SECT. XXXV. BIELA'S COMET. 367 



becomes evident if it be considered that both planets and comets 

 are retained in their orbits by two forces which exactly balance 

 one another ; namely, the centrifugal force producing the velocity 

 in the tangent, and the attraction of the gravitating force directed 

 to the centre of the sun. If one of these forces be diminished by 

 any cause, the other will be proportionally increased. Now, the 

 necessary effect of a resisting medium is to diminish the tan- 

 gential velocity, so that the balance is destroyed, gravity pre- 

 ponderates, the body descends towards the sun till equilibrium 

 is again restored between the two forces ; and, as it then describes 

 a smaller orbit, it moves with increased velocity. Thus, the 

 resistance of an ethereal medium actually accelerates the motion 

 of a body ; but, as the resisting force is confined to the plane of 

 the orbit, it has no influence whatever on the inclination of the 

 orbit, or on the place of the nodes. In computing its effect, M. 

 Encke assumed the increase to be inversely as the square of the 

 distance, and that its resistance acts as a tangential force pro- 

 portional to the squares of the comet's actual velocity in each 

 point of its orbit. Another comet belonging to our system, 

 which returns to its perihelion after a period of 6f years, has 

 been accelerated in its motion by a whole day during one revolu- 

 tion, which puts the existence of ether beyond a doubt, and con- 

 firms the undulatory theory of light. Since this comet, which 

 revolves nearly between the orbits of the earth and Jupiter, is 

 only accelerated one day at each revolution, while Encke's, 

 revolving nearly between the orbits of Mercury and Pallas, is 

 accelerated two, the ethereal medium must increase in density 

 towards the sun. The comet in question was discovered by M. 

 Biela at Josephstadt on the 27th of February, 1826, and ten 

 days afterwards it was seen by M. Gambart at Marseilles, who 

 computed its parabolic elements, and found that they agreed 

 with those of the comets which had appeared in the years 1 789 

 and 1795, whence he concluded them to be the same body mov- 

 ing in an ellipse, and accomplishing its revolution in 2460 days. 

 The perturbations of this comet were computed by M. Damoiseau, 

 who predicted that it would cross the plane of the ecliptic on the 

 29th of October, 1832, a little before midnight, at a point nearly 

 18,484 miles within the earth's orbit ; and as M. Gibers of Bremen, 

 in 1805, had determined the radius of the comet's head to be 

 about 21,136 miles, it was evident that its nebulosity would 



