1900.] LOWELL—MARS ON GLACIAL EPOCHS. 643 
creases with the increased rate of diminution of the cap and de- 
creases as that diminution falls off; meanwhile, it keeps pace with 
the cap, shrinking with it so as always to border its outer edge. 
It is difficult to conceive how anything could more conclusively 
proclaim itself the liquid product of the disintegration of the cap. 
This badge of blue ribbon seems to mark the substance as H,O. 
4. Against this pronounced appearance and decisive conduct on 
the part of the cap, what difficulties have we to oppose to its ac- 
ceptance for what it purports to be? ‘Two such present themselves, ° 
both on the score of general temperature: first, the less heat re- 
ceived by the planet due to its greater distance from the sun, a heat 
only the moiety of what falls to our lot ; secondly, a thinner air 
at the surface than that we know, perhaps in amount but a seventh 
that of our own. Are either of these objections fatal? Upon 
scrutiny I think we shall see that neither of them necessarily is so, 
on account of certain counterbalancing facts. ° 
5. In the first place, not all the heat intercepted by the earth 
reaches its surface which might do so, quite apart from what is 
necessarily reflected. From the commotional character of our sky, 
it is safe to say that the earth fails of half the heat it would re- 
ceive were that sky perfectly clear. What with storms, passing 
clouds and haze, fully more bar than passage is offered to the rays. 
Now the Martian sky is clear, perpetually so. All the heat a pure 
sky permits of passage falls unhindered upon the soil. Its frugal 
atmosphere wastes nothing. ‘Thus receptivity makes up what dis- 
tance. denies. 
6. As regards the second point, it used to be thought that air, 
pure and simple,.furnished the earth with the cloak that kept out 
the cold of space. But it is no longer considered thus effective. 
Tyndall made experiments on the subject to the deduction that not 
air, but water yapor it was that did the business. Since then the 
enormous preponderance of power ascribed by him to water vapor 
has been questioned ; but experiments to disprove it labor under 
the disquieting impossibility of excluding the vapor itself from the 
test. To get perfectly dry air is as difficult as to get a perfect 
vacuum ; and the least trace of water vapor is potent to vitiate the 
whole transaction. 
As figures are here of importance I shall quote him on the sub- 
ject. In his ‘‘Rede’’ lecture at Cambridge, in 1867, Tyndall 
Says : 
