Next to its great size, the most striking feature about it was its color. This, instead of being white or whitish, was ochre orange, a hue closely assimilated to the tint of the subjacent parts of the disk, which was the region known as Chryse. This distinctive complexion it kept throughout the period of its apparition. At the same time Baltia, a region to the north of it and synchronously visible close upon the terminator, showed whitish. The seeing was good enough to disclose the Phison and Euphrates double, the power a magnification of 310 and the aperture the full aperture of the 24-inch objective.
From the time it was first seen the detached patch of light crept in toward the disk, the illuminated body of the planet. Four minutes after I noted it the dark space separating it from the nearest point of the terminator had sensibly lessened. So it continued, with some fluctuations intrinsic to the atmospheric difficulties of observations generally and to the smallness of the object itself, to become gradually less and less salient. It lasted for about forty minutes from the moment it had first appeared to Mr. Slipher and then passed from sight to leave the edge of the planet smooth and commonplace again.
The measures made on it showed that it lay when first seen in longitude 39°.7, latitude 18°.5 north, and that its highest point stood seventeen miles above the surface of the planet. It was three hundred miles long. These are my own figures, from which Mr. Slipher’s do not substantially differ.
The return of the part of the planet where it had been seen was eagerly awaited the night after by both observers, to see if it would bring the projection with it. For only once a day is the same region of Mars similarly presented. But in order not to miss the projection, should it be ahead of time, observations were begun before it was due. Shortly after they were started, there appeared higher up the terminator and therefore farther north than had been the case the night before, a small projection. It was with difficulty made out and its position measured. Without careful watching it must have been missed altogether. As it was, it differed in every respect from that of the preceding day. It was not nearly so high, not nearly so large, and lay in a different place on the planet, being now in longitude 31°.7, latitude 25°.5. Either the two, therefore, were totally different things or the projection had moved in the elapsed interval of time over seven degrees of latitude and eight degrees of longitude, a distance of three hundred and ninety miles in twenty-four hours. Where the previous projection had been nothing showed. On the following night, May 28, no trace of anything unusual could be seen anywhere.
We are now concerned to inquire to what this series of appearances could have been due. The first observers of projections on Mars had unhesitatingly attributed them to the same cause that produces projections on the Moon, to wit, mountains. Such they were held to be in France and at the Lick. This view, however, was in 1892 disputed by W. H. Pickering who considered them to be not mountains, but cloud. And this view was strongly supported by A. E. Douglass in a discussion of a large number of them observed in 1894 at Flagstaff. The mountain theory of their generation was finally shown to be untenable and their ascription to cloud conclusively proved to be the correct solution by the observations of a remarkable one made in December, 1900, and the careful study to which by the writer they were subjected. We shall now explain how this was done and we will begin by pointing out that the fact that only a single specimen of the phenomenon was visible at each of the three oppositions of 1900, 1903, and 1905 was itself conclusive, rightly viewed, of their non-mountainous character. This conclusion follows at once from the isolateness of the phenomenon. For a mountain cannot change its place. Now, the shift in the aspect presented by the planet’s disk from one night to the next is not sufficient to alter perceptibly the appearance shown by anything upon its edge on the two occasions. If, then, a peak stood out upon it one evening, the peak should again show salient when the region reached the same position upon the succeeding night. That nothing then was seen where something had previously been visible proved the phenomenon not that of a mountain peak, since what produced the projection was clearly not fixed in place and therefore not attached to the soil. Now the only other thing capable of catching the light before it reached the surface would be something suspended in the air, that is, a cloud. Deduction, therefore, from the rarity of the phenomenon alone showed that the projections must be clouds.
Their behavior in detail entirely corroborates this deduction from their intermittence. Such was shown by the action of the projection of December 6, 1900, as set forth in a paper before the American Philosophical Society and such again by that of the one of May 26, 1903, as we shall now note. To begin with, we notice that the projection seen on May 26 was not found either in situ or in size on May 27 and had wholly vanished on May 28, though the seeing was substantially the same if not better on the two nights succeeding that of its original detection. Hence in its own instance this projection proved an alibi irreconcilable with the character of a mountain mass. But it did more. It not only was not on the second evening what and where it had been on the first, but the remains of it visible on the second occasion showed clearly that it had moved in the meantime. Furthermore it was disappearing as it went, for it was very much smaller after the lapse of twenty-four hours. The something that caused it was not only not attached to the soil, but was moving and dissipating as it moved. Only one class of bodies known to us can account for these metamorphoses and that is: cloud.
But what kind of cloud are we to conceive it to be? Our ordinary vapor clouds are whitish and this would be still more their color could they be looked at from above. The Martian cloud was not white but tawny, of the tint exhibited by a cloud of dust. Nor could this color have very well been lent it by its sunrise position, for other places equally situated to be tinged by the hues of that time of day, Baltia to wit, showed distinctly white. So that we must suppose it to be what it looked, a something of the soil, not beholden to atmospheric tinting for its hue; a vast dust-cloud traveling slowly over the desert and settling slowly again to the ground.
Precisely the same general course of drifting disappearance was taken by the projection of December 7 and 8, 1900. And this, too, stood an unique apparition in the annals of its opposition. Clouds, then, and not mountains are the explanation of the projections on Mars, differing thus completely from the lunar ones.