358 FROM NEBULA TO NEBULA 



polar cap and of the same whiteness and brilliancy, following the 

 Adicalium marsh stretched from the sunrise limb to within 45 of 

 the central meridian. It must have been an unusually heavy fall 

 for Mars, for this would indicate that it persisted until nearly n 

 o'clock in the Martian morning. Its size was rather difficult to 

 determine for, unlike the snow cap, it had no sharply defined 

 southern boundary. It stretched about 600 miles south from the 

 polar cap, or through some 16, to latitude 37, and must have 

 covered nearly a million square miles of surface in the visible 

 hemisphere, and very likely much more on the other side of the 

 planet. (Dec., 1915). 



The snow cap is now appreciably smaller than it was at the 

 last opposition, at the same season of the year. Since as we have 

 already seen it was much larger earlier in the season than at the 

 former opposition, we can hardly doubt but that they have been 

 having rather warmer weather on Mars this year. It will be of 

 interest to note if this means a warmer summer than usual for 

 our Earth. 



It may be mentioned here that although we have watched the 

 planet under very satisfactory circumstances throughout the 

 whole of the spring of its northern hemisphere, yet not a trace of 

 green has been detected in that region. The impression gathered 

 has been that all the dark northern areas, and most of the canals 

 observed have been simply marshes, or moistened soil. Possibly 

 greens will appear in their autumn or before. On the other hand 

 the greens of the southern hemisphere, during their autumn, have 

 been at times very striking. They have now all disappeared. 



The writer would point out that the clouds always lie over 

 the so-called desert regions of the planet, apparently being pre- 

 cipitated as soon as the fertile regions are reached. This is 

 clearly shown in Figure 14, where the cloud following Charontis 

 is also indicated, half covering Elysium. This was not an unusual 

 feature in this region in the Martian mornings, indicating doubt- 

 less that Charontis like Acidalium is composed of moist or marshy 

 land. Incomplete canals, i. e. elongated marshes, disappearing in 

 the desert, are shown in many of the drawings, by most of the 

 observers. Such a thing like an incomplete canal was formerly 

 supposed not to exist. (Dec., 1916). 



Unlike our polar ice caps, which are perpetual, Mars' 

 snow caps totally disappear every recurring summer sea- 

 son, and just as regularly reappear during the ensuing 

 autumn. This circumstance has led Lowell and astron- 

 omers in general to suppose that the snow sheet must be 

 exceedingly thin to yield so completely and so readily to 

 the sun's warmth, especially when it is remembered how 



