Page:The Moon (Pickering).djvu/92

62 drawings of Mars were made its apparent diameter was a little less than half the diameter of the crater of Eratosthenes, as shown in these drawings.

Besides this difference in actual size, there is another real point of difference between the lunar and the Martian formations. On Mars the so-called seas are green in the spring, gray in the summer and yellow in the autumn. On the Moon, gray and yellowish-white are the only shades visible; the markings merely darken and fade out again. This difference might very naturally be ascribed to the comparative lack of air and water-vapour found upon the Moon.

In 1888 the writer suggested that the markings known as the canals of Mars, as well as its seas, were in reality caused by processes of vegetation, and were in no way due to the presence of large bodies of water upon the planet. In 1892 our Arequipa observations showed that in some instances the canals crossed the seas. This fact somewhat added to the difficulties of explaining them on the supposition that both were due to water. Latterly the vegetation hypothesis has been advocated by several astronomers, Mr. Lowell among others, and so forcibly and so widely have they propagated this idea that it is believed that at the present time there are comparatively few astronomers who are adherents of the old theory that Mars is a marshy planet, peopled by a race who devote their lives chiefly to excavating ditches and then filling them up again.

Turning now to the observed facts, we shall begin by describing some of the changes that are found to take place in these dark areas of vegetation as the lunar day progresses. It must be premised that the four drawings. Figures 1, 3, 5 and 8, are representative of some thirty drawings in all, made chiefly between June 25 and September 1, 1901. Each of these four drawings is confirmed in all its essential details by at least two others, made upon different dates and frequently during different months. Therefore the more marked changes that are shown on them—for instance, between Figures 3 and 5—cannot be ascribed to mere errors of drawing nor to defective telescopic definition.

Perhaps the most marked change due to the growth of the lunar vegetation itself is shown in the darkening of the region situated just to the right of the central peaks of Eratosthenes. In Figures 1 and 2 this region is comparatively light. In Figure 3 it has appreciably darkened, although still retaining in part the shape shown in Figure 2. The inner wall of the crater to the right of the spot has now begun to darken. In Figure 4 the shape of the spot on the floor has changed, the darkening of the crater wall is now