422 



THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



mean density is much less than that of our own 

 earth, being only about three and a half times, 

 whereas the earth's is nearly five and three- 

 fourths times the density of water. We may 

 fairly conclude that the crust of the moon is far 

 less compact than that of the earth. Again, it 

 manifestly would require a much smaller force to 

 effect the overthrow of a steep wall of great height, 

 than to produce an up-flow of matter equal even 

 to but one-hundredth part of the wall in mass. 

 Mere shrinkage of the nucleus would account for 

 effects of the former kind, whereas to produce 

 effects of the latter kind energetic expansion and 

 therefore intense heat would be required. 



Apart, however, from any vulcanian forces, 

 forces are known to be at work on the moon, 

 which might fairly account for the overthrow of 

 steep and lofty walls. During the intense heat 

 of the long lunar day, a process of expansion 

 takes place, which must affect the moon's crust 

 to a depth of several hundred yards. Different 

 parts of the moon's surface must be differently 

 affected according to their substance, color, con- 

 tour, and so forth. After day has passed comes 

 the long lunar night, when intense cold affects 

 the moon's surface. The change from a heat 

 surpassing that of boiling water to a degree of 

 cold far exceeding that of our bitterest arctic 

 winters, cannot but produce a steady disintegra- 

 tion of the lunar crust. I observe, indeed, that 

 Prof. Newcomb, in his excellent "Popular Astron- 

 omy," expresses the opinion that these changes 

 of temperature are not sufficient of themselves to 

 produce any effect, though they powerfully reen- 

 force the action of causes due to the existence of 

 air, water, etc., on a planet. Since on the moon, 

 he says, there is " neither air, water, rain, frost, 

 nor organic matter, the causes of disintegration 

 and decay are all absent. A marble building 

 erected on the surface of the moon would remain 

 century after century, just as it was left. It is 

 true that there might be bodies so friable that 

 the expansions and contractions due to the great 

 changes of temperature to which the moon is ex- 

 posed would cause them to crumble. But what- 

 ever crumbling might thus be caused would soon 



2Tew Crater. 



Byginus. 



be done with, and then no further change would 

 occur." This view of the matter is, however, al- 

 together untenable ; expansions and contractions 

 in masses of different substance cannot take place 

 without some degree of friction ; and friction long 

 continued will, however gradually, destroy the 

 strongest material. It is only a question of time. 



Nevertheless, it must be admitted, I think, 

 that if Linne really has undergone the tremen- 

 dous change in which many astronomers believe, 

 some degree of vulcanian energy must have been 

 at work. The mere expansion and contraction 

 of the crust could scarcely have brought about 

 the downfall of a ringed wall nearly seven miles 

 in diameter at once, or (at the outside) in a few 

 weeks. The first impulse toward the process of 

 destruction may have been given in this way ; 

 but sublunarian forces must have helped to pro- 

 duce so complete an overthrow of the great wall 

 which seems to have existed when Lohrmann 

 and Madler made their independent observations. 



Let us turn now to the change which is sup- 

 posed to have more recently taken place on the 

 moon. 



We have to direct our attention to a spot 

 lying near the middle of that face which the 

 moon turns constantly toward the earth. Linne, 

 though not near the edge of the lunar disk, is yet 

 far enough removed from the centre to be con- 

 siderably affected by those apparent swayings to 

 and fro of the moon's globe, which are called her 

 librations. As I have shown in my treatise on 

 the moon (not in the first edition, but in the sec- 

 ond), any irregular surface lying at a considera- 

 ble distance from the centre of the moon's face 

 may be very much changed in aspect in conse- 

 quence of these librations, whereas a region 

 near the centre, though it maybe to some degree 

 affected, must be much less changed than a simi- 

 lar region near the edge. 



Not to trouble the reader with a long descrip- 

 tion of lunar topography, let me thus indicate, 

 sufficiently for my present purpose, the position 

 of the crater which is supposed by many to have 

 recently been formed in the lunar plain called 

 the Sea of Vapors. 



Copernicus. 



o 



o 



Eratosthenes. 



