January 26, 1 



NA TURE 



305 



the inclination of the axis of rotation of the moon to the plane 

 of the orbit has undergone important variations. 



In order to understand the mechanism of the depressions 

 which have caused the seas, the phenomena must be seen 

 at different stages of advancement, and for this purpose it is 

 necessary to study in detail how the transition between the great 

 southern cap, which is mountainous, and the plains of the equa- 

 torial region have been effected. This passage may be studied 

 on Plate XIV., of which nearly half, occupied by the Mcr des 

 Niiagss^ indicates numerous obliterated or submerged formations. 

 The traces of the most ancient depressions have naturally dis- 

 appeared by the overflowing of lava, but the more modern 

 depressions on a dry and resisting crust, often allow their con- 

 tour and size to be recognised. Some of these are connected 

 to the central portions of the seas, leaving a great exterior band 

 joining the mountains, which serve at the same time as limit and 

 fulcrum. The two portions of crust thus disjointed have only 

 been able to acquire a relative movement in a vertical direction. 

 The part which remained immovable constitutes, in relation to 

 the other, a sort of raised terrace. Thus must be interpreted 

 the celebrated formation known by the name of "The Straight 

 Wall. " As the same map shows it us a little distance off, the 

 rupture can also be accompanied by a tangential slip. It appears 

 then like a large crevice, similar to those we saw appearing 

 to the east of Hesiod. These fissures may be connected with 

 large portions of the crust, may even divide chains of hills, and 

 their form, generally rectilinear, seems independent of all the 

 small inequalities of the surface. 



It can easily be understood that such crevices rarely acquire 

 dimensions large enough to be visible from our earth. Further, 

 it is only near the boundaries of the seas that they have a chance 

 of remaining open. In the central parts, the submersion of the 

 surface has made every vestige disappear. The zones near the 

 limb are also, by reason of the variations of the interior pressure, 

 subject to encroachments of lava. But these inundations, which 

 are not so frequent, are not uniformly distributed. They give 

 rise to solid accumulations along the crevices from which they 

 issue, and they take the form of swollen nerves. Two of these 

 net-works can be studied in the present — and third — part ; 

 one stretching to the west of Bouillaud (Plate XIV.), the other 

 between Landsberg and Wichmann (Plate XV. ). In this last 

 system we see a large region composed alternately of hollows 

 and hills, as if to enable us to note the change. 



The evidence of volcanic action appears here with an ampli- 

 tude, a clearness, which leaves little to be desired. We have 

 had to content ourselves with a somewhat smaller enlargement 

 in order to comprise in two consecutive pages (Plates XV. and 

 XVI.) the largest portions of these brilliant aureoles, that are 

 seen shining round certain walled plains, as Lalande, Kepler, or 

 Copernicus. It seems to us that the comparison of these two 

 pages is very suggestive. One is convinced by it that the 

 diverging trails, becoming invisible by a very oblique illumin- 

 ation, cannot be interpreted as inequalities of the surface. Inter- 

 secting valleys and mountains without becoming fainter or 

 deviating from their course, they cannot have been produced by 

 subterranean or superficial means. An atmosphere agitated by 

 variable currents seems to be the only cause which can explain 

 the diffusion of tracks to such distances. This hypothesis further 

 agrees with what we know of the extreme tenuity of volcanic 

 dusts, with their capability of remaining for a long time in 

 suspension in very thin air. It is strengthened by the fact of 

 the existence of a relatively dark corona round the principal 

 centres of luminosity. It is possible that in a certain zone 

 round the more recent craters, like the region in the neighbour- 

 hood of terrestrial volcanoes, the largest projectiles, the streams 

 of lava have got mixed in the deposits of cinders, and have not 

 allowed them to remain clearly visible near these orifices. 



