A NEW GRATER IN THE MOON. 



421 



seem to present the crater much as it now ap- 

 pears. It has, however, been generally recog- 

 nized that more reliance can be placed on Lohr- 

 mann's and on Madler's drawings than on Schro- 

 ter's. If Lohrmann and Madler had not definite- 

 ly and independently described Linne, we might 

 hesitate to accept their drawings, and prefer 

 Schroter's, simply as agreeing better with the 

 present aspect of the crater. But it seems diffi- 

 cult to reject their concurrent testimony, while 

 Schroter's drawings have never had much weight 

 with lunarians, or selenographers, as they prefer 

 to be called. It is noteworthy, however, that 

 among Schroter's observations of Linne is one 

 recording that in November, 1*788, the crater was 

 occupied by a dark spot, instead of appearing as 

 usual somewhat brighter than the neighboring 

 regions. 



Are we, then, to believe that a crater some 

 seven miles across, and two or three miles deep 

 (nothing less would correspond with Lohrmann's 

 and Madler's description, " very deep ") bad be- 

 come in some way filled up to within a third of a 

 mile of its lips ? Sir John Herschel not only ac- 

 cepted this stupendous idea, but even went some- 

 what further. " The most plausible conjecture," 

 he said, " as to the cause of this disappearance, 

 seems to be the filling up of the crater from be- 

 neath, by an effusion of viscous lava, which, over- 

 flowing the rim on all sides, may have so flowed 

 down the outer slope as to efface its ruggedness, 

 and convert it into a gradual declivity, casting no 

 stray shadows." This, indeed, was my own notion 

 at the time when Schmidt's announcement was 

 first made, and the present aspect of Linne de- 

 termined. In an article which appeared in the 

 Temple Bar Magazine in 186*7, I even went so far 

 as to say that this was the only explanation avail- 

 able, viz., that a mass of matter had been poured 

 into the crater from below, and had overflowed 

 the barrier formed by the ring-mountain, so as to 

 cover the steep outer sides of the ring. 



Since that time, however, I have had occasion 

 to study closely a number of considerations which 

 I did not then take duly into account. Scarcely 

 any amount of evidence would in my opinion es- 

 tablish the existence of internal forces so tremen- 

 dous as would be implied by the theory I advanced 

 in 1867. Indeed, if it could be proved that the 

 reaction of the moon's interior against her crust 

 is capable of producing such effects as these, it is 

 quite certain that elsewhere much more obvious 

 effects would be exhibited. A crater seven miles 

 in diameter and very deep, so situated above a 

 region of vulcanian activity as to receive into its 



mighty basin sufficient lava to fill it, and, over- 

 flowing its sides, obliterate all traces of the for- 

 mer high walls, would be a safety-valve for such 

 a region. Ordinarily it could not happen that in- 

 ternal forces so tremendous would find so ready 

 an outlet. We should see regions much larger 

 than Linne completely riven and devastated by 

 the action of the moon's internal energies. Un- 

 less, indeed, we supposed that at two or three 

 spots only these tremendous energies are at work, 

 while elsewhere there is comparatively little dis- 

 turbance. But such a supposition is manifestly 

 opposed to all reasonable probability. 



Another explanation, which in 186*7 1 regarded 

 as less probable, or rather less accordant with 

 observed facts, than the theory enunciated above, 

 and subsequently adopted by Sir J. Herschel, 

 seems to me now the only admissible explanation 

 of the change in Linne. The ring-shaped wall 

 around the deep crater had not, I then thought, 

 been destroyed, because, if it had, its fragments 

 and their shadows would remain visible. But I 

 overlooked two things : first, the possibility that 

 the fragments of the destroyed wall would be too 

 small to be separately discernible ; and, secondly, 

 the probability that the downfall of the wall would 

 be accompanied by the development (for a short 

 time) of an intense heat, competent, if not to 

 liquefy, yet to render plastic, the matter which 

 had before formed the base of the wall, in such 

 sort that the fragments (themselves heated by 

 their downfall) would be more or less completely 

 imbedded. Either explanation would suffice to 

 remove the difficulty which I pointed out in 

 1867. 



But it may be asked whether the downfall 

 of a lofty and doubtless massive wall originally 

 surrounding the crater would not itself be in- 

 dicative of the action of tremendous forces of 

 upheaval. It appears to me that the opinion we 

 are to form on this point would depend consider- 

 ably on our estimate of the condition of the lunar 

 crust. If this crust were supposed to be very 

 thick or to rest immediately upon the lunar nu- 

 cleus, then we could hardly imagine that any save 

 some very energetic cause could effect the de- 

 struction of a wall so large as that which former- 

 ly surrounded Linne. But if in the long pro- 

 cesses of cooling and contraction to which both 

 crust and nucleus have been exposed large open 

 spaces have been formed between the nucleus and 

 the crust, in certain regions, we could readily un- 

 derstand that in some cases very moderate vul- 

 canian forces would suffice to overthrow very 

 large masses. Now, we know that the moon's 



