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Rh well shown, and the spot itself has grown very much darker. The fact that in Figure 4 the region is intensely illuminated by the almost vertical Sun, while in Figures 1 and 2 the Sun was comparatively low, makes these changes still more striking. In Figures 5, 6 and 7 the spot is still dark, but in the last two the shading of the eastern wall by the setting Sun has materially changed the shape of the spot as compared with its appearance in Figures 4 and 5. Figure 8 shows that the spot has now clearly faded, although a part of its area is already deeply enveloped in the heavy shadows that precede the absolute blackness of coming night.

Another spot, to the left and below the central peaks of the crater, first makes its appearance in Figure 2. In Figure 3 a dark marking connects it with the crater wall and a second dark marking appears to be forming. In Figures 4, 5 and 6 the second marking is well shown, although in the last it has begtui to fade. In Figures 7 and 8 it is no longer visible, and in the latter the spot itself has vanished. To express the matter more fully in words, we may say that a triangular area partly shown in Figure 2 gradually darkens. After reaching its maximum intensity the central portion begins to fade, leaving the canals, as shown in Figure 4. These later fade, as shown in Figures 6 and 7, and finally the darkest part—the lake—disappears. It is believed that we have here an illustration of the formation and destruction of a canal, a phenomenon never yet satisfactorily observed upon Mars.

A comparison of the upper portions of the central dark marking in Figures 2 and 4 further illustrates the formation and growth of a variable spot as the lunar morning advances, while a comparison of the upper portion of the same spot in Figures 6 and 7 shows how it fades out as the day declines.

Turning now from these changes that are so conspicuous that even the coarse, reliable photographs are capable of showing them, we will next examine a set of markings where, for the present, at all events, the human eye and hand must reign supreme. In the delicate system of canals shown in the upper part of Figure 3, by far the most conspicuous is the one which starts from a little lake near the centre of the system. This lake is well shown in Figure 1, where it is found to be situated upon the very crest of the crater wall. The canal flows from it in a northeasterly direction—that is, downward and to the right — for about twelve miles, when it heads northerly, still following the crater rim, but apparently remaining chiefly on the outer slopes. This darkening may be due in part to shadow, but that it is really a canal is shown by reference to Figure 3, where, as