94 Tlie A)nericnn Geolo(jist. Auj,'Ufc;t, ib95.- 
(IcM- pass back in a short time to their original ycHow color or 
invisibility. 
On the view advocated by Mr. Lowell the melting ol' the 
south polar snow-cap produces a glacial lake extending from 
the ice-front for two hundred miles to the northward and 
probably deepest where it is in contact with the ice. The 
steady increase of the lake at last enables it to throw otf its 
water through the so-called "canals," and with the arrival of 
these streams the growth of vegetation commences and gives 
the blue-green tint so much insisted on by that author. With 
the disappearance or great reduction of the polar cap the wa- 
ter disappears, vegetation dies and the surface of the planet 
returns to its wonted fiery tint. In Mr. Lowell's words the 
polar ice-dammed sea is the "Deus ex machina" to the sum- 
mer life of the planet. Mars being short of w^ater draws on 
its polar reservoir for an annual supply. 
Mars would on this view be in the same condition as some 
of the arid regions of the earth where a short wet season de- 
velops an equally short-lived period of vegetation and perhaps 
of animal life. The so-called seas would be the lowest parts 
of the surface through which water passes at annual intervals- 
but in which it does not continually remain. With his scanty 
allowance of air and w^ater the planet shows a stage of being- 
considerably more advanced than that of our own earth where 
water is still almost everj^where abundant and air the cheap- 
est necessity of life. This is in keeping with what might be 
expected fronr his great distance from the sun and his smaller 
size. Internal cooling may be assumed to be more nearly com- 
plete. 
There is no doubt concerning the excessive tenuity of the 
Martian atmosphere. Delicate tests have many times failed 
to detect it and this fact has led to the denial of its existence 
by many observers. But the peculiar hazy appearance that 
veils the structural details near the edge of the planet's disc 
justifies Mr. Lowell in the conclusion that a thin atmosphere 
of some kind must exist. And if we admit the presence of 
water as shown above, there must certainly be, at least locally 
and temporarily, an atmosphere of water- vapor of tension va- 
rying with the temperature. Assuming other conditions as on 
the earth, this tension would at 32° Fahrenheit equal 0.2 
