242 Dr. J. Croll on some Controverted Points 
But it is in reference to the influence of aqueous vapour, 
fogs, and clouds on the production and preservation of snow- 
that the greatest diversity of opinion has prevailed. The 
object of the present article is to examine at some length the 
principal objections which have been advanced in regard to 
this part of the inquiry. I shall also take the present op- 
portunity of discussing more fully some points on which I 
have been sometimes misunderstood, and which appear to 
have been treated rather too briefly on former occasions. 
In the 'American Journal of Science' for April 1876, 
Professor Newcomb has done me the honour to review at some 
length my work, 'Climate and Time;' and as his article is 
mainly devoted to a criticism of my reasoning in regard to 
those very points to which I refer, I shall begin with an 
examination of his objections. One reason for entering at 
some length into an examination of Professor Newcomb's 
objections is the fact that they embrace to a large extent those 
which have been urged by reviewers in Great Britain. Some 
of his objections, however, as will be seen, are based upon a 
misapprehension of my reasoning. 
Temperature of Space. — One of the most important factors 
in the theory of geological climate resulting from changes in 
the eccentricity of the earth's orbit is obviously the tempe- 
rature of stellar space. Unless we have, at least, some rough 
idea of the proportion which the heat derived from the stars 
bears to that derived from the sun, we cannot form any 
estimate of how much the temperature of our earth would be 
lowered or raised by a given decrease or increase of the sun's 
distance. 
The question of the temperature of space has been investi- 
gated in different ways by Pouillet and Herschel ; and the 
result arrived at was that space has a temperature of — 239° F., 
or an absolute temperature of 222°. The mean absolute tem- 
perature of our earth is about 521°. Consequently, according 
to these results, the heat received from the stars is to that 
received from the sun as 222 to 299. All my determinations 
of the change of temperature due to changes in the sun's 
distance were computed on these data, although I believe, for 
reasons stated, that space must have a much lower tempera- 
ture. Recent observations of Professor Langley made during 
the Mount-Whitney Expedition confirm the correctness of 
my belief. 
Professor Newcomb, however, wholly ignores all that has 
been done on that subject, for he commences his review by 
the statement that " practically there is but one source from 
which the surface of the earth receives heat — the sun, since 
