may be safely assumed that we cannot perceive the outline of an object whose average breadth subtends a less angle than a minute. Arago put this question into another shape:—The moon is distant from us 237,000 miles (mean). A magnifying power of a thousand would show us the moon as if she were distant 237 miles from the naked eye. 2000 would bring her within 118 miles. 4000 ” ” ” 59 ” 6000 ” ” ” 39 ” Mont Blanc is visible to the naked eye from Lyons, at the distance of about 100 miles; so that to see the mountains of the moon as Mont Blanc is seen from Lyons would require the impracticable power of 2500. CHAPTER VII. TOPOGRAPHY OF THE MOON. It is scarcely necessary to seek the reasons which prompted astronomers, soon after the invention of the telescope, to map the surface features of the moon. They may have considered it desirable to record the positions of the spots upon her disc, for the purpose of facilitating observations of the passage of the earth’s shadow over them in lunar eclipses; or they may have been actuated by a desire to register appearances then existing, in order that if changes took place in after years these might be readily detected. Scheiner was one of the earliest of lunar cartographers; he worked about the middle of the seventeenth century; but his delineations were very rough and exaggerated. Better maps—the best of the time, according to an old authority—were engraved by one Mellan, about the years 1634 or 1635. At about the same epoch, Langreen and Hevelius were working upon the same subject. Langreen executed some thirty maps of portions of the moon, and introduced the practice of naming the spots after philosophers and eminent men. Hevelius spent several years upon his task, the results of which he published in a bulky volume containing some 50 maps of the moon in various phases, and accompanied by 500 pages of letter-press. He rejected Langreen’s system of nomenclature, and called the spots after the seas and continents of the earth to which he conceived they bore resemblance. Riccioli, another selenographer, whose map was compiled from observations made by Grimaldi, restored Langreen’s nomenclature, but he confined himself to the names of eminent astronomers, and his system has gained the adhesion of the map-makers of later times. Cassini prepared a large map from his own observations, and it was engraved about the year 1692. It appears to have been regarded as a standard work, for a reduced copy of it was repeatedly issued with the yearly volumes of the _Connaissance des Temps_, (the “Nautical Almanac” of France) some time after its publication. These small copies have no great merit: the large copper plate of the original was, we are told by Arago, who received the statement from Bouvard, sold to a brazier by a director of the French Government Printing-Office, who thought proper to