138 THE ROYAL SOCIETY OF CANADA 

Direct Solar Radiation.| 1-502 | 1-390 | 1-060 
MAS | 
Total Sky Radiation.. . “115 -114 -106 





The numbers given in the above table refer to the radiation ex- 
pressed in calories per sq. cm. per minute incident on a horizontal 
plane. It is found that the total sky radiation per unit area of a hori- 
zontal plane in some 7-8 per cent of the radiation on a plane exposed 
normally to the sun’s rays at the earth’s surface (sea level). This 
proportion remains very nearly constant for all zenith distances up to 
80°. It will be noticed from the above table that sky-radiation is a 
feature of considerable importance in the meteorology of northern lati- 
tudes when the altitude of the sun remaims small during the winter 
months. 
7. The analysis of the paper sketched out in the present abstract 
seems to support the view that at levels above Mt. Wilson molecular 
scattering is sufficient to completely account for attenuation of solar 
radiation as well as for the intensity and quality of sky-radiation. 
Even the effect of atmospheric dust can be taken into account in the 
formulae for absorption and scattering. Should future observations 
support the existence of the simple law expressed by equation (iii) 
connecting the coefficient of attenuation with the wave-length, we shall 
be able with considerable assurance to make use of the law to obtain the 
coefficients of absorption for very short of for very long wave-lengths 
where the direct method of calculation from high and low sun observa- 
tions leaves room for considerable uncertainly owing to the small in- 
tensities in the solar spectrum at these wave-lengths and owing to 
other difficulties of observation. Schuster! points out the extreme im- 
portance of determining accurately the form of the solar intensity curve 
over short-wave lengths, since the effect of a solar atmosphere in ab- 
sorbing and scattering radiation is to give rise to an intensity-curve 
.which does not resemble in any way that given by Planck’s formula 
over this part of the spectrum. 
The problem of absorption and scattering in the sun’s atmosphere 
taken in conjunction with effects of self-illumination gives rise to a 
problem analogous to that just considered for the earth’s atmosphere. 

1 Schuster, “Radiation Through a Foggy Atmosphere,’ Astrophysical Journal, 
XXI, January, 1905, p. 20. 
