110 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 2 



thus depend. Were this trifling quantity of atmospheric ozone re- 

 moved, we should all perish. If it were ten times greater, we could 

 not live. Rickets would prevail universally. 



Other little-considered atmospheric gases of great importance to 

 our comfort are water vapor and carbonic acid, both minor constit- 

 uents of the air, quantitatively speaking. Without the water vapor, 

 our earth, like the moon, would cool far below freezing every night 

 within a few moments after the sun sinks below the horizon. Water 

 vapor hinders the incoming rays of the sun to the extent of 10 to 15 

 per cent, but it hinders the escape of the long-wave rays emitted by 

 the earth by over 70 per cent. Thus water vapor acts like the gar- 

 dener's glass cover of his hotbed. It lets sun rays in profusely, 

 but holds earth rays from escaping, and so is an eiiicient regulator 

 of climate. Carbonic-acid gas also acts in a somewhat similar way, 

 but less efficiently than water vapor. The indispensable function of 

 carbonic-acid gas in our atmosphere is to be the essential food for 

 plants. 



Other atmospheric constituents which greatly modify the income 

 and outgo of radiation for the earth's surface are clouds and dust. 

 The permanent gases, oxygen and nitrogen, alter the incoming sun 

 rays, it is true, but weaken them only slightly, and hardly alter the 

 outgoing earth rays at all. The molecules of the permanent atmos- 

 pheric gases, oxygen and nitrogen, scatter sun rays more and more 

 powerfully toward the violet end of the spectrum. But what is thus 

 lost to the direct solar beam comes to us almost wholly in the beauti- 

 ful blue rays of the sky. Dust also scatters sun rays, but without 

 much selection of color. A dusty slvy is therefore nearly white. 

 Sunset and sunrise owe their beautiful colors to the scattering of sun 

 rays by the atmosphere. When the sun is near the horizon, its rays 

 shine very obliquely through the atmosphere and therefore by 

 enormously long paths. The consequence is that the powerful tend- 

 ency to scattering of the blue and violet rays so far depletes that 

 part of the spectrum that the sky light which reaches us has a yel- 

 low or even a red tinge. S. P. Langley, third secretary of the 

 Smithsonian Institution, was a profoundly interested student of 

 solar radiation and atmospheric absorption. He wrote: 



If the observation of the amount of heat the suu sends the earth is among 

 the most important and difficult in astronomical physics, it may also be termed 

 the fundamental problem of meteorology, nearly all whose phenomena would 

 become predictable, if we knew both the original quantity and kind of this 

 heat; how it affects the constituents of the atmosphere on its passage earth- 

 ward ; how much of it reaches the soil ; how, through the aid of the atmosphere, 

 it maintains the surface temperature of this planet; and how, in diminished 

 quantity and altered kind, it is finally returned to outer space.' 



2 Langley, S. P., Report of the Mount Whitney Expedition, Prof. Pap. Signal Service, 

 No. 15, p. 11, 1884. 



