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 acqueous 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 Vogel². 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] (pages 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 degrees fahrenheit

Boiling point of water — 111 degrees fahrenheit

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 [signature]

Over

[4] P.S. Perhaps I should add that Marchand at the 'Pic du Midi'⁷was unable to detect lines of acqueous vapour in the spectrum of Mars.

Also that in 1905 Slipher⁸ by the line-in-sight test was unable to detect any terrestrial lines[?illeg] in the spectrum of Mars.⁹