400 THE POPULAR SCIENCE MONTHLY. 



would, consequently, find abundant employment in maintainino- a 

 difference of 189° * 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 aons of time to direct sun- 

 ghine. 



Among the most noteworthy results of Professor Langley's obser- 

 vations 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 rela- 

 tively great wave-length of 1,200 millionths of a millimetre. This, be 

 it noted, approaches rery closely to the theoretical limit set by Cauchy 

 to that end of the spectrum. The information was accordingly re- 

 ceived 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.f 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 circu- 

 lation, 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 



*SirJ. Herschel's estimate of the "temperature of space " was 239° Fahr. ; Pouil- 

 let'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, " Na- 

 ture," vol. xxi, p. 521. 



f 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 trans- 

 mitted. 



