SECT. XXXVI. ENCKE'S COMET. 345 



orbits have been traced, their elements were computed 

 from Chinese observations. 



Besides Halley's and Lexel's comets, two others are 

 now proved to form part of our system ; that is to say, 

 they return to the sun at intervals, one of three years, 

 and the other of 6J years nearly. The first, generally 

 called Encke's comet, or the comet of the short period, 

 was first seen by MM. Messier and Mechain, in 1786, 

 again by Miss Herschel hi 1805, and its returns, in the 

 years 1805 and 1819, were observed by other astrono- 

 mers, under the impression that all four were different 

 bodies. However, Professor Encke not only proved 

 their identity, but determined the circumstances of the 

 comet's motion. Its reappearance in the years 1825, 

 1828, and 1832, accorded with the orbit assigned by M. 

 Encke, who thus established the length of its period to 

 be 1204 days, nearly. This comet is very small, of 

 feeble light, and invisible to the naked eye, except 

 under very favorable circumstances, and in particular 

 positions. It has no tail, it revolves in an ellipse of 

 great eccentricity inclined at an angle of 13 22' to the 

 plane of the ecliptic, and is subject to considerable per- 

 turbations from the attraction of the planets, which 

 occasion variations in its periodic time. Among the 

 many perturbations to which the planets are liable, 

 their mean motions, and therefore the major axes of 

 their orbits, experience no change ; while on the con- 

 trary, the mean motion of the moon is accelerated from 

 age to age a circumstance at first attributed to the re- 

 sistance of an ethereal medium pervading space, but 

 subsequently proved to arise from the secular diminution 

 of the eccentricity of the terrestrial orbit. Although 

 the resistance of such a medium has not hitherto been 

 perceived in the motions of such dense bodies as the 

 planets and satellites, its effects on the revolutions of 

 the two small periodic comets hardly leave a doubt of 

 its existence. From the numerous observations that 

 have been made on each return of the comet of the 

 short period, the elements have been computed with 

 great accuracy on the hypothesis of its moving in vacua. 

 Its perturbations occasioned by the disturbing action of 

 the planets have been determined ; and after everything 



