I'Glucial Xofes fro)j/ the Planet Mar.s. — Cta ijpote. 99 
it is useless to look for its evidence in the spectrum with our 
present means. 
In general, therefore, it appears safe to conclude that the 
polar caps of Mars are really composed of snow and therefore 
prove the existence of water on his surface, — that they prove 
a variation of climate. with latitude like our own, — that they 
prove by melting on emergence into sunlight that api)roxi- 
mately similar laws control the freezing of water and the 
thawing of ice there and here. 
It is also obvious from what has here been summed up that 
INIars affords no evidence in support of the eccentricity theory 
-of glacial cold, though his conditions are at present such as 
to favor a state of intense glaciation in his southern hemi- 
sphere. But exactly how far this inference may be modified 
by the apparent scarcity of water and therefore of snow on 
the planet is a point not easy at present to be determined. 
For obviously if the svipply of vapor be small no degree of cold 
■or length of winter can produce more than a corresponding 
quantity of polar snoAV. For this reason the presence of a 
north polar snow-cap during an interglacial period is evidence 
of greater importance. 
It is further to be noted in the same direction that we find 
no evidence whatever of the intenser climate which the eccen- 
tricity theory would lead us to look for on the planet Mars. 
His snow-caps extend no farther south than do those of the 
earth. Indeed we cannot say that they extend so far. In a 
•severe American winter the snow field is often continuous from 
the pole to the middle states or to about the i)arallel of 35^ 
north. Not infrequently the whole northern part of the con- 
tinent is sheeted in white. Nothing of the kind has ever been 
seen on Mars. Yet a layer of snow a few inches in thickness 
would be just as conspicuous as one of greater depth. If also, 
according to Schiapai-elli's observation alread}^ referred to, 
the north polar ide-cap does not form until after the vernal 
■equinox of that hemisphere has passed, the fact shows a sur- 
prising power of retaining the heat of the long summer. It 
would also seem as if the climate of Mars, in spite of greater 
distance from the sun and the high eccentricity of his (u-hit, 
was really milder than our own. And it would be an inter- 
esting question in physics, but scarcely cognate with our pres- 
