THE PLANET MARS 221 



One of them is directly deducible and throws an interesting 

 light on the nature of the water channels. Assuming that the 

 Martian atmosphere exerts a pressure of 2\ inches of mercury 

 upon the surface — and it can scarcely be greater than this — 

 Lowell has shown that water could boil at a temperature of 

 ni°F. As the solar energy falling on Mars is certainly not 

 much less than that which heats the rocks of the Sahara to at 

 least 130 F., it is clear that evaporation is much more rapid 

 there than here ; and consequently water travelling in an open 

 channel would evaporate long before it reached the tropics of 

 the planet, a journey which we know occupies several weeks. 

 It is therefore probable that the water is carried in something 

 akin to pipes and this is rendered the more plausible by the 

 fact that the water does not flow naturally but is driven, a 

 conclusion to which the shape of the planet has led us. 



No apology is made for this last speculation. It is, I think, 

 directly justified by the observations and this one example 

 serves to illustrate the amount of detail which is possible in 

 constructing a picture of the happenings on the planet. Changes 

 speaking eloquently of activity are to be found among the 

 double canals, for they are not always double. The doubling 

 is seasonal in its nature but not entirely so, for there are canals 

 which sometimes double at the appropriate season and some- 

 times do not. That when they are not double their alter ego 

 is lying fallow is strongly suggested. 



Instances of this kind might be greatly multiplied but space 

 does not permit. 



There is yet another class of surface marking to be dealt 

 with — namely, the white spots which are seen in the equatorial 

 regions. They are intensely brilliant, often glistening but they 

 seem not to be snow, for they are often most conspicuous in the 

 height of the Martian summer ; and it has been noticed above 

 that they are not glaciated mountain tops. It seems natural 

 to surmise that they may be beds of salt left by the evaporated 

 seas. Their close association with the green areas strongly 

 suggests this explanation. As yet observational data are too 

 scanty to afford a firm base for conjecture but their increase 

 of brightness under a high sun forcibly suggests a mineral 

 origin. Further it may be remarked that vegetation would not 

 invade them but would probably be near them at the bottom 

 of the old marine depressions. A like instance on earth occurs 



