File:Solar radiation Langley 1903.png

English: "The first adequate determination of the character as well as amount of en:solar radiation was made by S. P. Langley in 1893 at Mount Whitney in California (14,000 ft.), with the bolometer, an exceedingly sensitive instrument which he invented, and which enabled him to feel his way thermally over the whole spectrum, noting all the chief Fraunhofer lines and bands, which were shown by sharp serrations, or more prolonged depressions of the curve which gave the emissions, and discovering the lines and bands of the invisible ultra-red portion. The bolograph thus obtained mus be cleared of the absorption of the earth's atmosphere, and that of the transmitting apparatus - a spectroscope and siderostat. The first in itself requires an elaborate study. The first essential is an elevated observatory; the next is a long series of bolographs taken at different times of the year and of the day, to examine the effect of interposing different thicknesses of air and its variation in transparency (chiefly due to water vapour). It is found that atmospheric absorption is generally greater in summer than in winter, a difference of 20% being found between March and August; morning hours show a rapid and often irregular increase of transparency, culminating shortly after noon, after which the diminution is slow and comparatively regular.

"The resulting allowances and conclusions are shown in fig. 12, taken from an article by Langley in the Astrophysical Journal (1903), xvii, 2. The integrated emission of energy is given by the area of the outer smoothed curve (4), and the conclusion from this one bolograph is that the "solar constant" is 2.54 calories. The meaning of this statement is that, arguing away the earth's atmosphere, which wastes about one-half what is received, a square cenimetre, exposed perpendicularly to the sun's rays, would receive sufficient energy per minute to raise 2.54 grams of water 1° C. Langley's general determination of the constant was greater than this - 3.0 to 3.5 calories; more recently C. G. Abbot at Mount Wilson, with instruments and methods in which Langley's experience is embodies, has reduced it greatly, having proved that one of Langley's corrections was erroneously applied. The results vary between 1.89 and 2.22, and the variation appears to be solar, not terrestrial. Taking the value at 2.1 the earth is therefore receiving energy at the rate of 1.47 kilowatts per square metre, or 1.70 horse-power per square yard."

(The above incorporates text from a publication now in the public domain: Chisholm, Hugh, ed (1911). "Sun". Encyclopædia Britannica (Eleventh ed.). Cambridge University Press.)