THE PLANET MARS 215 



the sun at least as much as falls upon the surface of our 

 earth. 



Now the average temperature of the surface of the earth is 

 about 6o° F. It seems probable that Mars should not be much 

 colder. No doubt the thinness of the atmosphere of the planet 

 will have a chilling effect but it seems certain that the con- 

 ditions are such that winter and summer, frost and thaw, as 

 well as vital changes like those which occur on our earth, 

 are to be expected. 



The low reflecting power of the planet itself is also evidence 

 that the atmosphere is scanty, though the strong whitish glare 

 on the limbs which there obliterates surface detail is clearly 

 seen in the middle of the disc and proves that there is an 

 atmosphere. 



I come now to speak in greater detail of the markings of 

 the disc and the changes they undergo. 



On Mars there are, roughly speaking, six different kinds of 

 markings — viz. : 



Greenish areas ; 



Ochreous areas ; 



White areas near the poles ; 



White areas which behave differently in the equatorial 



regions ; 

 A network of extremely fine lines called " canali " ; 

 Small round dark spots forming knots in the network of 

 the "canali." 

 After duly noting the changes in which all these features 

 share, I shall attempt to outline an hypothesis which will 

 consistently account for all of them simultaneously. 



The first features to be considered are the white patches 

 which, in their respective winters, are so conspicuous at the 

 poles. For many years it has been noticed that these polar 

 patches are smallest at the time when the summer heating of the 

 pole is greatest— a time corresponding to late July in the earth's 

 Northern Hemisphere— and that the maximum extent of white 

 occurs at midwinter. This in itself is an indication that the 

 material of which the caps are composed may be water. 

 As has been shown above, the temperature at the planet's 

 surface is such as to justify this view. I shall therefore assume 

 that the white patches consist of water and pass on to examine 

 other observations which have been made of their behaviour. 



