28o 



NATURE 



[July 22, 1897 



THE PHOTOGRAPHS OF THE MOON TAKEN 



AT THE PARIS OBSERVATORY. 

 T^HE second part of the Photographic Atlas of the Moon, 

 published by the Paris Observatory, has just appeared. 

 The photographs were taken at the Paris Observatory, by 

 MM. Lcewy and Puiseux, and the negatives were exhibited at 

 the Academie des Sciences in 1895. The satisfactory reception 

 of the first series has induced IVIM. Loevvy and Puiseux to 

 continue the work, and recently they have contributed two 

 papers to the Academie des Sciences, of which the more recent 

 deals with some new studies concerning the history of the lunar 

 surface. We cannot do better than give a translation of this 

 second communication. 



" We have recently had the honour of presenting to the 

 Academy the second part of the Photographic Atlas of the 

 Moon, published by the Paris Observatory. We endeavoured 

 on this occasion to enumerate, and to describe briefly, the 

 principal objects which are represented therein. The object of 

 this second note is to point out where the new maps seem 

 to us to usefully complete the old ones, and to throw new light 

 on the history of the formation of the lunar crust. 



" We endeavoured before to ascertain what density of atmo- 

 sphere might be conceded to the moon. This density being 

 very feeble, it follows that the surface of our satellite must 

 now be at a low temperature— at least, near the poles. There 

 is even cause to ask if it is not totally or partially covered 

 with ice. The most complete representation of the southern 

 region, which is shown on PI. vi., inclines us to the opposite 

 view — that is to say, that the presence of a great accumulation 

 of ice must be considered improbable, as well for the polar 

 caps as for the equatorial region. One is, therefore, led to 

 imagine that the total moisture on the surface must have dis- 

 appeared, undoubtedly by penetration into the interior of the 

 globe, before the polar regions sank permanently beneath 

 freezing-point. It is simple enough to imagine this great 

 absorptive capacity of the lunar crust for liquids. The cooling 

 of our satellite, more rapid than that of the earth, has shortened 

 the period of condensation of the vapours. The water was 

 filtered as fast as it was formed in innumerable volcanic orifices, 

 which seemed prepared to receive it. PI. vi. gives us an idea 

 of the abundance of these openings in the neighbourhood of the 

 pole, and one is led to think that the same constitution must 

 have existed on the whole of the moon previously to the for- 

 mation of seas. 



" The first pages of the Atlas gave us a fairly large number 

 of rectilinear furrows, running without deviation across moun- 

 tainous spaces, and subjected, in every region, to one or two 

 principal orientations, in such a way as to constitute a sort of 

 network. We have seen that these furrows could be replaced 

 exceptionally by ridges presenting a similar condition. This 

 appearance becomes, on the contrary, very frequent in Pis. vi. 

 and vii., comprised in the present part. We see there signs of 

 a strong lateral pressure, which has obliged two fragments of 

 crust, brought into contact, to straighten their borders, or to 

 encroach on one another. It is easy to see that the elevations 

 thus formed by a local increasing of thickness are an efficient 

 barrier to the formation and regular expansion of the walled 

 plains. These tracings can be followed nowhere better than in 

 the southern region, which is the best preserved of all, and has 

 not participated in the general depressions of the rest of the 

 crust. 



" These depressions, whence the seas originated, are seen by 

 crevices which practically fix the contour limit, and which are 

 visible to us under favourable conditions. PI. viii. shows a 

 curious example of parallelism between these crevices and the 

 prominent ridges which are seen to run great distances on the 

 surface of the seas. This fact, and several others, lead us to 

 think these undulations, in some respects inverse, as having a 

 common origin. 



" In fact, we observe in Pis. ix. and x. mountainous regions 

 modelled on the same plan as their neighbours, offering, in point 

 of view of relief, a perfect continuity with them, but distinguish- 

 ing themselves by a darker colour. The situation of these 

 markings on the edges of the seas gives us reason to believe 

 that they have been occupied temporarily by liquid surfaces, 

 and that these have retreated, before solidification, into more 

 restricted limits. There would thus have been, in the inundated 

 region, change of colour without appreciable alteration of relief. 

