Page:Popular Science Monthly Volume 11.djvu/727

Rh those of 1830, gave 24h 37m 23.7s. In regard to the exact period of rotation and the slight discrepancies between the results obtained by different observers, Prof. O. M. Mitchel remarks as follows:

Finally, Richard A. Proctor, Esq., by an exhaustive discussion of all the observations, has determined the period to be 24h 37m 22.735s.

The diameter of Mars is about 4,700 miles. Its surface is rather more than one-third that of the earth, while its volume is to that of our planet in the ratio of two to nine.

The persevering labors of Beer and Mädler proved beyond question that many of the lineaments observed in the aspect of this planet are permanent in their character, and not merely atmospheric. The same spots, with the same general outlines, and the same varieties of color, have been noticed at successive oppositions; not always, it is true, with precisely the same distinctness, but without any other changes than such as might be attributed to atmospheric variations. Two white circular spots are observed in the polar regions, which increase during the winter, and decrease in the summer, of each hemisphere respectively, and which may, therefore, be regarded as polar snows. These spots were noticed by Maraldi as early as 1716; their connection, however, with the change of seasons was first shown by Sir William Herschel. The same astronomer found the inclination of the axis of Mars to the plane of its orbit to be 61° 18'. The Martial tropics are therefore 28° 42' from the equator, making the torrid zone 10° wider than that of the earth. In so far, then, as climatic changes are dependent on the obliquity of the planets, the seasons of Mars may not differ, perhaps, except in their duration, very greatly from our own.

—We come now to the history of one of the most interesting discoveries of the nineteenth century. With the single exception of our own moon, Mars is the most favorably situated of all the heavenly bodies for telescopic observation. The most careful scrutiny, however, for more than two centuries, had failed to furnish any indication of the existence of a satellite. The opposition of Mars in August, 1877, occurred when the planet was