78 A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 



of series in extending his conceptions. It is important that this method of 

 inquiry should be applied to the features of the moon, so that we may have a 

 foundation for a sound knowledge as to the categories into which its structures 

 fall, and the limits of these groups. Thus with the group of vulcanoids a close 

 study of their features by making extended series of their forms will be likely 

 to bring out relations and diversities not now understood. Besides these notable 

 structures, the mountains, cones, and ragged peaks, the rills and valleys, the light 

 streaks and blotches of color, all bespeak the same treatment. It may, indeed, 

 be applied to many other groups of objects. 



It is obviously desirable to gather all the information we can concerning 

 the unseen -j^ of the lunar surface. This inquiry I undertook more than 

 thirty years ago, but the task was left incomplete. The method of my inquiry 

 was as follows : on the limb of the moon in the successive extremes of libration 

 so-called mountains appear. Several of these ranges have a continuity which 

 is found only with the ramparts of the great vulcanoids. Of these, beginning at 

 the north pole and passing by the west around the limb, I noted the range west 

 of the Mare Crisium, another near Neper, the Leibnitz range near the south 

 pole, the great range beyond the Doerfel Mountains, and a succession of like 

 ridges down to ten degrees north. These and other fainter undelineated features 

 appear to be resolvable into arcs of circular ramparts, such as enclose the larger 

 vulcanoids. Plotting these as circles, the result was to establish, by fair hypothesis, 

 over a considerable part of the unseen realm, the existence of a topography like 

 that we see. 



Looking closely at the limb of the full moon, observers with good eyes may 

 agree with me in the opinion that certain faint light rays there discernible, 

 though with difficulty, apparently converge to centers on the farther side of the 

 moon. I brought to book enough of these to establish about half a dozen of 

 these centers on the invisible field. A confirmation of these uncompleted 

 observations would reduce the region of the entirely unknown part of the moon 

 to less than one-fifth of its whole surface. I cannot hope to return to this inter- 

 esting task of looking around the edge of the moon, but it appears to be the 

 most interesting of the many inquiries that demand good eyes, and opportunities 

 for observation when the rays are most clearly visible. 



Owing to the difficulty of interpreting objects seen in very oblique con- 

 ditions, the fields within five degrees of the limb have been much neglected. 

 Among the problems there found is that concerning the existence of maria on 

 the margin of the observable part of the surface. Except possibly in the case of 

 the Mare Australe, the surface of such areas is not visible. I have never been 

 certain that I saw the characteristic dark plain of that mapped sea. The ques- 

 tion is whether it be only a little varied ancient portion of the crust or a true 

 mare. It is also a question whether the tips of high peaks are not to fee traced 

 on the other side of the comparative level ; if this be the case, then it is, if a mare, 

 one of small area. The so-called Mare Humboldtianum also needs close atten- 

 tion to determine whether it be a mare or, as it seems to me, an ancient vul- 



