Jupiter's Satellites. 103 



hasty investigation can possibly master its difficulties : many 

 of the clefts are extremely minute, and the shadows, by which 

 alone they can be detected, are in proportion fugitive, so as to 

 require watching, literally, almost from hour to hour, in order 

 to trace their continuation and connection. It would be well, 

 too, to check our results in the opposite illumination, though 

 the large clefts, at least, are not so distinct in the waning moon. 



A little way out of our diagram to the N. lies the fine 

 crater Manilius (24) 25 miles in diameter : the broad and 

 peak- and crater-besprinkled ring of which attains in general 

 a luminosity of 8°, and so becomes always a conspicuous 

 object ; it may be perceived even in the lunar night, and was 

 one of IjPs pseudo-volcanoes. The E. side attains 7700, the 

 W. 7500f. (Schr. found 7500 B., 8000 to 9900 W., and the 

 W. wall 2200f. above the plain). 



Between Manilius and Menelaus (15), Schr. has represented 

 a very dark spot of an oval form, to which he gave the name 

 Boscovich. L., though he represents it, seems not to have 

 thought it worthy of a name, and has transferred the appel- 

 lation, without notice (a most unusual procedure with him), to 

 another dark spot half-way between it and Agrijppa, where it 

 has been recognized by B. and M., and is shown in our 

 diagram. From Mr. Births observations it seems that B. and 

 M. have failed in delineating this region (the old site) ; Schr. 

 and L. seem to be nearer the present state of the surface, but 

 the dark tone given by Schr. seems either to be visible only 

 under certain circumstances, or to have subsequently faded. 

 In these districts the flat ring-form so common elsewhere, is 

 supplanted by openings, or intermissions among the parallel 

 ridges universally prevalent here, so that two of their sides 

 only are bordered by a rampart. Of this kind is Julius Gczsar, 

 a large steel-grey depression, of only 1° in reflective power at 

 the N. end, becoming as bright as 3° southwards. In its 

 peculiar aspect Ohacornac traces the effect of a retreating tidal 

 wave, which he thinks may have penetrated it from the neigh- 

 bouring sea ; and he considers this as one of the many 

 evidences of similar action. Several dark valleys unite it with 

 a similar, but much smaller and not quite so dark, formation 

 at some distance N. On its W. side the true crater Sosigenes, 

 14m. broad, and, according to Schr., 3700f. deep, intervenes 

 between it and the M. Tranquillitatis : at a little distance B. 

 is the Boscovich of L., and B. and M., another very dark spot 

 of the same kind. 



JUPITEK'S SATELLITES. 



As the planet Jupiter is now conspicuous, though not so 

 high as desirable, the following particulars of his system will 



