February 9, 1905 J 



NATURE 



349 



of the planet is an incandescent mass which is slowly 

 cooling' and consolidating^ from the surface inward, 

 and is enclosed within a comparatively thin solid crust. 

 Some distinguished physicists, however, have con- 

 tended that the first formed crust would break up, 

 sink down, and be re-melted; and thus that permanent 

 consolidation would begin at the centre, and would 

 gradually extend outwards, until eventually the whole 

 globe became practically solid, with only here and 

 there large vesicular spaces whence active volcanoes 

 are supplied. The densest and least fusible materials 

 would thus tend to accumulate towards the centre, 

 and the lightest and most fusible towards the out- 

 side. The geological belief rests upon a large body 

 of evidence from the structure of the terrestrial crust, 

 which it is difficult or impossible to explain except 

 on the supposition of an internal mass which at least 

 in its outer parts is sufficiently liquid to emerge at 

 the surface as molten lava. The physical argument 

 rests on certain mathematical assumptions the 

 validity of which has been contested. One of these 

 assumptions is that if the interior were liquid, tides 

 would be set up in its mass, and the crust would rise 

 and fall with the passage of the internal tidal wave. 

 Another objection is based on the supposition that 

 huge mountain-chains could not possibly be supported 

 by a thin crust, but would sink down into the interior. 

 More recently the idea has been suggested that the 

 internal core of the earth is gaseous. At the high 

 temperatures and enormous pressures in the interior 

 of the planet, gaseous iron or lava must be more 

 incompressible than steel is at the surface. On the 

 outside of this gaseous mass it is believed that the 

 materials pass into the liquid form or magma which 

 extends as a comparatively thin envelope round the 

 gaseous core, and shades off outward into a solid 

 crust which may not be more than twenty-five or thirty 

 miles in thickness. The most recent earthquake 

 observations have been quoted in support of this view. 



Messrs. Loewy and Puiseux approach the subject 

 impartially from a study of the phenomena presented 

 by the surface of the moon as recorded in a series 

 of photographs. They accept the general belief that 

 our satellite was once a liquid globe, and that traces 

 of its passage from that condition to its present state 

 of consolidation can be clearly recognised. They 

 cannot say whether its temperature increases with 

 depth from the surface, or if there is any variation in 

 density, but they find in their photographs various 

 particulars which, in their opinion, show that the 

 solidification started from the surface. 



The differences of level on the surface of the moon 

 are relatively greater and more abrupt than those on 

 the surface of the earth, and they display in many 

 ways the dynamic effects which a liquid when in 

 movement exerts on its solid containing walls, such 

 as the superficial outpourings which have covered two- 

 fifths of the visible lunar surface and have turned 

 these tracts into continuous plains, round the margins 

 of which numerous remains of the previous relief 

 have been left. Other effects are seen in the traces 

 of instability in the mountain ranges, the fractures, 

 sharply-defined terraces and marginal fissures so 

 often observable. The neighbourhood of a great 

 sheet of liquid material is required to account for 

 the undulations and horizontal displacements which 

 have affected large tracts of the surface, such as the 

 breaking down of the crest of the Apennines, the 

 separation of the rectangular blocks of the Caucasus, 

 and the formation of the rectilinear valleys of Rheita, 

 the Alps, and .'Kriadaeus- 



The most decisive argument in favour of the 

 gradual cooling of the moon from the outside towards 

 the interior appears to be furnished by some facts 

 which are brought out with great clearness by the 



NO. I 84 I, VOL. 71] 



recent photographs. Thus the two French 

 astronomers have satisfied themselves that after the 

 first establishment of a thin crust the inward retreat 

 of the liquid took place gradually, until the fatal 

 moment arrived when it partly lost connection with 

 the overlying solidified crust, so that an intermediate 

 vacant space was left between them. This temporary 

 interval, being filled with gas at a high pressure, 

 formed a cushion which was sufficiently elastic to 

 prevent any falling-in, but was too limited in extent to 

 affect isostatic compensations, so that the internal 

 tides might be developed without endangering the 

 external figure of the moon. When, for some un- 

 known reason, as happens also on our globe, the 

 lunar eruptive forces assumed special vigour, the crust, 

 yielding to the pressures along its least resisting parts, 

 was overflowed by the liquid intq,rior. Such local 

 subsidences gave rise to the great cirques and various 

 other features in the polar region, where the cooling 

 was most rapid, and where, for easily intelligible 

 reasons, the crust reached a considerably greater 

 thickness. But in the equatorial zone, where the tides 

 and the centrifugal force are most powerful, these 

 violent perturbations led to vast subsidences which 

 now form the lunar " seas." The survival of remains 

 of the earlier topographical relief, still visible along 

 the borders of these tracts, bears witness to the nature 

 of the gigantic changes. Each eruptive movement 

 has marked, by the level bottom of the formations, 

 the height of the level of the subjacent liquid. Five 

 such stages in the subsidence of the molten matter 

 are displayed in the photographs. We can under- 

 stand that the process would be repeated with 

 diminishing energy until the gradually thickening 

 crust presented too' great an obstacle to the eruptive 

 action. Vaiious striking examples are cited by the 

 authors; in particular one where the five platforms 

 are separated from each other by a step-like interval 

 of several thousand metres. Had the consolidation 

 begun at the centre of the moon, it is contended, the 

 result would have been altogether different, for then 

 only the latest level should have been seen, and the 

 eruptive forces would have had neither an opportunity 

 of manifesting themselves nor the means of leaving 

 permanent traces at very different stages. 



MM. Lcewy and Puiseux examine the argument 

 from the tides in favour of the consolidation of a 

 planet from the centre outwards, and remark that 

 it must be considered as doubtful, because we do not 

 know how far the coefficient of viscosity or internal 

 friction, which has been employed in the calculations, 

 agrees with the reality. They suggest that as the 

 niaterials in the interior are under enormous pressure 

 they may quite possibly have such viscosity, and 

 yield so slowly to planetary influences, which are con- 

 tinually changing in direction in consequence of the 

 diurnal movement, that no appreciable tidal deforma- 

 tion may result. In the case of the moon it is 

 admitted that the tides in the still liquid mass would 

 for a long time delay the formation of an outer 

 crust, which before its'final establishment must have 

 undergone many violent disruptions, when its broken- 

 up sheets were overflowed by the molten matter from 

 within. But in the course of time it has ended by 

 attaining a great thickness in consequence of con- 

 tinual cooling and the contraction of the outer layers. 



The argument that on the supposition of a com- 

 paratively thin crust the existence of mountainous 

 masses would be impossible is less applicable to the 

 moon, where the force of gravity is six times less 

 than on the earth. But in the opinion of the two 

 French astronomers the argument need not be 

 seriously considered, either for our planet or for our 

 satellite, inasmuch as it depends on a problematic 

 theory which is entirely based on an inaccurate 



