The various places on the moon's disk have received appropriate names. The dusky regions being formerly supposed to be seas, were named accordingly; and other remarkable places have each two names, one derived from some well-known spot on the earth, and the other from some distinguished personage. Thus, the same bright spot on the surface of the moon is called _Mount Sinai_ or _Tycho_, and another, _Mount Etna_ or _Copernicus_. The names of individuals, however, are more used than the others. The diagram, Fig. 36, (see page 159,) represents rudely, the telescopic appearance of the full moon. The reality is far more beautiful. A few of the most remarkable points have the following names corresponding to the numbers and letters on the map. 1. Tycho, 6. Eratosthenes, 2. Kepler, 7. Plato, 3. Copernicus, 8. Archimedes, 4. Aristarchus, 9. Eudoxus, 5. Helicon, 10. Aristotle. A. Mare Humorum, _Sea of Humors_, B. Mare Nubium, _Sea of Clouds_, C. Mare Imbrium, _Sea of Rains_, D. Mare Nectaris, _Sea of Nectar_, E. Mare Tranquillitatis, _Sea of Tranquillity_, F. Mare Serenitatis, _Sea of Serenity_, G. Mare Fecunditatis, _Sea of Plenty_, H. Mare Crisium, _Crisian Sea_. The heights of the lunar mountains, and the depths of the valleys, can be estimated with a considerable degree of accuracy. Some of the mountains are as high as five miles, and the valleys, in some instances, are four miles deep. Hence it is inferred, that the surface of the moon is more broken and irregular than that of the earth, its mountains being higher and its valleys deeper, in proportion to its magnitude, than those of the earth. The varieties of surface in the moon, as seen by the aid of large telescopes, have been well described by Dr. Dick, in his 'Celestial Scenery,' and I cannot give you a better idea of them, than to add a few extracts from his work. The lunar mountains in general exhibit an arrangement and an aspect very different from the mountain scenery of our globe. They may be arranged under the four following varieties: First, _insulated mountains_, which rise from plains nearly level, shaped like a sugar loaf, which may be supposed to present an appearance somewhat similar to Mount Etna, or the Peak of Teneriffe. The shadows of these mountains, in certain phases of the moon, are as distinctly perceived as the shadow of an upright staff, when placed opposite to the sun; and these heights can be calculated from the length of their shadows. Some of these mountains being elevated in the midst of extensive plains, would present to a spectator on their summits magnificent views of the surrounding regions.