394 ENCKE'S COMET. [SECT, 



pression that all four were different bodies. However, Pro- 

 fessor 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 favourable circumstances, and in particular positions. 

 It has no tail, it revolves in an ellipse of great excentricity 

 inclined at an angle of 13 22' to the plane of the ecliptic, 

 and is subjecj|to considerable perturbations from the attrac- 

 tion 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 contrary, 

 the mean motion of the moon is accelerated from age to age 

 a circumstance at first attributed to the resistance of an 

 ethereal medium pervading space, but subsequently proved 

 to arise from the secular diminution of the excentricity 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 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 vacuo. Its perturbations 

 occasioned by the disturbing action of the planets have been 

 determined ; and, after everything that could influence its 

 motion had been duly considered, M. Encke found that an 

 acceleration of about two days in each revolution has taken 

 place in its mean motion, precisely similar to that which 

 would be occasioned by the resistance of an ethereal medium. 

 And, as it cannot be attributed to a cause like that which pro- 

 duces the acceleration of the moon, it must be concluded that 

 the celestial bodies do not perform their revolutions in an 