 Taking into account the great delicacy of photography for 



NO. 1447, VOL. 56] 



differences of luminous intensity, it seems possible with its 

 aid to distinguish the successive periods in the retreat of the 

 seas of our satellite, as the palaeontologists have successfully- 

 done with our globe. 



" All the varieties of walled plains which we have come across 

 so far are represented in the second part. There are some, 

 like Gassendi and Eratosthenes, showing an irregular interior, 

 and where the movement of the surface, which has brought them 

 to their present state, has left very visible signs everywhere. 

 Others, like Archimedes and Plato, have been invaded by 

 interior overflowings, which have elevated and levelled the 

 bottom. Elsewhere, as in Stadius and Guerike, the side has 

 been corroded and partially destroyed. Copernicus and Aris- 

 tillus, the walls of which rise considerably above the seas, are 

 exceptionally interesting by the intensity of the upheaval, of 

 which they seem to have been the centre, by the violence and, 

 long duration of the eruptions which have taken place within 

 their walls, and which have modified, in a very extended way, the 

 colour of the surface around them. 



" However great the varieties in character of the walled 

 plains, they are not always sufficient to completely explain 

 their age and origin. We are only able to prove in each case 

 the last term of a series of metamorphoses. 



" The more or less perfect state of preservation of the walled 

 plains is, however, an important indication always worth noting. 

 The eruptive walled plains, with elevated borders and central 

 mountains, cannot very well be of a great age. This relative 

 soundness is apparently in favour of a more recent date, and 

 other indications can be added to this. We see, for example, 

 prominent formations, isolated in the middle of the seas, present- 

 ing generally a great regularity, as' if they had been created in 

 a crust, which became more homogeneous by liquid effusion. 

 The absence of parasitic walled plains on their borders seems 

 to show that at the time of their elevation the appearance of 

 volcanic orifices had already become an exceptional thing. The 

 great depression of their interior plains beneath the neighbouring; 

 level indicates a later solidification than that of the seas. The 

 white streams which escape from them and stretch out, remain- 

 ing visible on the surface of the seas, indicate notably Tycho, 

 Copernicus, Aristillus, as having been the seat of great eruptions 

 before all the parts of our satellite were fixed at their present 

 level. 



" The great walls totally effaced, partly destroyed, or filled up, 

 perchance belong to a more remote time, because they have been 

 subjected to greater modifications. The two principal causes, 

 of ruin have been the formation of parasitic orifices, and the 

 overflowing of lava in the interior cavity. This second cause is 

 without doubt that which has produced the most intense effects. 

 By it a number of the great walls have come to be mixed up with 

 the seas, and to possess only an obscure individuality. This 

 slow destruction, which can be recognised in all its degrees, 

 forces us to consider the majority of walled plains as more 

 ancient than the seas. But the part of the local eruptive actions 

 has, without doubt, extended over a much longer period than 

 the overflowing of the lava. The abundance of small orifices, 

 surrounded by white borders, on the elevated plateaus as well as 

 on the actual bed of the sea, leads us to look upon these violent 

 eruptions as one of the most persistent phenomena in lunar 

 history. 



" Perhaps the linear features of the surface of the moon, very 

 much neglected by selenographers in favour of the walled plains, 

 offer, as well as these, a solid basis for establishing a chronology. 

 " In the first place, we will begin with prominent elevations, 

 notably visible near Clavius, which have determined the polygon 

 outline of the walled plains, and offered an almost invincible re- 

 sistance to all the subsequent movements of the surface. By the 

 side of these may be placed the rectilinear furrows in the region 

 of Albategnius, representing, like the elevations, more or less 

 intimate joints between the broken fragments of a primitive 

 crust, but more easily destroyed or masked by recent eruptions. 

 The spacious valleys which are met with near Herschel and 

 Bode, across the Alps, are furrows enlarged by the movement 

 of one of the fragments. 



"The deep rectilinear crevices of Ariadseus and Hyginus, 

 progressively narrowed at their extremities, indicate dislocations 

 which have occurred in a crust already thicker and more 

 coherent. The intercrossing systems, like those of Triesnecher 

 and Ramsden, reveal the antagonistic influence of gravity and 

 eruptive elevations. 



" The parallel fissures which run to the outskirts of the 



