SECT, xxxvi.] ENCKE'S COMET. 395 



absolute void, and that, although the medium be too rare to 

 have a sensible effect on the masses of the planets and satel- 

 lites, it nevertheless has a considerable influence on so rare a 

 body as a comet. Contradictory as it may seem that the 

 motion of a body should be accelerated by the resistance of 

 an ethereal medium, the truth becomes evident if it be consi- 

 dered 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 

 tangential velocity, so that the balance is destroyed, gravity 

 preponderates, the body descends towards the sun till equi- 

 librium 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 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. Another 

 comet belonging to our system, which returns to its perihelion 

 after a period of 6| years, has been accelerated 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 corroboration of 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 Johannisberg on the 



