The same argument that applies to mountains applies to clouds, or to any opaque substance. Sporadic increase might be due to them; but for the increase to be systematic, it is necessary that the substance seen should also be seen through. It must be in part transparent. The measures, therefore, not only disclose the presence of an atmosphere, but do so directly.

Having thus seen first with the brain and then with the eye, and both in the simplest possible manner, that a Martian atmosphere exists, we will go on to consider what it may be like.

The first and most conspicuous of its characteristics is cloudlessness. A cloud is an event on Mars, a rare and unusual phenomenon, which should make it more fittingly appreciated there than Ruskin lamented was the case on Earth, for it is almost perpetually fine weather on our neighbor in space. From the day's beginning to its close, and from one end of the year to the other, nothing appears to veil the greater part of the planet's surface.

This would seem to be even more completely the case than has hitherto been supposed. We read sometimes in astronomical books and articles picturesque accounts of clouds and mists gathering over certain regions of the disk, hiding the coast-lines and continents from view, and then, some hours later, clearing off again. Very possibly this takes place, but not with the certainty imputed to it. It is also doubtful if certain effects of longer duration are really attributable to such cause. For closer study reveals another cause at work, as we shall see later, and the better our own air the more the Martian skies seem to clear. Certainly no instance of the blotting out of detail upon the surface of Mars has been seen this year at Flagstaff. Though the planet's face has been scanned there almost every night, from the last day of May to the end of November, not a single case of undoubted obscuration of any part of the central portions of the planet, from any Martian cause, has been detected by any one of three observers. Certain peculiar brightish patches have from time to time been noted, but, with a courtesy uncommon in clouds, they have carefully refrained from obscuring in the slightest degree any feature the observer might be engaged in looking at.

The only certain dimming of detail upon the Martian disk has been along its bright semicircular edge or edges, as the case may be,--what is technically called its limb. Fringing this is a permanent lune of light that swamps all except the very darkest markings in its glare. This limb-light has commonly been taken as evidence of sunrise or sunset mists on Mars. But observations at Flagstaff during last June show that such cannot be the case. In June Mars was gibbous,--that is, he showed a face like the Moon between the quarter and the full,--and along his limb, then upon his own western side, lay the bright limb-light, stretching inward about thirty degrees. Since the face turned toward us was only in part illumined by the Sun, the centre of it did not stand at noon, but some hours later, and the middle of the limb consequently not at sunrise, but at about nine o'clock of a Martian morning. As the limb-light extended in from this thirty degrees, or two hours in time, the mist, if mist it was, must have lasted till eleven o'clock in the day. Furthermore, it must have been mist of a singularly mathematical turn of mind, for it stretched from one pole to the other, quite oblivious of the fact that every hour from sunrise to sunset lay represented along the limb, including high noon. What is more, as the disk passed, in course of time, from the gibbous form to the full, and then to the gibbous form on the other side, the limb-light obligingly clung to the limb, regardless of everything except its geometric curve. But as it did so, the eleven o'clock meridian swung across it from one side of the disk to the other. As it passed the centre the regions there showed perfectly clear; not a trace of obscuration visible as it lay beneath the observer's eye.

From the first observation it is evident that Martian sunrise and sunset had nothing to do with the phenomenon, since it was not either Martian sunrise or sunset at the spot where it was seen; and, from both observations taken together, it is evident that the phenomenon did have to do with the position of the observer. For nothing on Mars had changed in the mean time, but only the point of view of the observer on Earth. It is clear, therefore, that it was not a case of Martian diurnal meteorological change, but a case of foreshortening of some sort.