GENEEAL DESCRIPTION OF THE MOON. 109 



the moon is extremel}^ rug-ged. 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 52 degrees. Though we have no such basis for reck- 

 oning 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 probably 

 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 

 exceedingh" numerous pits, generally with ring-like walls about them, 

 which slope very steeply to a central cavity and more gentl}^ toward 

 the surrounding country. These pits vary greatly in size; the largest 

 are more than a hundred miles in diameter, while the smallest discerni- 

 ble are less than a half mile across. The number 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 })it of small size or in its place a steep, rude 

 cone. When this plain is more than 20 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 60 miles in diameter, there are some scores of these lesser pits. 

 On the interior of the ring walls of the pits over 10 miles in diameter 

 there are usually more or less distinct terraces, which suggest, if the}' 

 do not clearly indicate, that the material now forming the solid floors 

 they inclose 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 sub- 

 stance 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 reader that these features are volcanoes. That view of their nature 

 was taken by the astronomers who first saw them with the telesco|)e 

 and has been generall}^ held by their successors. That they are in 



