176 



KNOWLEDGE 



[September 1, 1893. 



bayonet-shaped ridge which issues from the S.W. side 

 of the breached and half-eft'aced crater-ring of Janseu 

 (66). At first sight it might pass for a photographic 

 scratch, but the way in whicli it traverses first a tiny 

 crater at the bayonet's point, looping slightly to the crater's 

 shape, and then two sharply defined and larger craterlets 

 well out on the plain — all these, with the " scratch," being 

 in one straight line — leads one to regard it as the raised 

 ledge of a fault or dyke, touched sharply by the morning 

 light. 



Fainter still in its course, though still traceable, is 

 another system of rills to which the eye may be guided as 

 follows : — Nearly m the line of Sabine (65) and Maskelyne 

 (67), and lying at about an equal distance west of these, 

 is the southernmost of two small crater-pits ; a little to 

 the north of the northernmost of the two commences one 

 branch of the main rill, which the eye can follow down- 

 wards until opposite to a low, small, cup-topped hill. A 

 little east of this the branch, with some other still smaller 

 ones, faUs into a somewhat wider straight rill, trending 

 N.E. Following this until nearly opposite to a thhd pit 

 (having a smaller companion a little further west) is the 

 point of junction of another branch rill, which has its rise 

 in some minute flat depressions nearly opposite our starting 

 point, to the south. Leaving this junction, and continuing 

 N.E. on the main rill, we pass — on the left, and quite 

 close at hand — two minute pits in succession, which also 

 join in to our main rill by short channels. A little further, 

 the photo-engraving ceases to mark the course distinctly, 

 but upon the same N.E. line we observe numerous little 

 pits and depressions, which gradually guide the eye to a 

 small crater in the hill-country west of Jansen (66). 



Doubtless, by reference to the original photograph, you 

 will be able to check any error in the foregoing, or point 

 out any features which may be called photographic defects. 



There are other formations of both plains and moun- 

 tains which I will not here touch upon, but which all go 

 towards pro\'ing the theory as to craterlets being on the 

 line of faults, even when these do not at once strike the 

 eye — and there is also some evidence to show that the 

 larger craters have had to do with the pi'oduction of these 

 faults, fissures, rills, ridges, and ledges ; and all that we 

 can see upon the moon or its photographs tends to make us 

 feel inclined to put aside — at least as chief agent — the 

 projectile theory, the ice theory, and every other but the 

 volcanic theory. 



While in a general way agreeing with your observa- 

 tion that the earlier craters " do not present a distinctively 

 rounded appearance, indicating a longer period of 

 weathering," permit me still to say that although few 

 portions of the mountainous regions of the moon show 

 distinct traces of weathering (although on the plains, 

 smoothened and softened outlines prevail in the older 

 parts), yet there are to be found evidences of degradation, 

 of veUing with newer lava and scori*, and even of exten- 

 sive re-fusion from below, sufficient to mark an older date. 



To my eye — possibly prejudiced by great telescope 

 observations — Theophilus (819) is very markedly fresher 

 and sharper in outline, especially interiorly, than the older 

 and more degraded Cyrillus (320), upon which it trenches, 

 and this, again, is still superior in definition to Catharina 

 (321). Moiit degraded and age-softened of all, and, as it 

 were, half re-melted into the crust, is the difi'used fourth 

 great crater-ring beyond, with its central twin-crater of 

 somewhat later date. The order of formation would seem 

 to have begun here upon the course of a great fissure, 

 and the disruptive force to have acted in the curved line 

 indicated by Catharina and Cyrillus, ending with Theo- 

 philus and Miidler, whose newer and whiter debris have 



freshened the whole combined western slope down to the 



level of the Mare Nectaris. 



Pardon the length of this communication. 



Yours obediently, 



Riviera, 15th August, 1893. E. F. Macgeorge. 



— I • I — 



THE SUN AS A BRIG-HT-LTNE STAR. 

 To the Editor of Knowledge. 



Dear Sir, — In response to your courteous invitation to 

 comment upon Miss Clerke's most interesting and valuable 

 paper in the August number of Knowledge, I should like 

 to say that the most important point of the paper appears 

 to me to be the emphasis placed upon the singular spectrum 

 of Mira Ceti, with the suggestion for further observations 

 bearing on the questions which it raises. 



I may, perhaps, be allowed first of all to dift'er from one 

 expression in the earlier part of Miss Clerke's paper, viz., 

 the description of the facul;? as " enormously hot ; hotter, 

 certainly, than the photosphere." This does not appear 

 to me to be proved to be the case. Facuhe are elevations 

 above the photospheric level. They are subject, therefore, 

 to less of the iieneral absorption of the solar atmosphere, 

 that absorption evidenced by the diminution of light near 

 the limb ; they ai-e also subject to less of the selectire 

 absorption evidenced by the Fraunhofer lines. They also 

 have a special luminosity of their own as seen by the 

 reversal of the H and K lines. And yet with all these 

 advantages they are so little brighter than the photosphere 

 that they are ordinarily invisible near the centre of the 

 disc, and are only to be seen near the limb, where the 

 photosphere is seen through a great depth of absorbing 

 atmosphere. It seems to me that, could we view faculaj 

 and photosphere under equal conditions, the probability — 

 indeed, the certainty — is, that the photosphere would shine 

 out as the brighter of the two. 



This brings us to the cru.v of the entire problem, for it 

 accentuates the difference between the spectrum of Mira 

 Ceti and that of the sun. In the sun we trace the lines 

 of hydrogen and the supposed lines of calcium to the same 

 distance above the photosphere. The two elements are 

 therefore intermingled, and at the same temperature. 

 Nevertheless, in the region of the faculie we find the cal- 

 cium lines, rather than those of hydrogen, bright, whilst 

 Mira gives us the precise reverse. 



It seems impossible under these circumstances to adopt 

 the suggestion that in Mira we have an atmosphere of 

 mingled hydrogen and calcium which behaves in an exactly 

 opposite manner, the hydrogen lines being very bright — 

 far up in the ultra-violet, too, sure sign of an exalted 

 temperature — and the calcium lines very dark ; InU the 

 calcium atmosphere eliminating, for the H line of hydrogen, 

 the efi'ect of the radiation of the glowing hydrogen im- 

 mediately below it. If the calcium, rather than the 

 hydrogen, glows in the faculs of the sun, ought it not to 

 do the same in Mira ? 



On the other hand, how can we accept the conclusion 

 from which Miss Clerke tells us "there is no escape"? 

 Is it possible to conceive a star with a photosphere over- 

 laid by a stratum of much more highly-heated hydrogen, and 

 that again by a dense layer of cooler calcium '? If the two 

 gases are intermingled in the sun, why not in Mira ? 

 \Vhat conceivable power keeps the dense, the colder and 

 heavier gas in the upper stratum, and the hotter, lighter 

 gas below ';' 



There are two ways of escape which suggest them- 

 selves to me, but neither of which I feel myself free 

 to accept. The first is, that " the continuity of the liar- 

 monical progression " may, in the case of Mira, be really 

 broken. That the lines of hydrogen in the spectra of 



