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Rh ascribed to their being exposed to the sun. In the year 1781 the south polar spot was extremely large, which we might well expect, since that pole had but lately been involved in a whole twelvemonth's darkness and absence of the sun; but in 1783 I found it considerably smaller than before, and it decreased continually from the 20th of May till about the middle of September, when it seemed to be at a stand. During this last period the south pole had already been above eight months enjoying the benefit of summer, and still continued to receive the sunbeams; though, towards the latter end, in such an oblique direction as to be but little benefited by them. On the other hand, in the year 1781, the north polar spot, which had then been its twelvemonth in the sunshine, and was but lately returning to darkness, appeared small, though undoubtedly increasing in size." The length of the year in Mars is nearly two of our years, and the distance from us varies from about 230 to 50 millions of miles.

Astronomers, previous to Herschel's time, had found that Mars was surrounded by an atmosphere like the earth. One of them, Cassini, seems to have suspected the existence of an atmosphere of great density, and rising to a height of about 70,000 miles above the planet's surface. Herschel used the same means as Cassini to determine the height of the atmosphere of Mars by watching the fading or going out of starlight, when a star came up to its limb. At a distance of 30,000 miles there was no indication of an atmosphere. "It appears, however, that the planet is not without a