i873»] Condition of the Moon's Surface. 49 



contour of the surface would not be restored, — either an 

 excess of matter being forced up through the opening or a 

 part of the opening left unfilled, — and that the resulting 

 inequalities could not fail to be rendered discernible under 

 oblique illumination. According to any theory which ac- 

 counted for these features as due to internal forces acting 

 outwards, it was exceedingly difficult to interpret the fact 

 that along the whole length of these rays there can be ob- 

 served a peculiar difference of brightness under direct illu- 

 mination, while, nevertheless, such features of the surface 

 as craters, mountain-ranges, plains, and so on, extend un- 

 broken over the rays. I do not know that the theory of 

 contraction serves to meet the difficulty completely ; in fact, 

 the difference of tint in the rays and the circumstance that 

 the rays can only be well seen under full illumination 

 appear to me to be among the most perplexing of the many 

 perplexing phenomena presented by the moon's surface. 

 But so far as the mere formation of radiations of enormous 

 length is concerned, it seems to me that we have a far more 

 promising interpretation in the theory of contraction than 

 in any theory depending on the action of sublunarian forces. 

 For whenever an outer crust is forced to contract upon an 

 enclosed nucleus, a tendency can be recognised to the 

 formation of radially arranged corrugations. Nevertheless, 

 it may be questioned whether — when this tendency is most 

 clearly recognised — there is not always present some un- 

 yielding matter which forms a centre round which the 

 radiations are formed ; and it is somewhat difficult to see 

 how or why such centres of resistance should exist in the 

 case of the lunar crust. It is a little remarkable that here 

 again we find ourselves led to entertain the notion that 

 matter arriving from without has produced these sublunarian 

 knots, if one may so speak, whose presence is not directly 

 discernible, but is nevertheless strikingly indicated by these 

 series of radiating streaks. 



The circumstance already referred to, that these rays can 

 only be well seen when the moon is full, has long and justly 

 been regarded as among the most mysterious facts known 

 respecting the moon. It is difficult to understand how the 

 peculiarity is to be explained as due merely to a difference 

 of surface-contour in the streaks ; for it is as perplexing to 

 understand how the neighbouring regions could darken 

 from this cause just before full moon, and remain relatively 

 dark during two or three days, as to explain the peculiarity 

 by supposing that the rays themselves grow relatively 

 bright. It is true that there are certain surfaces which 



vol. in. (n.s.) h 



