172 MAN'S PLACE IN THE UNIVERSE 



of the globe of the earth. The tidal deformation of the 

 earth thus grew to large amplitude, ending in a cata- 

 clysm which separated the great lump of material that 

 formed the moon. Other planets escaped this dangerous 

 coincidence of period, and their satellites separated by- 

 more normal development. If ever I meet a being who 

 has lived in another world, I shall feel very humble in 

 most respects, but I expect to be able to boast a little 

 about the moon. 



Mars is the only planet whose solid surface can be 

 seen and studied; and it tempts us to consider the possi- 

 bility of life in more detail. Its smaller size leads to 

 considerably different conditions; but the two essentials, 

 air and water, are both present though scanty. The 

 Martian atmosphere is thinner than our own but it is 

 perhaps adequate. It has been proved to contain oxy- 

 gen. There is no ocean; the surface markings repre- 

 sent, not sea and land, but red desert and darker ground 

 which is perhaps moist and fertile. A conspicuous fea- 

 ture is the white cap covering the pole which is clearly 

 a deposit of snow; it must be quite shallow since it melts 

 away completely in the summer. Photographs show 

 from time to time indubitable clouds which blot out 

 temporarily large areas of surface detail; clear weather, 

 however, is more usual. The air, if cloudless, is slightly 

 hazy. W. H. Wright has shown this very convincingly 

 by comparing photographs taken with light of dif- 

 ferent wave-lengths. Light of short wave-length is 

 much scattered by haze and accordingly the ordinary 

 photographs are disappointingly blurry. Much sharper 

 surface-detail is shown when visual yellow light is 

 employed (a yellow screen being commonly used to 

 adapt visual telescopes for photography) ; being of 

 longer wave-length the visual rays penetrate the haze 



