Page:Mars - Lowell.djvu/117

Rh We may add to this the natural delight of the explorer, for we shall be gazing upon details of Martian geography never till last summer seen by man.

Areography is a true geography, as real as our own. Quite unlike the markings upon Jupiter or Saturn, where all we see is cloud, in the markings on Mars we gaze upon the actual surface features of the Martian globe. That we do so we know from the permanency of the spots and patches thus revealed to us. They change in appearance, indeed, according to times and seasons, but they alter as true surface features would, not like cloud-belts that gather to-day and vanish forever to-morrow. That the markings are essentially permanent has been known ever since Cassini in 1666 definitely discovered, what Huyghens had thought to detect in 1659, the rotation of the planet, by means of their periodic presentations.

The twelve views we shall here scan are of the nature of a map, made in November, 1894. They represent the ensemble of the drawings from this observatory, for about that date. The details from these drawings were plotted upon a globe, which was then tilted toward the observer at the angle at which the Martian south pole itself was tilted toward the Earth during November, and photographed at intervals of 30°. The negatives were then made to