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sion that it always presents the same face to the sun, as the moon presents the same face to the earth. This view was shared by Mr. Lowell, observing at the Flagstaff Observatory. But the observation is too difficult to permit us to regard the fact as established. All that a conservative astronomer would be willing to say is that as yet we know nothing of the revolution of Mercury on its axis.

Drawings showing the face of Mercury have been made by several astronomers. As it is seen under all ordinary conditions no special features are well marked. Very different is the case at the Lowell Observatory in Flagstaff, Ariz. The most singular feature of its surface in the latter picture consists in the dark lines which cross it. These have not been seen by other observers, and, until they are established by independent evidence, astronomers will be sceptical as to their reality. The reason of this will be stated later in connection with the planet Mars.

Owing to the various positions of Mercury relative to the sun it presents phases like those of the moon. These depend upon the relation of the dark and the illuminated hemispheres relative to the direction in which we see the planet. The hemisphere which is turned away from the sun, being in darkness, is always invisible to us. At superior conjunction the illuminated hemisphere is turned toward us and the planet seems round, like a full moon. As it moves from east elongation to inferior conjunction, more and more of the dark hemisphere is turned toward us, and less and less of the illuminated one. But this