that of the earth at nearly thirteen degrees. This comet may be considered 

 as a planet, revolving within the orbit of Jupiter, and nearly in the common 

 plane of the solar system. Its motion is in the same direction as that of the 

 planets. 



In the calculations of Encke for the determination of the movement of this 

 comet, the most scrupulous account was taken of the effects which the planets 

 must produce upon it. Nevertheless, a small discrepancy was found to exist 

 between its observed and computed returns ; and what was sull more remark- 

 able, this discrepancy was of the same nature in every case, so that it is im- 

 possible to suppose that it could have arisen from any casual error of compu- 

 tation or of observation ; since, had it so occurred, it would have affected the 

 result irregularly. We must therefore conclude that this comet does not pre- 

 cisely retrace its course each revolution. It is found, however, that this irregu- 

 larity, from whatever cause it may proceed, does not disturb the plane of the 

 comet's p^ath. It is, in fact, according to the observations and reasonings of 

 Professor Encke, precisely the effect which would be produced if the space 

 through which the comet moves was filled by a subtle fluid, offering a small 

 resistance to the motion of the comet : just as our atmosphere resists the motion 

 of any light body through it. 



The existence of an extremely subtle ethereal fluid which fills the infini- 

 tude of space, has been adopted hypothetically to explain the phenomena of 

 optics. In fact, light itself is, according to the undulatory theory, supposed to 

 consist in vibrations transmitted through such a fluid, just as sound is known 

 to consist in similar undulations transmitted through the atmosphere. Hith- 

 erto this assumed cause for light has been justly regarded as an ingenious hy- 

 pothesis not proved, but which accounts for the various phenomena more fully 

 and satisfactorily than the corpuscular theory, which, being open to the same 

 objection, completely fails when applied to some phenomena of light which 

 recent investigations have developed. If an effect similar to that which has 

 been observed in Encke's comet should be discovered on the approaching re- 

 turn of Halley's comet, and still more, if it be observed on the next return of 

 Biela's comet, the undulatory hypothesis will begin to assume the character of 

 a vera causa ; and that theory of light must, under such circumstances, be con- 

 sidered as established. 



The effect on the return of a comet produced by this resistance, contrary to 

 what might at lirst be expected, is to accelerate it, or to make the actual re- 

 turn anticipate the return as computed on the supposition that the comet moves 



