174 Prof. P. Lowell on a Method of Evaluating 



indraught along its edge, together with an absence of moisture 

 on the sunlit half of the planet. Dry winds of the sort blowing- 

 over a perpetual Sahara must be laden with dust, which Very's 

 investigation finds to be the chief cause of reflexion in our 

 own air. The high albedo of Venus thus stands accounted 

 for. 



Light round Venus. — A sidelight bearing on the albedo of 

 air comes from the prolongation of the crescent of Venus 

 when the planet passes in inferior conjunction before the Sun. 



It used to be thought that the fine circlet of light that then 

 crowns the disk was due to refraction in the Venusian air. 

 But in 1898 Russell at Princeton showed that it is rather 

 reflexion from that air than refraction through it which 

 reaches our eyes. Now that such should be the case follows 

 from the planet's albedo, if that albedo be of atmospheric and 

 not of nubial origin. This supports the conclusion reached 

 by the visual observations of Venus at Flagstaff. For refrac- 

 tion means transmission, and if the air of Venus reflects 

 90 per cent, of the incident light it can refract but 10 per cent, 

 at most. The light from it, therefore, must be reflected not 

 refracted light in the proportion of 9 to 1. The albedo, 

 Russell's observations, and the Flagstaff results, thus all 

 concur to the conclusion that Venus is not enveloped in 

 cloud. 



... Deduction as to amount of Martian Air. — Another outcome 

 of the consideration of albedoes is a means it gives us of 

 approximating to the density of the Martian air. Mars' 

 surface is chiefly Saharan, and dust, therefore, must be largely 

 present in its air. Now from the albedo of various rocks, of 

 forests, and of other superficies we may calculate the relative 

 quotas in the whole albedo of Mars of its surface and its air. 

 Five-eighths of its surface is desert and therefore of an albedo 

 of about 0*16, as its hue shows- three-eighths of a blue-green, 

 the colour of vegetation with an albedo of about 0*07 ; while 

 one-sixth is more or less permanently white, the white of the 

 polar caps. These would combine to give it an albedo of 0*13. 

 This, however, is illuminated by so much of the light as 

 penetrates the atmosphere only, about three-quarters of the 

 whole. Whence the apparent albedo of the surface must be 

 about 0*10. As the total albedo of the planet is 0'27, the 

 remaining 0*17 is the albedo of its air. 



Taking the density of the air as proportionate to its 

 brilliancy, which would seem to be something like the fact, 

 since the denser the air the more dust it would buoy up, we 

 have for the Martian air a density about 2/9 our own over 

 each square unit of surface. 



