i52 HISTORY OF PHYSICAL ASTRONOMY. 



Biela bad great merit in the discovery of his Comet's periodicity, hav 

 ing set about his search of it from an anticipation of its return founded 

 upon former observations. 



Also a Comet was discovered by De Vico at Rome on Aug. 22, 

 1844, -which was found to describe an elliptical orbit having its aphe- 

 lion near the orbit of Jupiter, which is consequently one of those of 

 short period. And on Feb. 26, 1846, M. Brorsen of Kiel discovered a 

 telescopic Comet whose orbit is found to be elliptical.] 



We may add to the history of Comets, that of Lexell's, which, in 

 1770, appeared to be revolving in a period of about five years, and 

 whose motion was predicted accordingly. The prediction was disap- 

 pointed ; but the failure was sufficiently explained by the comet's hav- 

 ing passed close to Jupiter, by which occurrence its orbit was utterly 

 deranged. 



It results from the theory of universal gravitation, that Comets are 

 collections of extremely attenuated matter. Lexell's is supposed to 

 have passed twice (in 1767 and 1779) through the system of Jupiter's 

 Satellites, without disturbing their motions, though suffering itself so 

 great a disturbance as to have its orbit entirely altered. The same 

 result is still more decidedly proved by the last appearance of Biela's 

 Comet. It appeared double, but the two bodies did not perceptibly 

 affect each other's motions, as I am informed by Professor Challis ot 

 Cambridge, who observed both of them from Jan. 23 to Mar. 25, 1840. 



O ' 



This proves the quantity of matter in each body to have been exceed- 

 ingly small. 



Thus, no verification of the Newtonian theory, which was possible 

 in the motions of the stars, has yet been wanting. The return of Hal- 

 ley's Comet again in 1835, and the extreme exactitude with which it 

 conformed to its predicted course, is a testimony of truth, which must 

 appear striking even to the most incurious respecting such matters. 39 



7. 



pplication of the Newtonian Theory to the Figure of the 

 Earth. 



THE Heavens had thus been consulted respecting the Newtonian 

 doctrine, and the answer given, over and over again, in a thousand 



' M. do Ilumbolclt (Kosmos, p. 116) speaks of nine returns of Ilulley's Comet, the 

 comet observed in China in 1378 being identified with this. But whether we take 

 1378 or 1380 for the appearance in that century, if we begin with that, we have only 

 seven appearances, namely, in 1373 or 1330, in 1456, in 1531, in 1607, in 1682. in 

 1759, and in 1835. 



