Decembee 1, 1894.] 



KNOWLEDGE. 



277 



obliterated by the grey material (whatever this may be) of 

 which the floor of Ptolemaus and other similar formations, 

 including the " seas," are composed, which was once most 

 probably of a semi-fluid or viscous consistency, though now 

 probably everywhere consolidated. On the north-western 

 side of the interior stands the only conspicuous object upon 

 it, a bright deep crater about four miles in diameter ; and 

 south of it two large obscure rings, extendmg to the border, 

 one of which in the plate is almost wholly obscured by its 

 shadow. This crater, including its slopes, cannot be less 

 than seven or eight miles across, and thus occupies an 

 area fully as large as Vesuvius, though it does not resemble 

 it in other respects — its flanks being much steeper and the 

 opening on the summit more than double the diameter of 

 the old crater-ring of Somma. Viewed imder a somewhat 

 higher sun than obtained when j\IM. Henry's photograph 

 was taken, the interior of Ptolemaus is seen to contain a 

 large number of small craters ; and, under a still more 

 vertical illumination, to be traversed by a curious arrange- 

 ment of light streaks, the nature of which still remains 

 enigmatical, though the fact that they are invariably 

 associated with small craters, both here and elsewhere, 

 points to an intimate physical connection. 



Among other features in the vicinity, beautifully dis- 

 played in the plate, is a great valley, flanking on the west 

 the bright circular formation Herschel. This elongated 

 gorge, eighty miles in length and in places fully ten miles 

 in width, is one of the finest on the moon, and is, perhaps, 

 only surpassed by two others, one being the valley east of 

 Ukert, and the second the great wedge-shaped depression 

 that cuts through the lunar Alps west of Plato, which is 

 shown with remarkable distinctness in a photograph of the 

 moon two hundi'ed and forty hours old, by MM. Henry, 

 published in Knowledge in December, 1890. Herschel is a 

 typical example of the "ring-plains," by far the most nume- 

 rous of the large crater-like objects on the moon. Here we 

 have a depressed floor, and a nearly circular, continuous and 

 massive wall, surmoimted by peaks considerably loftier 

 than any on the circumvallation of Ptolemaus, which it 

 adjoins. Hipparchus, whose low irregular outline can 

 only be well traced under a rising or setting sun, comes 

 out so plainly that the isolated sections of its apparently 

 ruined border can be seen nearly as well-defined as they 

 are in the telescope under good atmospheric conditions. 



No portion of the moon, excepting in high latitudes in 

 the south-western quadrant, exhibits to such au extent 

 the mutual deformation and interference of formations, or 

 so many departures from circularity, as that under 

 consideration. There is scai-cely a ring-mountain or 

 enclosure of any description that does not appear to 

 have been more or less modified in form by the propin- 

 quity of neighbouring objects. On the northern side 

 of the equator very few examples of this appearance 

 are found, except in the rugged highlands and in the 

 vicinity of the lunar north pole, where the enclosures 

 are more crowded. The region between Albategnius and 

 Purbach is particularly noteworthy and suggestive on this 

 account, as including a great number of small overlapping 

 rings and fragmental craters, many of them grouped and 

 massed together without any apparent governing principle, 

 though on a closer scrutiny a tendency to a more or less 

 linear arrangement in a meridional direction can be traced. 

 This tendency is much more obviously displayed in the 

 curious chain of seven or eight little misshapen ring-plains 

 extending in a sinuous line from the foot of the iilacis 

 of Albategnius, towards Purbach. It can hardly be 

 questioned that, like the smaller crater-rows, so plentifully 

 found in almost every part of the moon, they occupy the 

 site of a long-extending crack or fissure. It wiU also be noted 



that, with two or three exceptions, they have prominent 

 central hiUs, and that the most southerly constituent of 

 the chain consists of at least four inosculating rings of 

 unequal size. 



The sharpness and general excelleuce of MM. Henry's 

 photograph is otherwise manifested by the ease with which 

 many of those delicate cracks or furrows called " rills '" 

 can be followed. The well-known cleft passing through 

 Hyginus, with its local expansions and contractions, is 

 admirably distinct. The more attenuated but longer 

 object of the same class running towards the east from 

 AriadfBus, and the very remarkable group of rills, all of 

 considerable delicacy, situated on the grey plain west of 

 Triesnecker are also plainly traceable. 



Urttcv. 



[The Editor does not hold himself responsible for the opinions or 

 statements of correspondents. I 



> 



OX THE ELECTRIC ORiaiX OF THE SOLAK 

 CHROJIOSPHERE. 



To the Editor of Knowledge. 



Dear Sir, — The October number of Knowled(;e contains 

 the summary of Mr. Evershed's very interesting experi- 

 ments on the question whether gases can become luminous 

 by heat alone. This question is a very difficult, and a very 

 important one too, and the results given by the numerous 

 observers who have studied it are not alike. Two leading 

 spirits, Profs. Helmholz and Stokes, have set forth upon 

 this subject quite different opinions. 



From all the experiments it may be concluded, I believe, 

 that gases can emit from the single elevation of tempera- 

 ture a continuous spectral light, as the soUd and liquid 

 bodies do ; but does light, in the same conditions, show 

 the characteristic lines ? Some doubts are still allowed, 

 for sodium vapour, the only one that has been studied, 

 contains, as a rule, hydrogen in great quantity, and there- 

 fore is not absolutely pure ; besides, the recipient used is 

 either attacked by it, as porcelain, or else is porous for the 

 gases of the flame, as hot iron. 



These objections are raised with a view to obtaining 

 explanations. 



However, Mr. Evershed concludes from his experiments 

 on sodium vapour that all the gases may generally give their 

 characteristic vibrations by heat only, and he immediately 

 applies this result to the solar chromosphere, the light of 

 which might be due to the intense heat of the disc. Mr. 

 Brester explains the chromosphere by chemical combina- 

 tions, but, for my part, I have already owned the necessity of 

 au electric action ('■). As I have got to this result without 

 touching the great question of the luminescence of the 

 gases, I think I must expound it with some details. 



The characteristic lines of the gases are due either to 

 heat (I admit that heat may suffice) or to a chemical action, 

 or to an electric action. But, a priori, chemical and 

 electric actions bring in something more, that is, a 

 chemical combination, an electric and magnetic field, 

 and one understands that they may be attended by 

 special vibrations. This induction is confirmed by all the 

 facts of spectral analysis. With the same body, electricity 

 gives lines that a chemical flame does not show ; moreover, 

 the induction spark does not give the same spectrum aa 

 the electric arc, and the addition of a condenser in the 

 circuit of the spark causes the production of new and 

 special lines. 



(1) Comptes reiidus c/e V Academic de Paris, 27 November, 1893. 

 Knowledge, December, 1893. 



