NOTES TO BOOK VII. 309 



orbit having its aphelion near the orbit of Jupiter, which 

 is consequently one of those of short period. And in 

 the present year (Feb. 26, 1846) M. Brorsen of Kiel dis- 

 covered a telescopic Comet whose orbit is found to be 

 elliptical. 



M. de Humboldt (Kosmos, p. 116) speaks of nine re- 

 turns of Halley'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 1378 or 1380, in 1456, hi 1531, in 1607, 

 in 1682, in 1759, and in 1835. 



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, with- 

 out 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 of Cambridge, who 

 observed both of them from Jan. 23 to Mar. 25, 1846. 

 This proves the quantity of matter in each body to have 

 been exceedingly small. 



(Q.) p. 251. I ought not to omit another class of 

 phenomena in which the effects of the Earth's Oblateness, 

 acting according to the law of universal gravitation, have 

 manifested themselves ; I speak of the Moon's Motion, 

 as affected by the Earth's Ellipticity. In this case, as in 

 most others, observation anticipated theory. Mason had 

 inferred from lunar observations a certain Inequality, in 



