ASTRONOMY AND METEOROLOGY. 339 



her motion diflferent points of lier surface are presented to us at 

 different times, wliich is, in fact, the phenomenon of libration. 



Let us suppose that the centre of gravity and figure were both 

 visible to us, then it is plain that when the moon is ahead of her 

 mean place her centre of figure will appear ahead of her centre 

 of gravity, and vice versa. The irregularities of the apparent 

 angular motions of the two centres will be inversely as their dis- 

 tance from the earth. 



The centre of gravity of the moon is supposed to move around 

 the earth in accordance with the theory of gravitation. But it is 

 the motion of the centre of figure which is given by observations. 

 Hence, in this case supposed, the inequalities derived from obser- 

 vation will all be too great in a constant ratio. 



COLOR OF SUN-SPOTS. 



Mr. S. Broughton, on observing with high powers a group of 

 fine sun-spots, one of large size, tried the efi'ect of removing the 

 dark glass, and keeping the eye much beyond the focus of the 

 heating rays and at such a distance that the spot almost filled the 

 apparent field of the eye-piece. The spot at once appeared of a 

 dark blood-red. Thinking this might be from the strong contrast 

 of cohn", he projected the image of the spot on a disc of plaster 

 of Paris attached to the telescope. Under a common pocket mag- 

 nifier, the imao;e was seen to be of a dark blood-red, although tlie 

 observatory was not darkened, and the disc was merely protected 

 from the direct rays by an intervening opaque substance. Should 

 these observations be confirmed, it will confirm the opinion that 

 the spots are not black, but appear so by contrast, as it would 

 seem, from the intervention of the colored glass. 



Interesting information concerning solar physics may be found 

 in *' Comptes Rendus " for July, 1868, by M. Faye. 



COMETS OF 1868. 



Bi-orseri's Comet. — This comet is of the small number whose 

 periodicity is well established. It was first discovered the 26th of 

 February, 1846, by M. Brorsen, in Denmark, and remained visible 

 for about 8 weeks. Soon after it was found that the observa- 

 tions made upon it would be best satisfied by the assumption that 

 it revolved in an ellipse around the sun in about oh years; and in 

 this view the return of the comet September 26, 1851, was pre- 

 dicted. At that date, however, the portion of the heavens in 

 which it was supposed to be moving was unfavorably situated for 

 observation, and accordingly it was not detected. The next re- 

 turn, by theory, would take place in the spring of 1857, to which 

 time the astronomical world looked forward with great interest. 

 The comet was in fact rediscovered by Bruhns, at Berlin, on the 

 night of March 18, 1857, and a little later it was seen and assidu- 

 ously observed by many other astronomers. It followed very 



