504 



NA TURE 



[January 26, 1S99 



that the wiilih of ihc swarm had increased very considerably 

 since 1866. 



In I'ola, on the night of the iSth, the meteors seem to have 

 been well seen, eighty-three being observed, thirty-four of which 

 were Leonids. The maximum of the display is stated to have 

 taken place at i6h. 48m. Pola M.T., the radiant point de- 

 duced from twenty-two of the best observations being 

 K.A. loh. 12m. -h 26"'5. 



STUDIES OF THE LUNAR PHOTOGRAPHS 

 TAKEN WITH THE LARGE EOUATORIAL 



couDi:} 



T^HE new part of the " Atlas of the Moon," which we 

 offer to the public to-day, presents many points of com- 

 parison with sections previously published. But, on account 

 of changes of the moon's age and her libration, the portions 

 common to both serve no double purpose. The comparison 

 of the negatives, as we shall see more especially by studying 

 Plates X\'. and .W'l., enables us to interpret some dark mark- 

 ings, to establish a finer distinction between the unevenness of 

 the surface and the variations from the ordinary colour, and 

 finally to determine more precisely the points where the reality 

 of a periodic change may be presumed. 



Plate < is the third in our publication, in which the whole 

 of the moon, visible simultaneously from the earth and sun, is 

 represented. This portion is here much more restricted than 

 has already appeared in Plates a and b. The new sheet is, in 

 consequence, less rich in detail, but derives a particularly expres- 

 sive appearance, due to the rapid change from shadow to light. 

 It is naturally the general features of the physiognomy of our 

 satellite, the laws of distribution of plains and mountains, which 

 can be usefully studied on these unenlarged pictures. We call 

 special attention to the polygon form of the Mer des Crises, 

 which is the best defined of all the dark plains of the moon ; 

 to the traces of progressive depressions which are presented 

 in its central portions ; to the large rectilinear valleys which 

 are visible in the southern part of the disc ; to the alignment 

 following a meridian of the four most important circles of the 

 southern horn ; to the existence of a long series of dark spots 

 on another meridian near the limb. We find altogether, in 

 this part of the moon, a well-marked relation between the local 

 appearance of the surface and longitude. This relation is in- 

 teresting to note, owing to the probability of its being connected 

 with the mechanical work of tides caused by the earth. 



The three unenlarged proofs published as yet demonstrate 

 several important facts, which are confirmed by a number of 

 cliches in our possession. They are : — 



(i) A nearly continuous progression of light, extending from 

 the terminator to the illuminated limb, approximately coincident 

 with the curves of equal illumination and the meridians. 



(2) A recrudescence of illumination in the neighbourhood of 

 the poles, and principally of the .South Pole. 



(3) A marked increased of luminous intensity in the im- 

 mediate neighbourhood of the limb. 



A satisfactory theoretical explanation is possible for the first 

 law, if the moon be regarded as a uniform globe in its super- 

 ficial constitution, without appreciable atmosphere, beyond the 

 state of exercising a sensible specular reflection, and reflecting 

 indifferently in every direction the light received. Under these 

 conditions the formula which expresses the relative intensity, 

 and which will be found in the note relating to Plate 1, indicates 

 that the curves of equal illumination are the meridians, and that 

 the intensity must increase from the terminator to the limb. 



The slight exception to this rule, which is apparent near 

 the poles, already attracted our attention when we described 

 Plates I., II. and \ I. We there saw the proof that these 

 portions of crust very soon became solidified and rapidly ac- 

 quired a great thicknc-^s, liy this the polar caps have escaped 

 the inundations originating from the interior, which have changed 

 the appearance of the cijuatorial region. They are found to 

 receive more rapidly the deposits of white cinders of the volcanic 

 period, chiefcau.se of the difterencc of lints which we observe 

 to-day. 



One may attempt to go further back, and to account for the early 

 solidification of the polar regions. It is evident, to begin with, 



(Published by the 



1 Tr.inslation of a p,iper by MM. Lu.-wy .ind Pu 

 *aris Oljscrvatory.) 



