1900.] LOWELL—MARS ON GLACIAL EPOCHS. 647 
the way in which they are lit up into the night after the sun has set 
to the ground below. In like semblance of beacons he finds them 
out beyond the sunrise terminator, heralds of the dawn. 
Whether the cloud canopy has been continuous the night long 
we cannot of course positively affirm, That it has been so seems 
probable, inasmuch as the conditions which caused it to form would 
continue to some extent on through the night. The surface would 
grow colder and colder, thus keeping up the condensation into 
cloud above it. And this state of things would last till sunrise, as 
the coldest moment, so far as the heat received from the sun goes, 
is the moment before the dawn. ‘That at that time we find the 
clouds still there is strong presumption that they have not left their 
posts during the night. The effect of such a cloud canopy to the 
planet during the night hours is as important as the lack of it was 
important by day. It effectually shields the surface from depleting 
radiation. It thus helps husband what the day garnered of heat. 
It acts again the part of the glass in a greenhouse. 
11. So much on the score of authenticity of appearance presented 
by the Martian polar caps—so much, that is, toward the establish- 
ing that they are what they purport to be. 
From the constitution of the caps we pass now to the second 
point in which the planet recalls our own for purposes of glaciation, 
to wit: in the character of its orbit. The caps show us apparently 
that the necessary material is present ; the orbit assures us that the 
necessary cosmic conditions are fulfilled. 
At the present time the orbit of Mars is possessed of an eccen- 
tricity about five and a half times our own. Our earth’s is .0168 ; 
that of Mars .0933. ‘The planet’s axial tilt, too, is consonant with 
the conditions of a criterion; for it is closely accordant with the 
earth’s. According to Schiaparelli, who has made the last and 
undoubtedly the best determination of this tilt, the planet’s equator 
is inclined to its ecliptic 24° 52’. That is 25°-—for the quantity 
cannot be found to within the nicety of a few minutes—represents 
on Mars what 233° does on the earth, the tilt of the planet’s poles 
and the consequent breadth of its arctic regions. ‘The slight differ- 
ence between the two values would simply increase by so much the 
theoretical effect of the eccentricity. 
We have then in the case of Mars at the present moment both 
eccentricity and tilt, such as to enhance whatever effect might be 
expected. For by an odd coincidence it so chances that these essen- 
