In order to represent in a vivid and accurate manner the character and appearance of the Moon's surface, the construction of a large model of the Moon was undertaken a number of years ago by Th. Dickert, Curator of the Natural History Museum of Bonn, Germany and Dr. J. F. Julius Schmidt, Director of the Observatory of Athens, Greece and an eminent selenographer. The model was presented to the Museum by the late Lewis Reese of Chicago, and is installed at the west end of Hall 35 of the Museum. The model is 19.2 feet in diameter, and is by far the largest and most elaborate representation of the Moon's surface ever made. Its horizontal scale is 1:600,000, one inch on the model equaling 9-47/100 miles on the surface of the Moon, and its vertical scale is 1:200,000, one inch equaling 3-15/100 miles on the Moon. Some characteristic features of the Moon's surface which are especially well illustrated on the model are the following: 1. GRAY PLAINS or "SEAS." These are the darker portions of the Moon's surface as it is seen with the naked eye. They were thought by earlier observers to be seas and were so named. We now know, however, that there is no water on the Moon's surface and that the so-called "seas" are really low-land plains, some of them of vast extent. The Oceanus Procellarum, for instance, covers an area of 90,000 square miles. As seen from the Earth, the plains show a gray-green color, often of varying intensity and sometimes a little bluish in portions. The brightest green color is shown by the area known as Mare Serenitatis. Though appearing perfectly level, a close study shows that these plains have undulating surfaces. They occupy about one-third of the visible surface of the Moon. 2. MOUNTAINS and HIGHLANDS. These constitute the bright portions of the Moon's surface as it is seen with the naked eye. Although these elevated areas are conveniently called mountains, Shaler has drawn attention to the fact that they are unlike those on the Earth since they lack features due to erosion and there is absence of order in their association. The average declivity of their slopes is also much greater than that of the mountains on the Earth. It has been estimated that the average angle of the lunar surface to its horizon is 52°, while on the Earth it does not amount to more than one-tenth of that figure. This difference is probably due to the lack of water on the Moon, the work of which on the Earth tends continually to reduce slopes to a level. Using the term mountains for convenience, however, those on the Moon may be divided into the following classes: