The Surface of the Moon. [Illustration: Fig. 130.] 112. _Dusky Patches on the Disk of the Moon._--With the naked eye, large dusky patches are seen on the moon, in which popular fancy has detected a resemblance to a human face. With a telescope of low power, these dark patches appear as smooth as water, and they were once supposed to be seas. This theory was the origin of the name _mare_ (Latin for _sea_), which is still applied to the larger of these plains; but, if there were water on the surface of the moon, it could not fail to manifest its presence by its vapor, which would form an appreciable atmosphere. Moreover, with a high telescopic power, these plains present a more or less uneven surface; and, as the elevations and depressions are found to be permanent, they cannot, of course, belong to the surface of water. The chief of these plains are shown in Fig. 130. They are _Mare Crisium_, _Mare Foecunditatis_, _Mare Nectaris_, _Mare Tranquillitatis_, _Mare Serenitatis_, _Mare Imbrium_, _Mare Frigoris_, and _Oceanus Procellarum_. All these plains can easily be recognized on the surface of the full moon with the unaided eye. 113. _The Terminator of the Moon._--The terminator of the moon is the line which separates the bright and dark portions of its disk. When viewed with a telescope of even moderate power, the terminator is seen to be very irregular and uneven. Many bright points are seen just outside of the terminator in the dark portion of the disk, while all along in the neighborhood of the terminator are bright patches and dense shadows. These appearances are shown in Figs. 131 and 132, which represent the moon near the first and last quarters. They indicate that the surface of the moon is very rough and uneven. [Illustration: Fig. 131.] [Illustration: Fig. 132.] As it is always either sunrise or sunset along the terminator, the bright spots outside of it are clearly the tops of mountains, which catch the rays of the sun while their bases are in the shade. The bright patches in the neighborhood of the terminator are the sides of hills and mountains which are receiving the full light of the sun, while the dense shadows near by are cast by these elevations. 114. _Height of the Lunar Mountains._--There are two methods of finding the height of lunar mountains:-- (1) We may measure the length of the shadows, and then calculate the height of the mountains that would cast such shadows with the sun at the required height above the horizon.