Without misunderstanding the evident unity of origin of these 

 tracks of a same system, one might be surprised to see their 

 direction, their size, and their brightness sometimes undergoing 

 sudden changes without clear relation to the distance of the 

 central crater. Two disturbing causes seem to interfere : one is 

 the meeting of high mountains, capable of dividing the atmo- 

 spheric currents, of causing downfalls, and abundant condensa- 

 tion. The other, more frequent and more efficient, is the 

 presence of hollow basins, still liquid at the time the down- 

 falls of cinders took place, u.seless in con.sequence of receiving 

 or keeping superficial deposits. The tracks, therefore, behave in 

 the plains like a very sensitive reagent, being able to disclose 

 by a recrudescence of brightness the smallest unevenness of the 



NO. 1526, VOL. 59] 



surface, and, by a sudden weakening, the slowly solidified 

 lagoons. The comparative examination of a similar region, de- 

 scribed in the Plates XV. and .XVI. under contrary conditicns, 

 furnished numerous facts in support of this idea. 



Are some of these deep basins still imperfectly dry, and will 

 their physical slate consequently be changed by a prolonged 

 exposure to the solar rays ? The green and red tints that are 

 seen in the neighbourhood of the terminator, in the interior 

 of some walled plains, make one think that this is so. The eye 

 being more sensitive in the appreciation of tints, photography 

 has the advantage of the impartial registering of relative luminous 

 intensities, It has, without contradiction, the right to bring its 

 evidence into the question. Plate XVII., which represents a 

 region where the sun is setting, must be compared from this 

 point of view with Plate I., where the sun is rising on the 

 same parts. We find these dark spots of even tone, which 

 in the interval have obviously modified their tints relatively to 

 the neighbouring plateaus. The reality of this change has been 

 confirmed by the examination of a series of cliches arranged in 

 intermediate phases. 



Below we sum up the principal ideas which this third part 

 suggests or confirms, and which one will find developed in the 

 following pages containing the description of the different plates. 

 They are : — 



(i) The explanation of the relative stability and the moun- 

 tainous character of the polar caps. 



(2) The extension of these same characters to every region 

 which form the apparent limb. 



(3) The geometric reason of the approximate coincidence 

 that one .sees between the curves of equal illumination on the 

 disc and meridians. 



(4) The origin of the abnormal recrudescence of luminosity 

 which is shown at the apparent limb in contradiction to calcu- 

 lation. 



(5) The difference of constitution of the two poles seems to 

 indicate that the axis of rotation has undergone great displace- 

 ments in the interior of the planet. 



(6) The cause of the predominance of the seas in equatorial 

 regions. 



(7) The interpretation of the different tints that are apparent 

 in the tracks ; the use of the dark spots to recognise, amongst 

 the sunken basins of the lunar surface, those which have been 

 the last to solidify. 



Results of equal interest can apparently be deduced from the 

 fourth and fifth parts, the materials of which we have in hand. 

 We hope the studious public will not have very long to wait 

 for the rest of this work, for it has indeed been good enough 

 to see in the two first parts an appreciable addition to ourseleno- 

 graphic knowledge. However, we do not doubt that it is 

 possible to do more, and also better ; for if we think we have 

 brought the methods of reproduction to the desired degree of 

 perfection, it is not the case in the execution of direct photo- 

 graphs, which remains, by reason of the habitual movement 

 of the images, a very difficult operation. A single cliche, 

 available for enlarging, represents practically for us the only 

 result of several months' work, and in certain phases our best 

 proofs betray, in a very apparent way, the unsteadiness of the 

 atmosphere. 



OYSTERS AND DISEASE} 

 T^HIS research was commenced three years ago, and has 

 been carried on intermittently in the intervals of other 

 work. 



Preliminary reports on some of our results have been laid 

 before the British Association at the Ipswich, Liverpool, 

 Toronto, and Bristol meetings, and a short paper on one section 

 of the subject was communicated to the Royal Society and 

 printed in the Proceedings last year. In the present paper we 

 give a full account, with illustrations, of the detailed evidence 

 upon which our various conclusions are based. The following 

 is a brief statement of the more important results given in the 

 paper : — 



(I) Although our primary object was to study the oyster 

 under unhealthy conditions, in order to elucidate its supposed 

 connection with infective disease, we found it necessary to study 



1 "Observations upon the Normal and Pathological Histology and 

 Bacteriology of the Oyster." By Profs. W. A. Herdman, F.R.S., and 

 Riibert Boyce. (Abstract of a paper read before the Koyal Society, 

 January 19.) 



