1 82 The Planet Mars in 1873. [April, 



the hemisphere of Mars, which at the time of observation 

 is passing through its summer season, and readily interprets 

 the indistinctness of the other hemisphere as due to greater 

 prevalence of clouds during the Martial winter. He can even 

 recognise a difference in the colour of the planet as a whole, 

 as though at certain times there were a great increase or 

 diminution of the total quantity of cloud in the Martial air. 

 All these circumstances indicate a resemblance rather to 

 our earth than to the moon, where, as we know, there is 

 neither water nor any considerable atmosphere ; and when 

 we consider the physical relations involved by the circum- 

 stances thus far noted, we find much to suggest the idea 

 that Mars deserves to be regarded as a miniature of our 

 earth. It seems reasonable to infer that since the regions 

 where snow is constantly present, extend on Mars to lati- 

 tudes resembling those which limit our own regions of 

 perpetual snow, there must be a certain climatic resem- 

 blance between the two planets, notwithstanding the fact 

 that Mars receives so much less heat (on mile per mile of 

 surface) than the earth. If we remember that the mean 

 distance of Mars from the sun exceeds that of the earth in 

 the proportion of more than 15 to 10, so that the supply of 

 heat from the sun is less in the proportion of 100 to 225, we 

 cannot but be surprised to find that any resemblance of the 

 sort should exist. And yet, unless we adopt a view pre- 

 sently to be discussed, I apprehend that very little doubt 

 can exist upon the subject. For although, as has been well 

 pointed out by Prof. Tyndall, the presence of snow is an in- 

 dication of the action of heat, it is manifest that it must 

 indicate also the existence of cold, and that the relative 

 extent of the permanent snow regions of a planet must form 



skies of Mars are ordinarily clouded. But although the second explanation is 

 obviously in accordance with the whiteness near the edge of the disc (since 

 at the parts near the eastern edge day is breaking on Mars, while at the parts 

 near the western edge evening is approaching), the explanation must, never- 

 theless, be abandoned in presence of the facl: that near the terminator of 

 gibbous Mars there is a marked loss of brightness. For here if there were a 

 misty sky on Mars, the whitish light should be seen, and would compensate 

 for the greater obliquity of the sun's rays. As such light is not seen near the 

 terminator, the influence clearly is, that the morning and evening skies of 

 Mars are not specially cloudy, but that the white light seen near the edge of the 

 disc depends (according to the first explanation) on the obliquity with which 

 the line of sight falls on those parts. The illustration in my " Essays " shows 

 how this obliquity would result in causing the whole of the light here 

 received to be that reflected from clouds. I do not think any other explanation 

 is possible ; certainly I cannot conceive that any reliance can be placed on 

 the influence of Zollner, that Mars is covered over with hills having a mean 

 slope of 72 . The great point to be determined, however, is whether the edge 

 of the terminator does or does not show signs of evening or morning mists. 



