SEPTEMBER 21, 1899 | 
NATURE 
493 
the second communication to the Academy, which deals 
with the conclusions which the authors base on the 
photographs. 
| 
(1) There exists, from the point of view of relief, | 
a general similarity between the seas of the moon and | 
the plateaus which are covered to-day by terrestrial 
oceans. 
In these, convex surfaces are more frequent than con- 
cave basins, thrown back generally towards the limit of | 
the depressed space. In the same way, the seas of the 
moon present, generally at the edges, rather pronounced 
depressions. In one case, as in the other, we observe 
normal deformations of a shrinking globe shielded from 
the erosive action of rain, which tends, on the contrary, 
in all the abundantly watered parts of the earth to make 
the concave surfaces predominate. The explanation of 
this structure, such as is admitted to-day by geologists, 
seen’s to us equally valid for the moon. 
(2) In order to find an equivalent resemblance in the 
raised parts of the surface, one ought to be able to estab- 
lish on the moon features effaced by the volcanic erup- 
tions, on the earth those which have disappeared by 
erosion. We can supply this in a certain measure by 
comparing on the one side the lunar ranges relatively poor 
in walled-plains, on the other by terrestrial ranges of recent 
elevation, where the initial structure can be reconstituted 
without too much effort. We then observe, on the chains 
which surround the lunar seas, as on those which enclose 
the Mediterranean basin, the contrast of a rapid interior | 
slope and of a slightly inclined exterior one. This con- 
trast is often so clear on the moon, that the cause may | 
be put down toa rupture of the strata, without waiting 
for any stratigraphical confirmation, which up to the 
present time has not been realised. 
(3) The greater development in the seas of the eastern 
half of the lunar disc shows that the phenomena of de- 
pression must have manifested themselves at an earlier 
period than in the western part. If it were so, one must 
hold that the crust had there imprisoned gases in 
relatively greater quantities, and opposed a smaller re- 
sistance to their expansion. It is, in fact, on the east 
side that the isolated orifices show themselves in greater 
numbers on the surface of the seas, and that the volcanic 
forces have created radial systems stretching in all 
directions. 
The development of these phenomena has necessarily 
required a considerable time, and there is reason to 
admit that these plains, solidified before those of the 
western part of the moon, have long ago reached a 
configuration little different from that which they possess 
to-day. 
(4) The formation of the seas begins by the sinking of 
a vast region, which is soon isolated bya circular fracture. 
This fracture does not generally mark the future limit of 
the sea. We can mention cases where the depressed 
space entirely escapes submersion ; others where the 
central part only is invaded ; others, finally, where the 
primitive enclosure is covered, and where the sea in- 
creases by annexing marginal belts. It is by a series of 
analogous stages that the largest walled-plains seem to 
have arrived at their actual dimensions. 
(5) The epoch of the solidification of a sea does not | 
coincide with that of the positive fixing of the level in the 
central part. This may lower itself still more, and de- 
termine by its retreat the formation of a new crevasse, 
parallel like the first, to the borders of the sea. 
(6) The new photographs, as well as the first, furnish | 
us with several specimens of great walled-plains where | 
the solidification, due to the progressive cooling, has been 
effected at three or even four different levels, separated 
by intervals of several kilometres. The modern de- 
pressions, compared with the ancient ones, are nearly 
| parts of the seas. 
| this gaseous envelope ? 
always less extensive, and have a more rapid interior | 
NO. 1560, VOL. 60] 
° 
slope and a more regular circular form. The more 
modern ones, such as those which open on the bottom of 
| Longomontanus already very depressed, present no 
| trace of the surrounding bulge; that is to say, that 
their appearance does not seem to have been preceded 
by an upheaval. 
(7) Nevertheless, this intumescence phenomenon of 
the lunar crust, considered by us to be the habitual pre- 
liminary of the formation of walled-plains, has in certain 
exceptional but well verified cases given rise to convex 
figures, of which the central part has not sunk. 
(8) We have previously indicated how it was possible. 
in a fairly large number of cases, to assign the relative 
age of the walled plains according to the state of pre- 
servation of their ramparts, and the more or less com- 
plete submersion of their interior cavity. In the parts 
invaded by the trails, we can judge, by another character- 
istic, the epoch of the interior solidification of the 
walled-plain. It is convenient to place in the first line 
and in order of age the plains which have received 
and retained a uniform white covering; then those 
which only present some feeble and late trails, in the 
form of bands ; lastly, those which have remained com- 
pletely clear, and encroach to-day by their sombre tint 
on the neighbouring region. 
This chronological criterion, clearer than that which 
depends on the state of preservation of the ridges, informs 
us also of the relative time of solidification in the different 
Unfortunately, it fails us in the fairly 
numerous regions to which the trails have not extended. 
(9) In general, the great systems of trails cover indis- 
tinctly all the undulations of the soil in their path. This 
circumstance has already permitted us to conclude that 
the formidable volcanic eruptions, of which the moon has 
been the theatre, belong to a recent time in the history 
of our satellite. They must have been preceded by the 
almost complete solidification of the seas, and of the 
bottom of the walled-plains. It seems to us the same 
fact must be taken into consideration in the problem, so 
often discussed, of the atmosphere of the moon. In fact, 
not only have these eruptions set at liberty great quan- 
tities of gas or vapours, but the diffusion of cinders to 
great distances infers a gaseous envelope of a certain 
density. 
It is true that the relative feebleness of gravity helps 
one to understand their initial ascent to a considerable 
altitude. However, the resistance of the atmosphere 
must have been sufficient to retard the fall of this dust 
during its transport over a distance of more than 1000 
kilometres. 
Has the time which has elapsed since the great erup- 
tions sufficed to bring about the total disappearance of 
One is inclined to doubt it, on 
examining the mechanism of the two principal causes 
which could have operated in this direction. The crust, 
already everywhere solidified, could only have absorbed 
the gases slowly and with difficulty. The loss in space 
of molecules with a velocity great enough to carry them 
into the sphere of attraction of another body became of 
necessity less and less in proportion as the temperature 
became lower. We find, therefore, in the examination of 
the lunar surface serious ground to believe that there 
exists, at the present time, a residue of atmosphere 
of which the detection, surrounded as it is with great 
difficulties, may yet be realised. 
This induction adds itself to that which has been fur- 
nished, as we have seen by the discussion of eclipses 
and occultations. The care which astronomers have 
for some years given to the study of these pheno- 
mena, and the great number of occultations of small 
stars which may now be observed at each total eclipse, 
give reason to hope that this discussion may soon be 
resumed on a new basis, and lead to more precise con- 
clusions. 
