85 from observations on the
Faulhorn in 1842;[743] but they failed to obtain the confidence they
merited. Pouillet's result was not definitely superseded until Violle,
from actinometrical measures at the summit and base of Mont Blanc in
1875, computed the intensity of solar radiation at 2.54,[744] and Crova,
about the same time, at Montpellier, showed it to be above two
calories.[745] Langley went higher still. Working out the results of the
Mount Whitney expedition, he was led to conclude atmospheric absorption
to be fully twice as effective as had hitherto been supposed. Scarcely
60 per cent., in fact, of those solar radiations which strike
perpendicularly through a seemingly translucent sky, were estimated to
attain the sea-level. The rest are reflected, dispersed, or absorbed.
This discovery involved a large addition to the original supply so
mercilessly cut down in transmission, and the solar constant rose at
once to three calories. Nor did the rise stop there. M. Savelieff
deduced for it a value of 3.47 from actinometrical observations made at
Kieff in 1890;[746] and Knut Angstrom, taking account of the arrestive
power of carbonic acid, inferred enormous atmospheric absorption, and a
solar constant of four calories.[747] In other words, the sun's heat
reaching the outskirts of our atmosphere is capable of doing without
cessation the work of an engine of four-horse power for each square yard
of the earth's surface. Thus, modern inquiries tend to render more and
more evident the vastness of the thermal stores contained in the great
central reservoir of our system, while bringing into fair agreement the
estimates of its probable temperature. This is in great measure due to
the acquisition of a workable formula by which to connect temp