We now come to a highly interesting class of observations bearing upon the question of clouds,—Mr. Douglass’s terminator observations. During the last opposition, seven hundred and thirty-six irregularities upon the terminator of the planet were detected at Flagstaff. They were seen by one or more of three observers, but chiefly by Mr. Douglass, who made a systematic scrutiny of the terminator for almost every degree of Martian longitude. Their full presentation would be both too tabular and too technical for this book. The paper embodying them will be found among the published annals of this observatory. I shall here give only certain deductions from it.
Of the 736 irregularities observed, 694 were not only recorded but measured. Of these 403 were depressions. It is singular, in view of their easy visibility, that they never should have been noticed before. Schroeter, indeed, saw three appearances of the sort,—on September 21, 1798, November 12, 1800, and December 18, 1802,—but all on the limb, not the terminator, which shows them not to have been of those here meant. Nevertheless they are not difficult to see, and anything but rare. When the phase is large enough, several may be seen every night.
The projections number 291. As their number shows, they are less common than the depressions, but they are even less of a feature of