Abstract
THE statement attributed to Director Campbell on p. 317 of NATURE for September 8, to the effect that the nights in September, 1909, on which his spectrograms of Mars were taken, “were as perfect for the purpose as could be wished”, is open to question. Though the sky may have been clear and the surface humidity low, this does not prove that the aqueous vapour in the upper air was small in amount. September is the month when the total vapour-content of the atmosphere is a maximum, and February is the month when the vapour-content is a minimum, in north temperate latitudes. This is well shown in the curves of energy in the infra-red solar spectrum for February 19, 1903, and September 14, 1903, in the article on “The Absorption of Water Vapour in the Infra-red Solar Spectrum”, by F. E. Fowle, jun. (Smithsonian Miscellaneous Collections, quarterly issue, vol ii., part i., p. 1, 1904, Plate i.). The ratio of the intensities of the bands of aqueous absorption {(Mars + earth)/earth} will be greatest when the total absorbent column of the earth's atmosphere contains least water, that is, other things being equal, the ratio may be expected to be smallest in September and largest in February. Director Campbell has chosen the worst month, and Dr. Slipher, who observed in January and February, the best months for making the experiment.
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VERY, F. Water Vapour on Mars. Nature 84, 495 (1910). https://doi.org/10.1038/084495c0
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DOI: https://doi.org/10.1038/084495c0
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