PORTION OF THE COSMOS. COMETS. 403 



of May, 1454. (See Jacobs in Zach's Monatl. Corresp. 

 Ed. xxiii. 1811, S. 196202). 



The facts relative to LexelFs comet and Jupiter's satel- 

 lites, i. c. the disturbances which it sustained from them 

 without sensibly influencing their periods of revolution 

 (Kosmos, Bd. i. S. 117; English edit. Vol. i. p. 103), 

 have undergone more accurate investigation by Le Verrier. 

 Messier discovered this remarkable comet on the 14th of 

 June, 1770, as a faint nebula in Sagittarius; but eight 

 days later its nucleus shone already like a star of th 

 2d magnitude. Previous to the perihelion no tail was 

 visible ; but afterwards one developed itself, by slight ema- 

 nations, to a length of barely 1. Lexell found for his 

 comet an elliptic path, and a period of revolution of 5*585 

 years, a result which was confirmed by Burckhardt in his 

 excellent prize memoir of 1806. According to Clausen it 

 approached the Earth, on the 1st of July, 1770, within 363 

 semi-diameters of the Earth (311000 German, or 1244000 

 English geographical miles, or six distances of the Moon 

 from the Earth). That the comet should not have been 

 seen earlier (March 1776), and later (October 1781), has, 

 in accordance with Lexell's previous conjecture, been made 

 out analytically by Laplace, in the 4th volume of the Me- 

 canique celeste, as the effect of perturbations proceeding 

 from the direction occupied by Jupiter's system at the time 

 of the approximations, in the two years 1767 and 1779. 

 Le Yerrier finds that, according to one hypothesis respecting 

 the comet's path, it should have passed, in 1779, through 

 the orbits of the satellites of Jupiter ; arid that, according 

 to another hypothesis, it should have passed far outside 

 the 4th or outermost satellite ( 663 ). 



