WCP2879

[1]¹

Graveley

Huntingdon.

Aug 3 1907

Dear D^r Wallace

Lowell's¹ argument appears to me to be as follows. He divides the light received from the sun by any planet into the spectrum chiefly luminous and that chiefly heat-producing. Then dealing with the luminous portion the light upon the outside of the atmosphere of a planet does not all reach the surface of the planet but part of it is arrested by dust & in passing through the atmosphere. Of that which is arrested part is reflected into space and goes to produce the albedo of the planet and part is ^also absorbed. A small part of the albedo is ^also derived from reflection from the solid surface of the planet. Thus for the Earth considering only the light effects of the luminous rays [2] Let S be the sun's light falling on units area of the outside of the earth's atmosphere then

S' =	reflection from the earth's atmosphere + reflection from the earth's surface + energy of the luminous rays absorbed by the earth's atmosphere and surface

That is to say

S’ =	the albedo of the earth + the energy absorbed

At Mars the sun's light per unit area is less than on the earth. Call it S - X. (I make it 0.43 x S)

Then we shall have

S – X =

albedo of Mars + the energy of the luminiferous rays absorbed by Mars' atmosphere & surface

Equating these two values of S we get

Albedo of Earth + energy absorbed by the earth = albedo of Mars + energy absorbed by Mars atmosphere & surface + X.

I do not see where Lowell has allowed [3] for the difference in the light on account of the difference in distance of the planet from the sun that is for X in my equation. Can you find that this has been allowed for?

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Supposing this all right we arrive at the conclusion that if the albedo and light absorbed by the earth are known that those of Mars can be estimated.

Now the light absorbed by the earth can be approximately estimated from our knowledge of its mean temperature so that of the albedo of the earth can be estimated that of Mars + its temperature can also be estimated. But th ^Mars' albedo is known from observation. Hence the temperature can be estimated.

This seems to be the line of his argument. The data ^{from which he deduces the albedo of the earth} are taken from Langley's¹ observations [illeg.] in the 2 nos.^2,3 of "Nature" 1885 to which I referred you. You ought by all means to read these which are very interesting.

I will not write more though I have much to say. But I want to know whether you can find any allowance [4] made for the difference of the suns [sic] energy per unit area on account of the difference of distances of the earth & Mars. This seems to me very important.

Believe me | sincerely yours | Osmond Fisher⁴ [signature]