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place. From the Martian last of April to the Martian middle of August, the bluish-green areas have been steadily undergoing a most marked transformation. There proves, in fact, to be a wave of seasonal change that sweeps over the face of the planet from pole to pole. We will examine this more in detail when we take up the question of water. For the present point it suffices that it takes place; for it constitutes proof positive of the presence of an atmosphere. A moment's consideration will show how absolutely positive this proof is; for it is the inevitable deduction from the simplest of observed facts. Its cogency consists in its simplicity. It is independent of difficult detail or of doubtful interpretation. It is not concerned with what may be the constitution of the polar caps, nor with the character of the transformation that sweeps, wavelike, over the rest of the planet. It merely states that change occurs, and that statement is conclusive. Having thus seen with the brain as much as with the eye, and in the simplest possible manner, that a Martian atmosphere exists, we will go on to consider what it is like. The first and most conspicuous of its characteristics is its 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 a year to the other, nothing appears to veil the greater part of the planet's c surface. This is more completely the case than has hitherto been supposed. We read sometimes in astronomical books and articles picturesque accounts of clouds and mist gathering over certain regions of the disc, hiding the coast lines and continents from view, and then, some hours later, clearing off again. No instance of such blotting out of detail 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 case of 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 detail the observer might be engaged in looking at. The only dimming of detail upon the Martian disc has been along its bright edge, 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 of mine 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