152 



KNOWLEDGE, 



[August 1, 18^8. 



Eiphaean Moun- 

 tains and tlie 

 crater Euelides. 



showed considerable ellipticitj'. He assumes that, owing 

 to the flat character of the Saturnian ring about the earth, 

 the mooulets must ha%'e approached the moon approxi- 

 mately in the plane of its equator ; but the fact is not 

 attested by the grouping of the craters in a medial zone. 

 Mr. Gilbert therefore assumes that the axis of the moon's 

 rotation has shifted under the successive impulses of the 

 bombardment, and that the moon's equator has occupied 

 successively all parts of its surface. He assumes that the 

 velocity of impact due to the moon's gravity would be 

 sufficient to melt the rocks of the lunar surface, and that 

 they would during a short period behave as if they were 

 composed of plastic material, but would become hardened 

 before the crater could subside. 



The theory does not at all commend itself to my mind. 

 M. Koche, of Montpellier, showed that a ring about a 

 planet would break up if it extended 

 laeyond a distance of Sixths the radius 

 from the centre of the planet ; and if the 

 density of the planet increased towards 

 its centre, the maximum limit to which 

 a ring could extend would be still further 

 contracted. A moon formed just outside 

 such a ring would have an ellipticity greater 

 than that of an ordinary hen's egg ; and as 

 tidal action carried the moon away from its 

 primary it would gradually approximate to 

 a spherical form. One can hardly conceive that such a 

 change of shape could take place without obliterating 

 scars on its surface ; but there is another objection to the 

 theory, which, to my mind, is even more conclusive. 

 There are upon the moon many lines or strings of small 

 craterlets which fall very evidently into line with one 

 another. If we are forced to treat them as scars upon a 

 target, we must regard their alineation as the result of 

 mere chance distribution ; but the number of such strings 

 precludes any such assumption ; there must therefore be a 

 physical reason for the alineation, and the most obvious 

 assumption seems to be that the craterlets mark out a line 

 of weakness in the crust of the moon and lie along a 

 volcanic fissure or lunar fault. 



There is every gradation La size and in type from the 

 small craterlets or cup-shaped depressions up to the 

 gigantic walled rings, and any theory which professes 

 to account for craterlets must account for the types of 

 crater into which they gradually merge. We therefore 

 seem driven back to the volcanic hypothesis, and have to 

 explain why upon the moon, which is so much smaller 

 than the earth, the volcanic outbreaks have been on so 

 colossal a scale. We are not even in a position to say that 

 the moon is made of similar materials to the earth — indeed, 

 we know that its average density is considerably less, the 

 earth being about 6-60 times as heavy as a similar globe 

 of water, while the moon is only about 3"39 times as 

 dense as water, or, according to L)r. Gill's recent deter- 

 mination, about one per cent. less. We must not, however, 

 conclude from this difference that the moon is made of 

 different materials from the earth, for we know too 

 little as to the behaviour of solids under the enormous 

 pressures that they must be subjected to at even a few 

 miles beneath the earth's surface. The average density of 

 the rocks of which the earth's surface is composed is only 

 about two and a half times that of water, but it does not 

 follow that the central parts of the earth are composed of 

 different and heavier material. The great rigidity of the 

 earth under the tidal strains imposed upon it by the sun 

 and moon points to the conclusion that the solid materials 

 of which the earth is built up are rendered rigid by com- 

 pression, and that the idea of a fluid interior must be 



abandoned. Mr. George F. Becker, of the United States 

 Geological Survey, has recently pointed out that the slags 

 into which most of the stratified rocks of the earth's , 

 surface ' would be reduced by melting, increase Ln bulk 

 on fusion, and are not like iron and water, which expand 

 on solidifying ; consequently, he argues that any crust 

 which formed on the surface of a molten sphere of slag 

 would speedily break up by its own weight and sink, and 

 that the process would go on until the whole mass had 

 been reduced in temperature by such upheavals to near 

 the melting point of the slag. But if the liquid slag, or 

 other materials of which the earth is composed, were 

 capable of being reduced by the pressure of the super- 

 incumbent mass to the solid condition, such upheavals 

 would not take place, and under such circumstances it is 

 possible that the heat of the earth may go on increasing 

 to its centre. 



If below the surface of the earth large masses of highly- 

 heated rock are kept solid by the enormous pressure of 

 overlying rocks, earth movements caused by the cooling 

 and contraction which crumple up the stratified rocks of 

 the surface and give rise to the upheaving of mountain 

 chains, may occasionally take off some of the weight from 

 the rocks beneath, causing the highly-heated rocks to run ' 

 off into the liquid state again and find their way to the 

 surface, causing the phenomena we know as volcanic action. , 



If we adopt this theory of volcanic action, and assume 

 that the moon is made of similar materials to the earth, 

 we shall with lunar gravity only equal to one-sixth of 

 terrestrial gravity need to pass to a depth six times as 

 great upon the moon in order to obtain the pressure neces- 

 sary to solidify liquid lava at a temperature equivalent to 

 that at which it is solidified beneath the earth's surface, 

 and any change of pressure that releases a stratum of rock 

 from the solid to the liquid state would upon the moon 

 release a stratum approximately six times as thick, other 



Thebit 



Hilt 



Straight lilm '. ( liuni' I lictwccn Hie craters Birt and Thebit. 

 From two plioto>;ia|ihs taken by the Krotliers ][eMrv. 



conditions being similar, and would presumably give rise 

 to lava flows on a gigantic scale compared with terrestrial 

 evolutions. Added to these considerations, we must 

 remember that under the feeble action of lunar gravity 

 crater rings and cliffs may be built up of similar material 

 much more steeply upon the moon than at the earth's 

 surface. 



There are many formations upon the moon which do 

 not take the form of crater rings. The Eiphiean Moun- 

 tains, shown in our photograph of May, 1890, is a very 

 good instance to cite. I should like to draw special atten- 

 tion to a curious straight black streak between the craters 

 liirt and Thebit. It is spoken of by Webb as a wall, but 

 it rather seems to be a narrow valley or fault. It is shown 

 on several of the photographs taken by the Brothers Henry. 

 I would also draw the reader's attention to three dark spots 

 on the floor of the crater Alphonsus, which are shown in 

 all good photographs, as well as the curious marking, like 

 a capital G, near to the centre of the Mare Nubium. 



• In an artiek' on the subjeet by Mr. Hci^kerin the Nnrlh Avie.riraii 

 Review for April, 18i(.'!, he states that granite and allied roeks increase 

 in bnlk on fusion by about ten per cent. 



