3-873-] Condition of the Moon's Surface. 47 



precise^ in the same proportion that gravity is less at the 

 moon's surface. It is important to notice that, as Mr. Mallet 

 himself points out, his view of the origin of volcanic heat 

 " is independent of any particular thickness being assigned 

 to the earth's solid crust, or to whether there is at present 

 a liquid fused nucleus, — all that is necessary being a hotter 

 nucleus than crust, so that the rate of contraction is greater 

 for the former than for the latter." Moreover, " as the play 

 of tangential pressures has elevated the mountain-chains in 

 past epochs, the nature of the forces employed sets a limit " 

 to the possible height of mountains on our globe. This 

 brings Mr. Mallet's views into connection with " vulcanicity 

 produced in like manner in other planets, or in our own 

 satellite, and supplies an adequate solution of the singular, 

 and so far unexplained, fact, that the elevations upon our 

 moon's surface and the evidences of former volcanic activity 

 are upon a scale so vast when compared with those upon 

 our globe." 



All that seems wanted to make the explanation of the 

 general condition of the moon's surface complete, according 

 to this theory, is the presence of water in former ages, over 

 a large extent of the moon's surface, — unless we combine 

 with the theory of contraction the further supposition that 

 the downfall of large masses on the moon produced that 

 local fusion which is necessary to account for the crateriform 

 surface-contour. It is impossible to contemplate the great 

 mountain-ranges of the moon (as, for instance, the Apen- 

 nines under favourable circumstances of illumination), with- 

 out seeing that Mallet's theory accords perfectly with their 

 peculiar corrugated aspect (the same aspect, doubtless, 

 which terrestrial mountain-ranges would exhibit if they 

 could be viewed as a whole from any suitable station). 

 Again, the aspect of the regions surrounding the great lunar 

 craters — and especially the well-studied crater Copernicus — 

 accords closely, when sufficient telescopic power is employed, 

 with the theory that there has been a general contraction 

 of the outer crust of the moon, resulting in foldings and 

 cross-foldings, wrinkles, corrugations, and nodules. But 

 the multiplicity of smaller craters does not seem to be 

 explained at all satisfactorily ; while the present absence of 

 water, as well as the want of any positive or direct evidence 

 that water ever existed upon the moon, compels us to regard 

 even the general condition of the moon's surface as a problem 

 which has still to be explained. If, however, it be admitted 

 that the processes of contraction proceeded with sufficient 

 activity to produce fusion in the central part of a great 



