90 
ASTRONOMY: C. G. ABBOT 
Proc. N. a. S. 
which reflects the solar beam into the long focus telescope, so that the 
solar image drifts across the slit of the spectrobolometer in an east and 
west fashion. The question was, therefore, open for some time whether 
the distribution at right angles to this, that is, along the north and south 
diameter of the sun, would be substantially the same. In 1918, Mr. 
Aldrich investigated this matter by the aid of special mechanism associated 
with the coelostat and was able to show that if any difference in distribu- 
tion exists between the north and south and the east and west diameter 
of the solar image, respectively, it is too small to recognize by this method 
and, indeed, is less than one per cent. 
INVESTIGATIONS OF THK RELATIONS OF TBRRESTRIAI^ CUMATE TO RADIATION 
Survey of the Present State of the Field 
1. Kinds of Radiation Concerned. — ^The temperature of the world is 
maintained by the balance between the income of solar radiation and 
the outgo to space of terrestrial radiation. The former is principally 
included between the wave-lengths 0.3 and 3.0 microns. The latter is 
principally included between the wave-lengths 5.0 and 50 microns. Sub- 
stances which are transparent for the one may be opaque for the other, 
though in these long ranges of wave-lengths substances may pass through 
several minima or maxima of opacity and transmissibility. Apart from 
cloudiness, which is the principal factor, the radiation of the sun is princi- 
pally hindered in its passage to the surface of the earth by the water 
vapor of the terrestrial atmosphere and by the scattering of the mole- 
cules of the air and by the dust which the air carries. The losses of the 
direct solar beam by scattering from air molecules and dust are largely 
compensated for by the indirect solar radiation received from the whole 
hemisphere of the sky. On the other hand the rays lost from the direct 
beam by absorption in the water vapor of the earth's atmosphere, while 
they go to warm the atmosphere itself, are very differently applied to 
maintain the heat of the earth as a planet than they would be if they reached 
the earth's surface. 
2. Absorbents of Terrestrial Radiation. — Apart from cloudiness, which 
is the principal hindering factor, the outgoing radiation of the earth's 
surface to space is hindered mainly by water vapor, ozone, and carbon 
dioxide. Of these the principal obstruction is water vapor which absorbs 
powerfully over great ranges of spectrum. The other two absorbents 
are each confined in their absorbing regions to comparatively narrow ranges 
of spectrum, but the ozone absorption band, at about 10 microns, occurs 
in a region where water vapor absorbs scarcely anything while the carbon 
dioxide absorption band at about 14 microns occurs in a region where water 
vapor is also powerfully absorbing. The atmospheric proportion of carbon 
dioxide is sensibly constant, while water vapor and ozone are variable. 
Accordingly, while water vapor is certainly the most important of the three, 
