PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 721 



off by the sun, and all luminous bodies with the above velocity, 

 through the empty planetary space ; Huyghens, a Hollander, 

 maintained that it was transmitted like sound, by vibrations in an 

 elastic medium, which fitted the planetary space ; and this last 

 opinion was adopted as the most probable by the ablest philoso- 

 phers, till at last the existence of such a medium was proved in 

 two mutually independent ways. 



Arago devised a method to determine if the vibratory theory of 

 Huyghens was correct, and his ideas were executed b}^ Foucault 

 and Fizeau, in France, and it was established beyond a shadow of 

 doubt that light is propagated like sound, by vibrations, and that 

 consequently a medium must exist in the space through which it 

 raa}^ propagate these vibrations, as sound is propagated in air as 

 its medium. 



The other proof was this : The effect of a medium actually 

 obstructing free motion, was first discovered on a telescopic comet, 

 called after its discoverer the comet of Encke, and has afterward 

 been verified on the comet of Biela, and on most other comets, and 

 is now an established fact. But why are we not able to detect the 

 effect of such a retarding medium on the revolution of the planets ? 

 For two reasons: First, that the planets are so much heavier bodies 

 than the comets, which are light above our conception, thousands 

 of times lighter than hydrogen, which is the lightest substance we 

 know, and is 12,000 times lighter than water ; heavier bodies with 

 much inertia are not apt to be so easily affected by a light resist- 

 ing medium than lighter bodies, having little inertia ; but the 

 principal reason is, that this resistant medium revolves in one 

 plane, probably in the same direction and with about the same 

 velocity as the planets ; as now the comets move in different planes 

 and all possible directions, they must, of course, be affected by the 

 resistant medium. 



This is verified by the motions of the moon, which, in its monthly 

 course around the earth, moves one-quarter of the time, just as the 

 earth, and one-quarter of the time slower, another quarter toward 

 the sun, and in another from the sun. The retardation in the 

 moon's motion has long been doubted by astronomers, and traces 

 of such a retardation formed, till finally the English astronomer 

 Adams (the same who also calculated about the existence of the 

 planet Neptune, discovered by Lc Verrier), has quite recently not 

 only proved that the moon retards in its motion, but he has even 

 measured the amount of that retardation, of which the necessar\^ 



[Am. Inst.J * TT 



