LOWELL. — TEMPERATURE OF MARS. 665 



Flagstaff, 1896, and at Mexico in 1897, from which it appeared that 

 the planet's markings were not obscured by cloud, but seen, as it were, 

 through a veil, and which also showed the correctness of Schiaparelli's 

 deduction that Venus in all probability turned in perpetuity the same 

 face to the sun. That she did so was evident from the long-continued 

 observations at Flagstaff and Mexico. Now such a facing always of 

 one hemisphere sunward would cause convection currents upward in 

 the centre of the disk, and an 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 reflection in 

 our own air. The high albedo of Venus thus stands accounted for. 



Light around 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 reflection 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 propor- 

 tion of nine to one. 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 of approximating to the density of the Martian air. Mars 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 super- 

 ficies, 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 .16, as its hue shows three eighths of a 

 blue-green, the color of vegetation, with an albedo of about .7, while 

 one sixth is more or less permanently of a glistening white in the 



