To these strange formations we have scarcely the faintest analogy on earth. Their multitude will at once strike even the most casual observer. Galileo compared them to the 'eyes' in a peacock's tail, and the comparison is not inapt, especially when the moon is viewed with a small telescope and low powers. In the Southern Hemisphere particularly, they simply swarm to such an extent that the district near the terminator presents much the appearance of a honeycomb with very irregular cells, or a piece of very porous pumice (Plate XIV.). Their vast size is not less remarkable than their number. One of the most conspicuous, for example, is the great walled-plain Ptolemäus, which is well-placed for observation near the centre of the visible hemisphere. It measures 115 miles from side to side of its great rampart, which, in at least one peak, towers more than 9,000 feet above the floor of the plain within. The area of this enormous enclosure is about equal to the combined areas of Yorkshire, Lancashire, and Westmorland--an extent so vast that an observer stationed at its centre would see no trace of the mountain-wall which bounds it, save at one point towards the West, where the upper part of the great 9,000-feet peak already referred to would break the line of the horizon (Plate XIX., No. 111; Plate XIII.). Nor is Ptolemäus by any means the largest of these objects. Clavius, lying towards the South Pole, measures no less than 142 miles from wall to wall, and includes within its tremendous rampart an area of at least 16,000 square miles. The great wall which encloses this space, itself no mean range of mountains, stands some 12,000 feet above the surface of the plain within, while in one peak it rises to a height of 17,000 feet. Clavius is remarkable also for the number of smaller craters associated with it. There are two conspicuous ones, one on the north, one on the south side of its wall, each about twenty-five miles in diameter, while the floor is broken by a chain of four large craters and a considerable number of smaller ones.