3G6 . ENCKE'S COMET. SECT. XXXV. 



in the years 1805 and 1819, were observed by other astronomers, 

 under the impression that all four were different bodies. However, 

 Professor Encke not only proved their identity, but determined 

 the eircumstances 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 subject to considerable 

 perturbations 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 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 ter- 

 restrial 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 leave no 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 per- 

 turbations 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 produces the 

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

 bodies do not perform their revolutions in an absolute void, and 

 that, although the medium be too rare to have a sensible effect 

 on the masses of the planets and satellites, it nevertheless has a 

 considerable influence on so rare a body as a comet. Contra- 

 dictory as it may seem that the motion of a body should be 

 accelerated by the resistance of an ethereal medium, the truth 



