serve 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°, and probably less than 1°. 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