262 Joty—On the Origin of the Canals of Mars. 
Something going on in Mars determines this coming and going of the lines. 
The changes are not sudden; and the conspicuousness of the lines appears 
attendant on the liberation of the polar snows of Mars. The white deposit upon 
the polar regions of Mars must represent but a shallow deposit of frost or snow. 
From Lowell’s observations we find in fact that a period of twelve days sufficed to 
free many hundreds of thousands of square miles of its coverig. And three 
months of the solar radiation at Mars (of but half the intensity of that received 
upon the Earth) sufficed to practically clear off the entire polar covering. It is 
very probable that this small amount of water (if such be its nature) will be for 
the most part, or entirely, diffused through the Martian atmosphere, and in this 
way gradually conveyed towards the equatorial regions. The diffusion of water 
in this manner will be greatly aided by the rarity of the Martian atmosphere. 
Mr. Lowell urges that it is improbable that rain or snow will form in so rare 
an atmosphere, but that the natural distribution of water will be by distilla- 
tion from place to place, or by dew-fall. This is, it may be said, a certainty, 
if the atmosphere is indeed but one-seventh the density of our own. 
In the serene atmosphere which observation appears to ascribe to Mars, his 
mountain ranges would exhibit phenomena depending upon aqueous distillation 
such as are only in a feeble degree familiar to our experience. In the extreme 
cold of the Martian night the water in the atmosphere would be precipitated 
principally upon the higher ground, which of course will first cool by radi- 
ation. This deposit will take the form of frost, which with the return of solar 
radiation will melt. In the Alps of Europe, travellers meet with this phenomenon 
on a small scale. 
If it be the case, as Mr. Lowell thinks, that the light areas of Mars are now 
arid wastes, the effects of the daily liberation of water on the higher ground 
will lead to just such contrasting darkening of the ground as will render the 
“canals” conspicuous. An early riser in summer time will often see a fallow field 
similarly darkened by the dews of night. We must, in short, remember that, in a 
world wherein water is as scarce as it is supposed to be in Mars, the elevated 
ground would receive the larger supply of water. Our valleys, indeed, only possess 
more water than our hills because the fall is so abundant as to gravitate from 
the higher ground to the lower. 
The seasonal increase of definition of the lines downwards over the planet’s 
surface, from the scene of the melting polar ice-cap, would naturally follow from 
this explanation. Quite recently M. Janssen has remarked, with regard to M. 
Perrotin’s observation, that the canals are visible to a greater distance along a 
meridian than along a parallel, that this points to the presence of bodies which 
condense the water from the Martian atmosphere, and increase its transparency 
as the polar regions are approached. (Nature, February 25th, 1897.) 
