646 LOWELL—MARS ON GLACIAL EPOCHS. [Nov. 16, 
occur, and that such change does take place is an indisputable fact 
of observation—such moisture would be rendered more potent there 
than it is on the earth by two Martian specialties in the matter of 
climate: first, the condition of the sky by day and, second, the state 
of the sky by night. 
The day-sky on Mars is distinguished by being almost perpetually 
clear. From dawn to dusk daily and from the year’s beginning to 
its close, the sun shines down upon the planet’s surface out of a 
heaven unflecked by cloud. I have mentioned this above, but one 
deduction from it I want to bring forward more prominently. Not 
only does a clear sky give ingress to warmth by the absence of 
cloud, ‘but if it contain water vapor it plays in addition the part 
of cloud itself. It exerts the efficiency of cloud and the efficiency 
of sunshine combined. For it lets in the warmth and then will not 
let it out again. In short, this transparent moisture suspended in 
mid-air is as a glass to make of the planet a conservatory. And 
when we reflect that this is true all over the planet at all times, we 
see that even the chief disturber of such a state of heat accumula- 
tion, the influx of cold winds from elsewhere, is as much as possible 
stopped. 
The importance of this solidarity in fair weather climate can 
hardly be overestimated. It is the indraught of colder winds that 
thwarts that hothouse heightening of the temperature which we 
experience in the days called weather-breeders. It is true that the 
coldness of the incoming winds is consequent on many causes, and 
universal sunshine would not avail to prevent a fall of temperature 
due to such circulation; but a general heating of the ground, espe- 
cially to the northward, would certainly temper its effects, not so 
much by equalizing the extremes, if indeed it would do this at all, 
as by raising the means. 
to. The night-sky on the planet abets the action of the day one. 
Just as the day sky is cloudless so the night-sky apparently is 
cloudy. This is the outcome of Mr. Douglass’ study of projections 
upon the terminator. By first observing and then classifying over 
four hundred of such projections, he came to the conclusion that the 
appearances could not be produced by bodies contiguous with the 
planet’s surface. In other aords, the optical effects were not such 
as could be caused by mountains, but were such as could be caused 
by cloud. Thus interpreted he finds that, though the sky is fleckless 
all day, at sunset clouds begin to form, showing their presence by 
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