LOWELL, — TEiMPERATUllE OF MARS. C57 



The albedo of various rocks and of the ocean is as follows : 



White quartzite . . .25 

 Clay shale 16 



For forest we may perhaps take 



and snow according to purity 



The percentages of distribution of surfaces being about 



Ocean • 72 per cent Steppes & Desert . 10 per cent 



Forest . 10 per cent Polar Caps ... 6 per cent, 



we deduce 1 1 for the albedo of the surface. But this being illumi- 

 nated by only 25 per cent of the light outside the air gives about 3 for 

 its quota to the planet's illumination. When finally the earth's whole 

 albedo to one viewing it from space becomes .74 + .3 = .77 albedo of 

 the earth for a clear sky. 



As the earth's is about 50 per cent cloud-covered (see the researches 

 of Teisserinc de Bord on Nebulosity) and the albedo of cloud is .72, we 

 get .75 for the mean albedo of the earth. 



Value of Loss of Light a Minimal One. 



That the value above found for the percentage transmission of solar 

 radiation to the earth's surface is a maximal rather than a minimal 

 amount, and the albedo a minimal rather than a maximal one, is hinted 

 by the fact that the higher the observer ascends above the surface the 

 greater his estimate of the solar constant becomes. Thus Langley in 

 his memoir on the Mt. Whitney expedition says : 



" In accordance with the results of previous observers, then, and of 

 our own with other instruments, we find a larger value for the Solar 

 Constant as we deduce it from observations through a smaller air 

 mass." The italics are his.'* 



Depletion by Water-vapor on Mars. 



We are now in position to estimate the heat actually received respec- 

 tively at the surfaces of Mars and the earth. The visual part of the 

 spectrum containing 32 per cent of the incident solar radiation gives 

 us its quota directly from the albedo, since the heat received = 1 

 albedo. The infra-red portion containing 65 per cent of the whole de- 

 pends upon the character of the air and of what it holds in suspension. 



* Researches on Solar Ileat, p. 68. 



VOL. XLII. — 42 



