Pagina:Scientia - Vol. VII.djvu/261

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Two centuries and a half have passed since the surface of the planet Mars was first subjected to a serious examination in the telescope. Since then our knowledge of the planet has made great progress, a progress which, for the sake of convenience, we may regard as having passed through seven principal stages.

I. — In 1666, Cassini detected several distinct dark spots on Mars, and from observing these ascertained that the planet had a rotation on its axis in about 24 hours 40 minutes.

II. — In the oppositions of 1777, 1779, 1781, and 1783, Sir William Herschel determined the inclination of the axis of Mars to the plane of its orbit; measured its polar and equatorial diameters; and ascertained the amount of the polar flattening. He showed also that the white spots which formed round the poles of the planet, increased with the approach of winter, and diminished with the approach of summer, behaving therefore as the snow does in our own Arctic and Antarctic regions.

III. — In the oppositions of 1830, 1832, and 1837, Beer and Mädler, observing with a telescope of 4 inches aperture, made a series of drawings from which they were able to construct a chart of the entire globe of Mars. The features which they then drew have been recognised at every succeeding opposition, and some of them can be identified in the rough sketches of Sir William Herschel, and even in those of the year 1666, made by Hooke and Cassini. The surface of Mars therefore possesses permanent features.