Page:Mars - Lowell.djvu/58

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appeared not only that he had seen an atmosphere, but actually measured it, although he was quite unaware of doing so at the time. The occasion was the measuring of the diameters of the planet, polar and equatorial. Micrometric measures of these were begun as early as the beginning of July, and kept up at intervals till the latter part of November. But the ones that proved specially tell-tale were those made from September 20th to November 22nd, a set of polar and a set of equatorial ones having been taken throughout that interval on twenty-six nights.

Now, when these measures came to be worked out by me, corrected for all known sources of error and reduced to distance unity, a curious result made its appearance. As they stood arranged in their table chronologically, it was at once evident, even before taking the means, that, as time went on, something had affected the equatorial diameter which had not affected the polar one.

The values for the polar diameter were nearly the same from first to last. The equatorial values, on the other hand, showed, apparently, a systematic increase as the eye followed down the column. Something, therefore, had been at work on the one, which had not been at work on the other. Almost as instantaneously, it was evident what this something was, to wit,