346 ENCKE'S COMET. SECT. XXXVL 



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 evolutions 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 influ- 

 ence on so rare a body as a comet. Contradictory as it 

 may seem, that the motion of a body should be accele- 

 rated by the resistance of an ethereal medium, the 

 truth 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 center of the sun. If one of these forces be dimin- 

 ished by any cause, the other will be proportionally 

 increased. Now, the necessaiy effect of a resisting 

 medium is to diminish the tangential velocity, so that 

 the balance is destroyed, gravity preponderates, the 

 body descends toward the sun till equilibrium is again 

 restored between the two forces; and as it then de- 

 scribes 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 in- 

 fluence 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 

 squares of the distances, and that its resistance acts as a 

 tangential force proportional to the squares of the 

 comet's actual velocity in each point of its orbit. The 

 other comet belonging to our system, which returns to 

 its perihelion after a period of 6| years, has been ac- 

 celerated in its motion by a whole day during its last 

 revolution, which puts the existence of ether nearly 

 beyond a doubt, and forms a strong presumption in cor- 

 roboration of the undulatory theory of light. Since this 



