estimate to about three miles and a quarter. Herschel attacked the problem, armed with a telescope of six feet eight inches focal length, which he speaks of as "a very excellent instrument, equal to any that was ever made." He brought to it also the same "uncommon diligence and attention," which made up in some measure for the imperfect instruments of previous astronomers; and he had confidence in himself, in his eyesight, and in the goodness of the work he had done.
He was struck by the "deep shadows" cast by mountains on the moon's surface. Probably these shadows were then a puzzle to him. But he made one sagacious observation, which subsequent observers have developed into a view of the moon's face altogether different from what he started with. On Mona Lacer he writes: "I am almost certain there are two very considerable cavities or places where the ground descends below the level of the convexity, just before these mountains." The moon's face is now known to be pitted with hollows of great extent and depth. Herschel's predecessors called them seas and oceans, of which there are none on the moon. The hills and mountains that rise from these vast cavities do not at the utmost greatly exceed the estimate come to by Herschel, a mile and a half, or a mile and three-quarters in height. But if the height be reckoned above the hollow from which they rise, it may be nearer three times as much. We count the heights of mountains on the earth from the level of the sea. If we reckoned from the bottom of the ocean, our mountains will be found considerably to exceed in height those of the moon. It is now known that these
