rates; while the
atmosphere itself sends back to the surface an ever varying portion of
both this radiant and reflected heat according to distinct laws. Further
difficulties arise from the fact that much of the sun's heat consists of
dark or invisible rays, and it cannot therefore be measured by the
quantity of light only.

From this rough statement it will be seen that the problem is an
exceedingly complex one, not to be decided off-hand, or by any simple
method. It has in fact been usually considered as (strictly speaking)
insoluble, and only to be estimated by a more or less rough
approximation, or by the method of general analogy from certain known
facts. It will be seen, from what has been said in previous chapters,
that Mr. Lowell, in his book, has used the latter method, and, by taking
the presence of water and water-vapour in Mars as proved by the
behaviour of the snow-caps and the bluish colour that results from their
melting, has deduced a temperature above the freezing point of water, as
prevalent in the equatorial regions permanently, and in the temperate
and arctic zones during a portion of each year.

_Mr. Lowell's Mathematical Investigation of the Problem._

But as this result has been held to be both improbable in itself and
founded on no valid evidence, he has now, in the _London, Edinburgh, and
Dublin Philosophical Magazine_ of July 1907, published an elaborate
paper of 15 pages, entitled _A General Method for Evaluating the
Surface-Temperatures of the Planets; with special reference to the
Temperature of Mars_, by Professor Percival Lowell; and in this paper,
by what purports to be strict mathematical reasoning based on the most
recent discoveries as to the laws of heat, as well as on measurements or
estimates of the various elements and constants used in the
calculations, he arrives at a conclusion strikingly accordant with that
put forward in the recently published volume. Having myself neither
mathematical nor physical knowledge sufficient to enable me to criticise
this elaborate paper, except on a few points, I will here limit myself
to giving a short account of it, so as to explain its method of
procedure; after which I may add a few notes on what seem to me doubtful
points; while I also hope to be able to give the opinions of some more
competent critics than myself.

_Mr. Lowell's Mode of Estimating the Surface-temperature of Mars._

The author first states, that Professor Young, in his _Ge