long to appear as an elevation, as the rotation of the planet would carry it in due course from the position r to the position s, and there it would be forced to masquerade as a depression. The same, reversely, would happen to a valley. In order that a depression should appear continuously, there must be a belt of lower level along its circle, and this could not be made visible as in the former case by projection, since projection depends upon difference of level along the same surface contour, not as between adjacent ones. It could, therefore, only be noted by its actual profile,—a very small affair, still further diminished by reason of the angle under which that profile was viewed. The resulting quantity in the case of Mars would be exceedingly minute. We perceive therefore, on the very threshold of our inquiry, reason to doubt the mountainous character of the irregularities. Such inference becomes the more probable on a more detailed investigation, into which we will now enter. This investigation depends upon a very important principle; namely, that if we have, as in this case, a great number of observations, it is possible, by dividing them into classes according to their kind and then taking the mean value of each class, to discover characteristics not otherwise exposed.
Means are very telling things. They are so from the fact of simplifying the effects of the
