TRANSACTIONS OF THE SECTIONS. 21 



ing the three works for elucidating the history of any given object, such results as 

 these are frequently obtained. An object is found in Schrciter designated by a 

 Greek or other character, and its appearance described in his text. This object 

 may be altogether omitted by Lohrmann, but given on Beer and Madler's map ; and 

 objects are by no means rare which may be found on Lohrmann, but omitted by 

 Beer and Miidler, and vice versa. 



Were the results of the laboui's of Julius Schmidt during a period of nearly 

 thirty years given to the public, there can be no doubt that our knowledge of seleno- 

 graphy would be greatly advanced. His chart must contain a large proportion of 

 the objects previously recorded by Schroter, Lohrmann, and Beer and Madler ; and, 

 judging from the instances already alluded to, of apparent omissions by one or other 

 of the above-named observers, it is highly probable that the number of such 

 instances would be much increased. The value of his measures (4000) of the alti- 

 tudes of lunar mountains for comparison with or addition to those of Schroter and 

 Madler cannot admit of a doubt. His published catalogue of rills is very valuable 

 in this respect. It is to Schmidt that we are indebted for one of the most import- 

 ant announcements bearing on the subject of lunar activity, that of a change in the 

 crater Linne, "which," says Madler (Report Brit. Assoc. 1868, p. 517), "has 

 hitherto offered the only authentic example of an admitted change." He had pre- 

 viously said (same Report), " what has lately been observed in the crater Linne 

 proves at all events that there real changes have taken place, and that too imder 

 circumstances even visible to us." Further on, however, the great selenographer 

 remarks that on the 10th of May, 1867, his eye having imdergone an operation for 

 cataract, he attempted an observation oi Linne in the heliometer of the observatoi-y 

 at Bonn, and found it shaped exactly, and with the same throw of shadow, as he 

 remembered to have seen it in 1831. '•' The event," he says, " of whatever nature 

 it might have been, must have passed away without leaving any trace observable 

 by me." The doubt still hanging over this object is well known, and it may be 

 regarded as furnishing at least one of the instances of the present state of the ques- 

 tion of activity. The uncertainty attaching to the question of change in this parti- 

 cular instance mainly arises from the diificidty of deciding upon the accuracy or 

 otherwise of the delineations of Lohrmann and Beer and Madler, although both 

 describe it as sho-s^ang a diameter of five or six English miles. Generally speak- 

 ing, the observations between October 1866 and July 1870 all agree in its present 

 appearance, differing greatly from that which it must have presented according to 

 the delineations and descriptions of the two selenographers just named ; also that 

 no change of a physical character has taken place in it during the 3| years it has 

 been under constant observation. 



It has been supposed that photography would solve all such difficulties, and that 

 photograms of the lunar surface taken imder similar angles of illumination and 

 visual ray would agree with each other ; but here, again, precisely the same diffi- 

 culties present themselves which perplexed Schroter, and which have been met 

 with in comparing Lohrmann's and Beer and ^Madler's works. Objects figured by 

 the earlier selenogi-aphers occur on some photograms, but not on others of about 

 the same phase of illumination. There appears to be an agency capable of affecting 

 the visibility of objects, rendering them indistinct or innsible on some occasions, 

 while on others they are distinctly seen on the photograms. Whatever opera- 

 tions may have taken place in the crater Linne, producing phenomena the recur- 

 rence of which is rare, in all the examples above mentioned, from Schroter's time 

 to the present we have phenomena of a different character, exceedingly difficult of 

 explanation, and constituting an important element in the solution of the question 

 of present activity or quiescence ; for unless it be fully proved that all these instances 

 depend upon ch.anges of visual and illuminating angles, a strong suspicion will 

 exist of tneir being more immediately connected with the moon itself. To effect 

 such a proof, however, is a matter of no small difficulty. Madler alludes to the per- 

 formance of calculations of the most varied kind as necessary for the delineation 

 of lunar featiu-es ; and in the case before us the calculation of several elements for 

 each sepa7-ate observation (and they are very numerous) is absolutely essential for 

 the purpose of referring the phenomena observed to changes of illumination and 

 visual ray. Calculations of this kind have not yet been made to any great extent, 



