THE 



AMERICAN JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



Aet. I. — Observations on Invisible Heat- Spectra and the recog- 

 nition of hitherto .unmeasured Wave-lengths, made at the Alle- 

 gheny Observatory ;* by S. P. Langley, Allegheny, Pa. 

 With Plates I to IV. 



It is known to all, that the surface temperature of this 

 planet depends upon the properties of radiant heat and the 

 relation to them of the action of its atmosphere. It has been 

 usual to compare this action to that of the glass cover of a hot- 

 bed ; for glass, it is also well known, grows opaque to dark 

 heat, and continuously so, as its wave-length increases, thus 

 letting the solar light-heat pass freely through it to the soil, 

 while it is comparatively impervious to the dark heat returned 

 from the latter ; but this analogy must not be interpreted too 

 literally. Whether the atmosphere is pervious to the soil's 

 heat we do not here discuss, but it has of latef been shown that 

 the air does not behave otherwise like glass as it was supposed 

 to do, but except for the absorption bands, grows — not more 

 opaque — but more transmissible, to solar heat, up to its greatest 

 observed wave-length, and that hence our views of the nature 

 of the yet uncomprehended heat-storing action which maintains 

 organic life on the earth must be modified. The little that 

 the spectroscope tells us about the atmospheres of other planets, 

 leads us to think that we can best understand their relations 

 to solar energy by studying the atmosphere of our own ; for 

 our non-comprehension of these relations is largely due to our 



* Read before the American Association for the Advancement of Science, at 

 Ann Arbor. 



f See this Journal, March, 1883, Professional Papers of U. S. Signal Service. 

 No. 15, Expedition to Mount Whitney. 



Am. Jour. Sci.— Tuird Series, Vol. XXXI, No. 181.— Jan., 1886. 

 1 



