A COMPARISON OF THE FEATURES OF THE EARTH AXD THE MOON. 7 



is occupied by broad plains which in their general shape are more nearly level 

 than any equally extensive areas of the land, or, so far as we know, of the ocean 

 floor of the earth, though they are beset with very many slight irregularities. 

 These areas of rough, dark-hued plains are the seas or ma^-ia of selenographers, 

 so termed because of old they were, from their relatively level nature, supposed 

 to be areas of water. These maria occupy about one-third of the visible sur- 

 face. Their height is somewhat less than that of the crust outside of their area. 

 The remaining portion of the moon is extremely rugged. It is evident that the 

 average declivity of the slopes is far greater than on the earth. This is apparent 

 in all the features made visible by the telescope, and it likely extends to others 

 too minute to be seen by the most powerful instruments. Zollner, by a very 

 ingenious computation based on the amount of sunlight reflected, estimates 

 that the average angle of the lunar surface to its horizon is fifty-two degrees. 

 Though we have no such basis for reckoning the average slope of the lands and 

 sea bottoms of the earth, it is eminently probable that it does not amount to more 

 than a tenth of that declivity. This difference, as well as many others, is prob- 

 ably due to the lack on the moon of the work of water, which so effectively 

 breaks down the steeps of the earth, tending ever to bring the surface to a 

 uniform level. 



The most notable feature on the lunar surface is the existence of exceed- 

 ingly numerous pits, generally with ring-like walls about them, which slope very 

 steeply to a central cavity and more gently towards the surrounding country. 

 These pits vary greatly in size ; the largest are more than a hundred miles in 

 diameter, while the smallest discernible are less than a half-mile across. The num- 

 ber increases as the size diminishes ; there are many thousands of them, so small 

 that they are revealed only when sought for with the most powerful telescopes 

 and with the best seeing. In all these pits, except those of the smallest size, and 

 possibly in these also, there is within the ring-wall and at a considerable though 

 variable depth below its summit a nearly flat floor, which often has a central pit 

 of small size or in its place a steep rude cone. When this plain is more than 

 twenty miles in diameter, and with increasing numbers as the floor is wider, there 

 are generally other irregularly scattered pits and cones. Thus in the case of Plato, 

 a ring about sixty miles in diameter, there are some scores of these lesser pits. 

 On the interior of the ring-walls of the pits over ten miles in diameter there are 

 usually more or less distinct terraces, which suggest, if they do not clearly indi- 

 cate, that the material now forming the solid floors they enclose was once fluid 

 and stood at greater heights in the pit than that at which it became permanently 

 frozen. It is, indeed, tolerably certain that the last movement of this material of 

 the floors was one of interrupted subsidence from an originally greater elevation 

 on the outside of the ring-wall, which is commonly of irregular height with many 

 peaks. There are sometimes tongues or protrusions of the substance which 

 forms the ring, as if it had flowed a short distance and then had cooled with steep 

 slopes. 



The foregoing account of the pits on the lunar surface suggests to the 



