diameter, with its system of fractures and its central mountains, which rise from 3,000 to 4,000 feet above its floor. This crater slopes southward towards the plain, showing the subsidence to which it has been submitted. While the northern portion of the wall of this crater rises to 10,000 feet, that on the plain is only 500 feet high, and is even wholly demolished at one place where the floor of the crater is in direct communication with the plain. In the lower part of the _mare_, and a little to the west of the middle line, is found the crater Agatharchides, which shows below its north wall the marks of rills impressed by a flood of lava, which once issued from the side of the crater. On the left-hand side of the plain, is seen the half-demolished crater Hippalus, resembling a large bay, which has its interior strewn with peaks and mountains. On this same side can be seen one of the most important systems of clefts and fractures visible on the Moon, these clefts varying in length from 150 to 200 miles. ECLIPSES OF THE MOON PLATE VII Since the Moon is not a self-luminous body, but shines by the light which it borrows from the Sun, it follows that when the Sun's light is prevented from reaching its surface, our satellite becomes obscured. The Earth, like all opaque bodies exposed to sunlight, casts a shadow in space, the direction of which is always opposite to the Sun's place. The form of the Earth's shadow is that of a long, sharply-pointed cone, which has our globe for its base. Its length, varying with the distance of the Earth from the Sun, is, on an average, 855,000 miles, or 108 times the terrestrial diameter. This conical shadow of the Earth, divided longitudinally by the plane of the ecliptic, lies half above and half below that plane, on which the summit of the shadow describes a whole circumference in the course of a year. If the Moon's orbit were not inclined to the ecliptic, our satellite would pass at every Full Moon directly through the Earth's shadow; but, owing to that inclination, it usually passes above or below the shadow. Twice, however, during each of its revolutions, it must cross the plane of the ecliptic, the points of its orbit where this happens being called nodes. Accordingly, if it is near a node at the time of Full Moon, it will enter the shadow of the Earth, and become either partly or wholly obscured, according to the distance of its centre from the plane of the ecliptic. The partial or total obscuration of the Moon's disk thus produced constitutes a partial or total eclipse of the Moon. The essential conditions for an eclipse of the Moon are, therefore, that our satellite must not only be full, but must also be at or very near one of its nodes.