LEXELL'S COMET. 



and that consequently it could never have been visible. It 

 followed also that, after suffering the disturbing action of Jupiter 

 in 1779, the comet passed into an elliptic orbit whose semi-axis 

 major was 7 '3, that its period was consequently twenty years, and 

 that its eccentricity was such that its perihelion distance was more 

 than twice the distance of Mars, and that in such an orbit it 

 could not become visible. 



40. This investigation was afterwards revised by M. Le Terrier,* 

 which showed that the observations of 1770 were not suffi- 

 ciently definite and accurate to justify conclusions so absolute. 

 He has shown that the orbit of 1770 is subject to an uncer- 

 tainty comprised between certain definite limits ; that tracing 

 the consequences of this to the positions of the comet in 1767 

 and 1779, these positions are subject to still wider limits of 

 uncertainty. Thus he shows that, compatibly with the obser- 

 vations of 1770, the comet might in 1779 pass either consi- 

 derably outside, or considerably inside Jupiter's orbit, or might, 

 as it was supposed to have done, have passed actually within the 

 orbits of his satellites. He deduces in fine the following general 

 conclusions : 



1 . That if the comet had passed within the orbits of the satellites, 

 it must have fallen down upon the planet and coalesced with it ; 

 an incident which he thinks highly improbable, though not 

 absolutely impossible. 



2. The action of Jupiter may have thrown the comet into a 

 parabolic or hyperbolic orbit, in which case it must have 

 departed from our system altogether, never to return, except by 

 the consequence of some disturbance produced in another sphere 

 of attraction. 



3. It may have been thrown into an elliptic orbit, having a 

 great axis and long period, and so placed and formed that the 

 comet could never become visible ; a supposition within which 

 comes the solution of Laplace. 



4. It may have had merely its elliptic elements more or less 

 modified by the action of the planet, without losing its character 

 of short periodicity ; a result which M. Le Yerrier thinks the 

 most probable, and which would render it possible that this 

 comet may still be identified with some one of the many comets of 

 short period, which the activity and sagacity of observers are 

 every year discovering. 



To facilitate such researches, M. Le Yerrier has given a Table, 

 including all the possible systems of elliptic elements of short 

 period which the comet could have assumed, subject to the dis- 



* See Mem. Acad. des Sciences, 1847, 1848. 



169 



