opposite state of things from that with which we were confronted in the short and sharp class. Here, as compared with the projections, instead of relatively few depressions of greater height, we have relatively many depressions of less height. Furthermore, there are a great many more of both projections and depressions than there were of the former variety, and they are both of much less height or depth. Evidently, therefore, we have here, in part at least, a different class of phenomena from what we have previously considered. Now we perceive at once that two factors enter here which did not enter in the case of the short and sharp irregularities. The long and low depressions occur, as we shall recall, almost always over the dark areas, while the short and sharp ones do not. In the next place, the average height or depth of the long and low irregularities is much nearer the value of the irradiation constant, that is, the amount by which a bright object seems bigger on account of its brightness; which would cause the dark areas to seem depressed. From these facts we infer that most of the depressions of this class are due to the character, not to the contour, of the surface where they occur; partly to the direct effect of lack of irradiation, partly to sombreness of the surface, which would cause the light to fade from them at a greater relative distance from the terminator. On eliminating these depressions, therefore, we find ourselves left with very few depressions as against nearly 200 projections. The excess in number of the latter shows, as in the case of the other variety, that we are here dealing chiefly with long and relatively low clouds formed after sunset or before sunrise; those so formed during daylight being few if any.

One more observation made at Flagstaff, on the subject of cloud, is as peculiar as it is important. It was made by Mr. Douglass, and I shall give it in his own words. A more detailed account of it, together with his tables of figures, will appear in his paper upon it in the Observatory annals: --

"On November 25 and 26 a bright spot was seen in the unilluminated portion of Mars, to which, in my opinion, no other name than cloud can be applied. Its great height, size, and brilliancy, and, on the second evening, its singular fluctuations, render it of importance in the study of the Martian atmosphere.

"I first saw it at 16h. 35m., G.M.T., of November 25, and made an estimate of its height. It seemed to be rapidly increasing in length in a direction parallel to the terminator at that point. Subsequent estimates of its height gave a different and greater value than at first; until its sudden disappearance at 17h. 6m., or perhaps a minute later. After once attaining its size, it seemed to remain with little change, presenting the appearance of a line 115 miles long by 33 miles wide at the centre and lying parallel to 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 degrees 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.

"A subsequent computation showed that this phenomenon took place over the southern part of Schiaparelli's Protei Regio. Other reasons lead me to think, however, that he has placed that island some 5 degrees too far south.

"On November 26 the cloud promptly appeared at 17h. 15m., G. M. T., but about 12 degrees farther north. Instead of remaining continuously visible, it dissipated and reformed at irregular intervals. The first appearance lasted sixteen minutes. After somewhat over four minutes had passed, it reappeared momentarily, and six minutes elapsed before it appeared again, lasting then but two and one half minutes. Then followed an absence of three minutes, presence for two minutes, absence for three minutes, presence