tlie Surface-Temperature of tdie Planets. 167 



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



White quartzite 0*25 



( lay shale 0*16 



Dark slate 0'0? 



Ocean 0'075 



For Forest we may perhaps take 0*07 



and Snow according to purity... 0*50 to 0*78 



The percentages of distribution of surface being- about : — 



Ocean 72 



Forest 10 



Steppes and Desert 10 



Polar caps (j 



We deduce *11 for the albedo of the surface. But this 

 being illuminated by only 20 per cent, of the light outside the 

 air, gives about '03 for its quota to the planet's illumination. 

 Whence finally the Earth's whole albedo to one viewing it 

 from space becomes '74 + *03 = 0*77. This is the albedo of 

 the Earth for a clear sky. 



< As the Earth is about 50 per cent, cloud-covered (see 

 the researches of Teisserinc de Bord on Kebulosity) and the 

 albedo of cloud is 0*72, we get finally for the albedo of the 

 Earth 0-75. 



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 : — 



M In accordance with the results of previous observers, 

 then, and of our own with other instruments,^ 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-vapour on Mars. — We are now in a 

 position to estimate the heat actually received respectively 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 invisible portion containing 

 (65 per cent, of the whole depends upon the character of the 

 air and of what it holds in suspension. The greater bulk of 

 the depletion in this part of the spectrum comes from the 

 'absorption by water-vapour, water itself or ice and carbon 



