Page:Mars - Lowell.djvu/93

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71
CLOUDS

the terminator, but separated from it by an apparent space of over 80 miles. It was generally yellowish in color, like the limb, but of less brilliancy than the centre of the disk, though distinctly surpassing in that respect the adjacent terminator. I estimated it to have the brilliancy of the bright areas of the disk at a distance of 9° from the terminator. In one view it appeared to be a very small whitish point, and I am inclined to think that there may have been a real diminution in its size at that moment. This idea is partly sustained by the following night’s observations. At 16h. 54m. it was observed by Professor Pickering, whose estimate gave 11 miles for its height At 17h. 5m., after obtaining two readings of the micrometer screw for latitude, the seeing, which had been quite steadily at the figure 7 (on a scale of 10), dropped to 4, and in attempting the next setting I could not find the ‘cloud,’ although once before it had remained visible when the seeing dropped instantaneously to that figure. Nor did it reappear in the next half hour. This sudden disappearance, without any previous lessening of its height above the terminator or of its size, made its cloud character unmistakable, since a mountain beyond the sunrise terminator must either constantly decrease in height, or soon join the illuminated disk.