The Lunar Cassini, 147 



neighbourhood of each celestial body. Thi3 idea, however, of 

 the enormous distance of his meteor was merely an optical 

 impression, and however probable it might be, it would be in- 

 teresting to test it by calculating at what distance from the 

 earth the average velocity of shooting stars would cause them to 

 traverse 15' of space in Is. of time. The data for this are not 

 sufficiently certain to lead to an accurate result, because the 

 speed of individual meteors is very variable, but we may make 

 a rough approximation to it. It appears from Professor Grant's 

 recent observations on the November meteor-shower, that an 

 arc of 60° described in 3s. would represent a great proportion 

 of their courses : taking this as a basis we shall find by an easy 

 reckoning that Schroter's meteor had 80 times less apparent 

 velocity and therefore may be supposed to have been at 80 times 

 greater distance. With every allowance for the uncertainty of 

 such a computation it certainly seems probable that he had 

 reason for supposing that it existed in the depths of space, and 

 that it must have been of a self-luminous character. 



We will now, after a long intermission, proceed with our 

 topographical illustration of the lunar surface. The W. extremity 

 of the M. Imbrium was called by Riccioli the Talus Nebularum 

 and the Palus Putredinis, the former lying N.E., the latter 

 S.E., of the No. 16 in our map. These plains are of very level 

 character, and elevations of 250 feet are there of importance, 

 especially in the former, where the terminator forms a clear 

 line, and risings of even 50 feet, if broad enough, could not 

 escape the eye. At the edge of this level, we come upon a very 

 peculiar walled plain, Gassini (20), the omission of which from 

 the maps of Hevel and Riccioli led Schroter to the precarious 

 idea that it was of a subsequent date.* How little confidence 

 can be placed in such a negative argument will presently ap- 

 pear, in a smaller but sharply- defined instance. The ring of 

 Gassini is very narrow, and lowest towards the four cardinal 

 points, but rises on N.W. to nearly 4400 feet above the plain, 

 the interior being very little, if at all, depressed. I have seen 

 it casting three grand obelisks of shade, two very near to- 

 gether, across the external level. It contains, according to B. 



* B. and M. have made it a charge against Schr. that in his anxiety to 

 discover change, he had asserted that Cassini was as obvious (augenfallig) as 

 Aristillus and Autolycus ; and they draw attention to the fact that those craters 

 throw shadows visible even in a comet-finder, at a time when Cassini would benearly 

 imperceptible. What Schr., however, actually says is something quite different; 

 viz., that there is no trice of Cassini in those two maps, though they contain the 

 neighbouring Aristillus anrt Autolycus, which is not so large as Cassini, with the 

 yet smaller Calippus and Tkecetetus, and the border of the M. Imbrium, and that 

 it is not found in the " Phases" of Hevel close to the terminator, though Autolycus 

 and Aristillus are shown under much higher illumination. Judicet Lector. J 