NO. 1526, VOL. 59] 



that the cooling must progress more quickly under the influence 

 of a less efficient solar radiation. We will add that the tides of 

 terrestrial origin caused smaller oscillations than in the equatorial 

 regions, and having a slower velocity of propagation. The 

 congealing of the superficial scoria is, therefore, much more 

 easily effected near the poles. 



The third fact, that is to say the abnormal increase of lumin- 

 osity near the limb, merits more attention, because it is not the 

 result of the mode of operation adopted, which, on the contrary, 

 would be of a nature to weaken it. It shows itself in every 

 latitude, and in all phases. In particular, the photographs 

 taken during the partial eclipse of January 7, 1S9S, show that 

 the increase of intensity on the edge is still very appreciable at 

 opposition — that is to say, at the moment when calculation »ould 

 assign a uniform brilliancy to the lunar disc. 



It seems that no purely geometrical theory accounts for this 

 appearance if it is not supposed that it is really connected with 

 the physical .state of the surface — that is to say, that not only the 

 polar caps, but all the regions which form the apparent "con- 

 tour " of the moon are, collectively, of a lighter colour than the 

 other parts of the disc. 



Here, again, the tides of terrestrial origin, already studied 

 from other points of view by MM. Faye and Poincarc, 

 appear to have played an essential part. Their character is 

 entirely modified, since the day when the rotation of the moon 

 upon its axis was equal to the period of revolution. The 

 periodical flow, which formerly invaded the whole equatorial 

 region, finished by accumulating in the portion of the disc which 

 the earth sees at the present day near its zenith. Besides this 

 our globe, still incandescent, was really a source of great heat for 

 its satellite. The regions near the limb have therefore entered, in 

 their turn, into this period of low temperature and relative calm 

 which is favourable to the consolidation of the polar regions. 



The real characters of the high latitudes, already notified 

 in the previous part, can be more completely studied here on 

 Plates XIII. and XVII. The latter shows us the South Pole 

 covered with mountains, varying in height from 6000 to 7000 

 metres, the highest that have been measured on the moon. 

 These depths are not entirely due to the hollow of the walled 

 plains. Although very numerous, sufficient space is left between 

 them to allow us to judge of what was the previous contour. It 

 can be seen that it comprised very high ridges which the craters 

 have encroached on without destroying, and without themselves 

 losing the regularity of their contour. There have thus beei> 

 formed between the different parts of the same enclosure, differ- 

 ences of level which mount up to 1500 or 2000 metres. The 

 most elevated points, which seem to correspond to a very thick 

 crust, and capable of oflering great resistance are, on the con- 

 trary, often full of little craters. The general appearance of the 

 region gives rise to the thought, as we have already said of- 

 Plate VI., that there does not exist a covering of ice at the 

 pole-s, and that it has not produced an active erosion there. 



If we now consider Plate XIII. in the neighbourhood of the 

 North Pole, we see the walled plains occupying a still more 

 secondary place. Here the seas advance to very high latitudes. 

 Long mountainous masses exist between them, as, for instance, 

 the Alps and the Caucasus. These ranges, situated at a higher 

 level than that of the seas, are strewn with summits presenting 

 well-marked alignments, but no sign of ramified valleys, and very 

 little circular formation. They are broken up into several frag- 

 ments by rectilinear fissures, of which the great valley of the Alps 

 constitutes the most celebrated and best example. The portions 

 thus separated seem to have undergone sliding movements in 

 relation to one another. Considerable diflerence of level is 

 manifested in one massif, in such a manner that it ends at one 

 side by a very high and very steep descent, while the other de- 

 scends insensibly to the seas. The signs of primitive level have 

 no other common feature with the terrestrial mountains than 

 their great relative altitude, and prolonged atmospheric agencies 

 would be necessary to make them acquire new features of re- 

 semblance with them. If the North Pole is approached, it will 

 be seen that at the surface a net-work of furrows are formed in 

 such a manner as to produce rectangular basins. The higher 

 the latitude, the more important these movements of the ground 

 become ; and it is credible that, if we could sue past the 

 apparent contour of the moon, we would observe a relief com- 

 parable with that of the South Pole. The undeniable difference 

 which to-day exists between the appearance of the two poles is 

 favourable to the views of G. 11. Darwin and other geometers, 

 who estimate, for reasons derived from celestial mechanics, that 



