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 _General Astronomy_ (1898), makes the mean temperature of Mars 223.6 deg.

absolute, by using Newton's law of heat being radiated in proportion to temperature, and 363 deg.abs.