A noted astronomer, Cerulli, observed, when he directed a glass of moderate power, such as opera glasses, toward the Moon that the spots seemingly arranged themselves in rows forming intersecting lines similar to the canal-system on Mars. As the regularity disappeared with greater enlargement, Cerulli believed that the canal-system on Mars also would dissolve into small spots if a sufficiently powerful telescope were used. His idea, which partly has been verified, was more recently adopted by the Englishman, Maunder, who denies the existence of canals on Mars. Photography, however, has proved their reality (Fig. 18). If we disregard the illusory reticulation, there are nevertheless on the surface of the Moon numerous designs of a nearly rectilinear outline. There are to begin with the sinuses, extended trenches, often dotted along their sides with minor volcanoes. Fig. 27 shows, in the upper right corner, two such sinuses, the right one with a small volcano, Hyginus, in the middle. There are, further, five such volcanoes in its left arm, not visible on the photograph, and two in the right arm. The second, “Sinus Ariadaeus,” commences to the left with the Volcano Ariadaeus, not visible on the figure. The explanation of the origin of these sinuses is probably to be found in the different contraction of the Moon’s surface layer and of the hotter substrata immediately after the solid crust was formed. In a way, they correspond therefore to cracks in the glazing on porcelain. Like the two sinuses just mentioned they frequently commence and end with small craters which formed weak spots in the crust that facilitated the original break. Later on, volcanoes broke through along the sinuses themselves. In several regions of the Moon, and particularly in the equatorial belt, observers have claimed discoveries of new sinuses and occasionally of minor craters, “which could not possibly have escaped notice if they had existed before.” At present, the almost unanimous verdict is that such changes are very improbable, and that the visibility of the “new” objects largely depends on favourable sidelight, so that they might well have been overlooked if the region in question previously was examined under less advantageous illumination.