183 . EEPORT--1861. 



Astronomical Society. The remainder, twenty-three, bringing the observa- 

 tions to May 12, 1862, are at present in my hands, and are intended to form 

 part of a second volume, should I be able to pursue the observations. The 

 arrangement of the volume is such that it can be used as an ephemeris of 

 the successive appearances of the crater, as well as being indicative of those 

 objects that require careful and steady watching. 



One of the most interesting objects among those newly pointed out is a 

 terrace on the south-west interior slope. It, with a ravine in the same neigh- 

 bourhood, is of an exceedingly delicate character, being brought out (espe- 

 cially the terrace) by the gradual change in the direction of the incident solar 



Accompanying the synopsis are two illustrative figures. Fig. 1 is a some- 

 what rough key-plan of the crater, the ellipse being that of the greatest open- 

 ing presented by Plato. This key-plan possesses no pretensions either to 

 accuracy of detail or correctness of locality, micrometrically considered; it is 

 only offered as a guide to the general and relative positions of the objects 

 included in the synopsis. Fig. 2 is a section indicated by observation of the 

 south-west interior slope of Plato, showing the terrace or ledge Y, one of the 

 new features brought to light by this series of observations. The reader is re- 

 ferred to Beer and Madler's large map of the moon, and is specially requested 

 to compare the delineation of the crater as they have given it with the key- 

 plan accompanying this licport. A careful comparison of them will show 

 the features they have in common, and the departures that may exist in those 

 determined by the present scries of observations from the representations of 

 the same features as given by Beer and Miidler. Schroter has given some 

 of the features mentioned, especially the mountain-range (w), which he marks 

 H, the mountain r, the shadows of the three peaks y, c, and e, the mountain 

 c, which in Scbroter's drawing is marked D, and the crater x. which is no 

 longer in existence — if Schroter really saw a perfect crater as he has deline- 

 ated it. In another delineation of Plato by Schroter, showing the two mark- 

 ings i and k on the interior of the north-east slope as he observed them on 

 December 11, 1788, he also gives a remarkably round cloud-like appear- 

 ance, not unlike in character to the one that has been so constantly a subject 

 of my own observation, marked/ in the key-plan. These delineations may 

 be found in his ' Selcnotopographische Fragmente,' t. xxi. 



To render the results of the inquiry of greater value, a careful microme- 

 trical survey of Plato, when presented under the greatest visual angle, would 

 be important. Every well-determined spot would be laid down in its accu- 

 rate position as seen from the earth under that angle ; and if such a survey 

 were executed with the requisite precision, one epoch only being fixed on, 

 and no reduction to a mean state of libration admitted, it would not be dif- 

 ficult, after a few years' observations, to judge of the probable fixity of aspect 

 presented by the most prominent features, and changes, if any, would soou 

 render themselves apparent. 



Synopsis of objects in Plato suitable for telescopic observation, tvitk reference 

 to fixity or variability of absolute aspect. 



By absolute aspect, I mean the aspect dependent on the object itself, its 

 form and constitution, — not an aspect dependent on the variability of the 

 incidence of solar light, or on the variability of the direction of the visual 

 ray as the object is seen from the earth, the one indicated by the moon's age, 

 the other by the libration of the moon. 



