WCP2875

[1]¹

90, UPPER TULSE HILL. SW

18 July. 1907.

Dear Dr. Wallace,

I am always pleased to see your handwriting.

The answer to your question whether spectroscopic proof is conclusive for aqueous vapour in the atmosphere of Mars must be in the negative.

My early spectroscopic observations in 1867 appeared to me to be strongly in favour of the presence of the [2] vapours of water. They were confirmed in 1873 by Vogel.² But more recent observations (about 1895) at the Lick³ by Campbell⁴ and Keeler⁵ were negative. No doubt the tenuous air of Mars makes such observations difficult.

In case you may not have seen it, will you permit me to refer you to Caswell's⁶most recent paper, in this month's Philosophical Magazine, [3] (pp. 161-176) "On a method of evaluating the surface-temperature of the Planets". In respect of Mars he gets out the following values.

Mean temperature — 48° F

Boiling point of water — 111° [F]

Amount of air per unit surface — 2.9 of the Earth's

Density of air at surface — 1/12^th of the earth's

Lady Huggins writes in kind remembrances and all good wishes,

yours sincerely | William Huggins.⁷ [signature]

Over

[4] P.S. Perhaps I should add that Marchand at the Pic du Midi was unable to detect lines of aqueous vapour in the spectrum of Mars[.]

Also that in 1905 Slipher⁸ by the line-in-sight test was unable to detect any terrestrial lives in Mars the spectrum of Mars.