for one minute, and a final brief appearance eight minutes later at 18h. 1m. Its presence was suspected five minutes before that hour, and again at 18h. 11m., but with great uncertainty.

"At this time it presented in general the same characteristics as the night before, though its appearances were too brief to permit such careful observations as were hoped for. The seeing, too, was not so good as before, varying from 4 to 7; and if the cloud happened to appear under the former figure, its observation was difficult. It is needless to remark that under such conditions it was impossible to observe its appearance or disappearance to the second. In general, it seemed to exhibit a less elevation than the night before. A careful estimate of its latitude placed it precisely at the centre of the terminator. I believe these latitude observations, though made rapidly, cannot be subject to an error greater than 2 degrees, at 18h., probably less than 1 degree. On November 27, at 18h., I searched for the cloud, but was not rewarded by finding any trace of it.

"Estimates of the size and height of this cloud were made with reference to a glass thread in the micrometer, whose diameter is 0".6. One tenth of the thread was found to represent on Mars a little less than twenty miles. This gives us an elevation above the surface of between 10 and 11 miles. In this process we have taken the apparent centre of the cloud, and have assumed the seeing to have no influence. We obtain, therefore, the smallest possible mean height of the centre of the cloud. If we assume that the seeing was not perfect, its effect would be to lessen the separation, but not to change the total height. Supposing, for example, that the apparent extension of the cloud was due to poor seeing enlarging a point, then our terminator distance would be 245 miles, and our minimum elevation 15 miles. Therefore we can assume 15 miles to be the smallest probable mean elevation of this cloud. The average height of our cirrus clouds is five and one half miles.

"One more idea requires mention, namely, the movement of this cloud in latitude. From the extreme rarity of clouds on Mars I am inclined to connect intimately the appearances of the two evenings, and consider them as due to one source, presumably a large body of air moving northward. Such an advance would be at the rate of 18.7 miles per hour."

I may add to this that the height of the cloud--relatively to those of the Earth--is what direct deduction from the less rapid thinning out of the air above the Martian surface, which must result from the smaller mass of Mars, would lead us to expect. The air at the surface would be thinner than at the surface of the Earth, but the rate at which it diminished with the height above that surface would not be so great. At no very great elevation the two densities would come to be the same.

One deduction from this thin air we must be careful not to make--that because it is thin it is incapable of supporting intelligent life. That beings constituted physically as we are would find it a most uncomfortable habitat is pretty certain. But lungs are not wedded to logic, as public speeches show, and there is nothing in the world or beyond it to prevent, so far as we know, a being with gills, for example from being a most superior person. A fish doubtless imagines life out of water to be impossible; and similarly to argue that life of an order as high as our own, or higher, is impossible because of less air to breathe than that to which we are locally accustomed, is, as Flammarion happily expresses it, to argue, not as a philosopher, but as a fish.

To sum up, now, what we know about the atmosphere of Mars: we have proof positive that Mars has an atmosphere; we have reason to believe this atmosphere to be very thin,--thinner at least by half than the air upon the summit of the Himalayas,--and in constitution not to differ greatly from our own.