PLURALITY OF WORLDS 173 



more easily.* Still clearer detail is obtained by photo- 

 graphing with the long infra-red waves. 



Great attention has lately been paid to the deter- 

 mination of the temperature of the surface of Mars; it 

 is possible to find this by direct measurement of the heat 

 rediated to us from different parts of the surface. The 

 results, though in many respects informative, are 

 scarcely accurate and accordant enough to give a defi- 

 nite idea of the climatology. Naturally the tempera- 

 ture varies a great deal between day and night and in 

 different latitudes; but on the average the conditions 

 are decidedly chilly. Even at the equator the tempera- 

 ture falls below freezing point at sunset. If we accepted 

 the present determinations as definitive we should have 

 some doubt as to whether life could endure the con- 

 ditions. 



In one of Huxley's Essays there occurs the passage 

 "Until human life is longer and the duties of the 

 present press less heavily I do not think that wise men 

 will occupy themselves with Jovian or Martian natural 

 history." To-day it would seem that Martian natural 

 history is not altogether beyond the limits of serious 

 science. At least the surface of Mars shows a seasonal 

 change such as we might well imagine the forest-clad 

 earth would show to an outside onlooker. This seasonal 

 change of appearance is very conspicuous to the atten- 

 tive observer. As the spring in one hemisphere advances 

 (I mean, of course, the Martian spring), the darker 

 areas, which are at first few and faint, extend and 

 deepen in contrast. The same regions darken year after 



* It seems to have been a fortunate circumstance that the pioneers 

 of Martian photography had no suitable photographic telescopes and 

 had to adapt visual telescopes — thus employing visual (yellow) light 

 which, as it turned out, was essential for good results. 



