would, consequently, find abundant employment in maintaining a difference of 189°[1] between the temperature of the mercury and that of its environment. What we may with perfect accuracy call the clothing function of our atmosphere is thus vividly brought home to us; for it protects the teeming surface of our planet against the cold of space exactly in the same way as, and much more effectually than, a lady's seal-skin mantle keeps her warm in frosty weather. That is to say, it impedes radiation. Or, again, to borrow another comparison, the gaseous envelop we breathe in (and chiefly the watery part of it) may be literally described as a "trap for sunbeams." It permits their entrance (exacting, it is true, a heavy toll), but almost totally bars their exit. It is now easy to understand why it is that on the airless moon no vapors rise to soften the hard shadow-outlines of craters or ridges throughout the fierce blaze of the long lunar day. In immediate contact with space (if we may be allowed the expression) water, should such a substance exist on our enigmatical satellite, must remain frozen, though exposed for endless roons of time to direct sunshine.
Among the most noteworthy results of Professor Langley's observations in the Sierra Nevada was the enormous extension give by them to the solar spectrum in the invisible region below the red. The first to make any detailed acquaintance with these obscure beams was Captain Abney, whose success in obtaining a substance—the so-called "blue bromide" of silver—sensitive to their chemical action enabled him to derive photographic impressions from rays possessing the relatively great wave-length of 1,200 millionths of a millimetre. This, be it noted, approaches very closely to the theoretical limit set by Cauchy to that end of the spectrum. The information was accordingly received with no small surprise that the bolometer showed entirely unmistakable heating effects from vibrations of the wave-length 2,800. The "dark continent" of the solar spectrum was thus demonstrated to cover an expanse nearly eight times that of the bright or visible part.[2] And in this newly discovered region lie three fifths of the entire energy received from the sun—three fifths of the vital force imparted to our planet for keeping its atmosphere and ocean in circulation, its streams rippling and running, its forests growing, its grain ripening. Throughout this wide range of vibrations the modifying power of our atmosphere is little felt. It is, indeed, interrupted by
- ↑ Sir J. Herschel's estimate of the "temperature of space" was 239° Fahr.; Pouillet's 224° Fahr. below zero. Both are almost certainly much too high. See Taylor, "Bulletin of the Philosophical Society of Washington," vol. ii, p. 73; and Croll, "Nature," vol. xxi, p. 521.
- ↑ This is true only of the "normal spectrum," formed by reflection from a "grating" on the principle of interference. In the spectrum produced by refraction, the red rays are huddled together by the distorting effect of the prism through which they are transmitted.
