under observation, to watch the changes occur; thus conclusively proving them to be changes of a seasonal character.

From early in June, which corresponded to the Martian last of April, to the end of November, which corresponded to the Martian last of August, the bluish-green areas underwent a marked transformation. During the summer of the Martian southern hemisphere, a wave of seasonal change swept down from the pole over the face of the planet. What and why it was we will examine in detail when we take up the question of water. Like the changes in the polar cap, it suffices here to chronicle the fact that it took place; for the fact of its occurrence constitutes proof positive of the presence of an atmosphere.

A moment's consideration will show how absolutely positive this proof is. It is the inevitable deduction from the simplest of observed facts. Its cogency gains from its very simplicity. For 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, wave-like, over the rest of the planet's face. It merely takes note that change occurs, and that note is final.

Now, since this was originally written, certain observations made at this observatory by Mr. Douglass have resulted apparently, most unexpectedly, in actually revealing this atmosphere to sight. Although the existence of an atmosphere is absolutely established by the above considerations, it is interesting to have ocular demonstration of it to boot; and this the more, that it would not have been thought possible to detect what, so to speak, disclosed itself. For the discovery was quite unconsciously made, being of the nature of a by-product to the outcome of another investigation. So systematically was his general search conducted that when the results came to be worked out it appeared not only that he had seen an atmosphere, but actually measured it, although he was quite unaware of doing so at the time. The occasion was the measuring of the diameters of the planet, polar and equatorial. Micrometric measures of these were begun as early as the beginning of July, and kept up at intervals till the latter part of November. But the ones that proved specially tell-tale were those made from September 20th to November 22nd, a set of polar and a set of equatorial ones having been taken throughout that interval on twenty-six nights.

Now, when these measures came to be worked out by me, corrected for all known sources of error and reduced to distance unity, a curious result made its appearance. As they stood arranged in their table chronologically, it was at once evident, even before taking the means, that, as time went on, something had affected the equatorial diameter which had not affected the polar one.

The values for the polar diameter were nearly the same from first to last. The equatorial values, on the other hand, showed, apparently, a systematic increase as the eye followed down the column. Something, therefore, had been at work on the one, which had not been at work on the other. Almost as instantaneously, it was evident what this something was, to wit, a visible twilight unconsciously measured for a part of the planet's surface. Like the Downeaster who shingled fifty feet on to the fog, Mr. Douglass had measured several miles into the Martian air.

A word or two will explain this. The planet came to opposition on October 20. The mid-measures of the series, therefore, were taken within a few days of opposition, just before and just after that event. The subsequent ones, on the other hand, were made at a gradually increasing distance from this position, as the planet passed toward quadrature. Now, at opposition, the disk of the planet is full, like the full Moon; while, as it passes to quadrature, it loses something of itself, becoming gibbous, as the Moon does two or three days after the full. This loss from phase chiefly affects the equatorial diameter, the polar one remaining substantially unchanged by it. It would remain absolutely unchanged if the planet moved in the plane of the ecliptic. It does not so move, but the quantity resulting from lack of accordance is so small that for the present explanation it may be neglected. Now, this question of phase was the only point, practically, in which the equatorial and polar diameters differed during the interval under consideration. This, then, was the clue to the discrepancy.