RESISTING MEDIUM. 195 



the observed places, Encke concluded that the 

 observations could not be exactly explained, 

 without supposing a resisting medium. This 

 comet was again generally observed in Europe 

 in 1825 and 1828, and the circumstances of the 

 last appearance were particularly favourable for 

 determining the absolute amount of the retarda- 

 tion arising from the medium, which the other 

 observations had left undetermined. 



The effect of this retarding influence is, as 

 might be supposed from what has already been 

 said, extremely slight ; and would probably not 

 have been perceptible at all, but for the loose 

 texture and small quantity of matter of the re- 

 volving body. It will easily be conceived that 

 a body which has perhaps no more solidity or 

 coherence than a cloud of dust, or a wreath of 

 smoke, will have less force to make its way 

 through a fluid medium, however thin, than a 

 more dense and compact body would have. In 

 atmospheric air much rarefied, a bullet might 

 proceed for miles without losing any of its ve- 

 locity, while such a loose mass as the comet is 

 supposed to be would lose its projectile motion 

 in the space of a few yards. This consideration 

 will account for the circumstance, that the ex- 

 istence of such a medium has been detected by 

 observing the motions of Encke's comet, though 

 the motions of the heavenly bodies previously ob- 

 served showed no trace of such an impediment. 



