RESISTING MEDIUM. 103 



cessities. And in the same manner in which 

 observation and calculation thus showed that 

 there could be none but a very rare medium 

 pervading the solar system, it was left open to 

 observation and calculation to prove that there 

 was such a medium, if any facts could be dis- 

 covered which offered suitable evidence. 



Within the last few years, facts have been ob- 

 served which show, in the opinion of some of the 

 best mathematicians of Europe, that such a very 

 rare medium does really occupy the spaces in 

 which the planets move ; and it may be proper 

 and interesting to consider the bearing of this 

 opinion upon the views and arguments which 

 we have had here to present. 



1. Reasons might be offered, founded on the 

 universal diffusion of light and on other grounds* 

 for believing that the planetary spaces cannot be 

 entirely free from matter of some kind ; and 

 wherever matter is, we should expect resistance. 

 But the facts which have thus led astronomers to 

 the conviction that such a resisting medium really 

 exists, are certain circumstances occurring in the 

 motion of a body revolving round the sun, which 

 is now usually called Enckes comet. This body 

 revolves in a very excentric or oblong orbit, its 

 greatest or aphelion distance from the sun, and 

 its nearest or perihelion distance, being in the 

 proportion of more than ten to one. In this re- 

 spect it agrees with other comets ; but its time of 



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