z ^73'] Changes in the Moon's Surface. 501 



very obliquely, there is a continual change of aspect 

 precisely resembling that which can be recognised when 

 certain kinds of rock surfaces, and especially crystalline 

 formations, are viewed under oblique illumination. We 

 know that in such cases the tints vary not only absolutely, 

 but relatively, insomuch that a part which is darker than 

 another with one oblique illumination will be lighter under 

 another and but slightly different oblique illumination. 



It appears to me that no other explanation can reasonably 

 be suggested ; because, in point of fact, we have to choose 

 between the theory that there has been a definite change 

 of surface in this part of the moon, or that the change is 

 only apparent. Now if there has been a definite change at 

 anytime, fresh changes must have restored, either from time 

 to time or definitely, the former condition of the surface. 

 But this seems extremely unlikely; while such a change as 

 Sir John Herschel considered to afford the best explanation 

 of Schmidt's observations may be regarded as one which no 

 subsequent process could so modify as to restore, or nearly 

 restore, the original appearance of the region. For, says 

 Herschel, " the most plausible conjecture as to the cause 

 of" the disappearance noted by Schmidt, '' seems to be the 

 filling up of the crater from beneath by an effusion of 

 viscous lava, which, overflowing the rim on all sides, may 

 have so flowed down the outer slope as to efface its rugged- 

 ness, and convert it into a gradual declivity casting no true 

 shadow." Such a change would doubtless account well for 

 the observed appearances ; but it leaves the restoration of 

 the crater afterwards unexplained. 



It remains to be noticed that recently the crater has been 

 observed again somewhat attentively by Dr. Erck and Mr. 

 Burton. Mr. Birt thus speaks of their observations of 

 Linne : — " During the present year this celebrated object 

 has appeared more of a crater form than for some years 

 previously, at least since 1866. . . . On June 4, 1873, Dr. 

 Erck measured the largest diameter of the elliptical spot, 

 and found the mean of several measures to be equal to 

 4 seconds, or 28,226 English feet, which but little exceeds 

 half the length of the largest diameter as measured by Dr. 

 Huggins. Mr. Burton gives a drawing to scale, which 

 differs very materially from Dr. Huggins's, in the '"'Monthly 

 Notices." In Dr. Huggins's drawing, the small crater is 

 situated in the western part of the white spot, its exact posi- 

 tion being indicated by the following numbers : Length, 35 ; 

 small crater, 5, or one-seventh of the length of the white 

 spot ; west rim of small crater distant from west edge 



vol. in. (n.s.) 3 T 



