Croll's Theory Redivivus. 175 
variations of the seasons, and are hence due to changes of 
temperature, and are to be attributed to the same cause or 
causes that produce the increase and diminution of the white 
spots. Water is the only known substance that will remain in 
the form of liquid sufficiently long and in sufficient quantities 
to answer all the conditions. Certainly it will not be allowed 
to assume an ocean nor rivers of liquid carbonic acid. 
The state of things seems to be this : so soon as either cap begins 
to shrink, there proceeds to surround it a blue belt. The belt increases 
with the increased rate of diminution of the cap and decreases as that 
diminution falls off. Meanwhile it keeps pace with the cap, shrinking 
with it so as to always border its outer edge. 
It is difficult to conceive how anything could more conclusively pro- 
claim itself the liquid product of the disintegration of the cap. This 
badge of blue ribbon seems to mark the substance as H2O. 
To this conclusion, however, there are two drawbacks. 
One consists in the less heat received by Mars from the sun, 
which, as compared with the earth, distance for distance, would 
be only a moiety, and would interfere with any assumption of 
H2O in liquid state on Mars in such large quantity. The other 
is a thinner air at the surface than we know, and therefore the 
absence of a blanketing atmosphere to keep out the cold of 
space. But these objections are not fatal to the assumption 
that the liquid substance that follows the margin of the white 
cap consists of water. 
In the first place the greater amount of clouds and vapor 
of water in the earth's atmosphere causes the earth to reject 
a far greater proportion of the sun's heat. Much of the sun's 
heat intercepted by the earth does not reach its surface, quite 
apart from what is necessarily reflected. But the Martian sky 
is clear from clouds, perpetually. "All the heat a pure sky per- 
mits to pass falls unhindered upon the soil. Thus receptivity 
makes up what distance denies." 
Secondly, the earth's blanket consists not in its atmosphere, 
contrary to what has been thought, but in the watery vapor that 
it contains. This has been shown by Tyndall. He stated that 
on an average day in England the atmospheric vapor of water 
exerts a hundred times as much reaction against the sun, and 
hence against the cold of space, as the atmosphere itself. Mr. 
Lowell concludes that if one seventieth part of the Martian at- 
mosphere is watery vapor, and the atmosphere one-seventh of 
