Mr. Elger then proceeds to describe his discovery in 1883, in the ring-plain Mersenius, of a cleft never noticed before, and which seems to have been of recent formation.[15] [Footnote 15: The Moon, a Full Description and Map of its Principal Features, by Thomas Gwyn Elger, 1895. Those who desire to read detailed descriptions of lunar scenery may consult, in addition to Mr. Elger's book, the following: The Moon, considered as a Planet, a World, and a Satellite, by James Nasmyth and James Carpenter, 1874; The Moon, and the Condition and Configurations of its Surface, by Edmund Neison, 1876. See also Annals of Harvard College Observatory, vol. xxxii, part ii, 1900, for observations made by Prof. William H. Pickering at the Arequipa Observatory.] We now return to the question of the force of lunar gravity. This we find to be only one sixth as great as gravity on the surface of the earth. It is by far the smallest force of gravity that we have found anywhere except on the asteroids. Employing the same method of comparison that was made in the case of Mars, we compute that a man on the moon could attain a height of thirty-six feet without being relatively more unwieldy than a six-foot descendant of Adam is on the earth. Whether this furnishes a sound reason for assuming that the lunar inhabitants, if any exist or have ever existed, should be preposterous giants is questionable; yet such an assumption receives a certain degree of support from the observed fact that the natural features of the moon are framed on an exaggerated scale as compared with the earth's. We have just observed that the moon is characterized by vast mountain rings, attaining in many cases a diameter exceeding fifty miles. If these are volcanic craters, it is evident, at a glance, that the mightiest volcanoes of the earth fall into insignificance beside them. Now, the slight force of gravity on the moon has been appealed to as a reason why volcanic explosions on the lunar globe should produce incomparably greater effects than upon the earth, where the ejected materials are so much heavier. The same force that would throw a volcanic bomb a mile high on the earth could throw it six miles high on the moon. The giant cannon that we have placed in one of our coast forts, which is said to be able to hurl a projectile to a distance of fifteen miles, could send the same projectile ninety miles on the moon. An athlete who can clear a horizontal bar at a height of six feet on the earth could clear the same bar at a height of thirty-six feet on the moon. In other words, he could jump over a house, unless, indeed, the lunarians really are giants, and live in houses proportioned to their own dimensions and to the size of their mountains. In that case, our athlete would have to content himself with jumping over a lunarian, whose head he could just clear--with the hat off.