434 RISE AND PKOGRESS OP 



system, which, we have now reason to believe, are less dense than 

 the clouds which float above us ; and it is among these that we 

 must seek the effects of resistance. The singular body, now- 

 known by the name of Encke's comet, was first observed in the 

 year 1786; and has since that time been frequently seen, its 

 periodic time being only about three years and one-third. It was 

 not until after its return in 1819, however, that its identity was 

 established by Encke, and its subsequent returns calculated. This 

 comet reappeared in 1822, and on comparing the observed with the 

 calculated places, Encke found that there was a difference, which 

 seemed to be completely accounted for only on the supposition of 

 a resisting medium. He has been thus led to examine in detail 

 the effects of such a medium on the motion of the comet ; and 

 its subsequent appearances in 1825 and 1828 completely fall in 

 with the supposition. 



The effect of this resistance is indeed very small : according to 

 the observations hitherto made, the diminution of velocity, in ten 

 revolutions of Encke's comet, has not amounted to the one- 

 thousandth part of the entire. And when we proceed from this 

 to speculate upon the probable amount of the effects produced 

 upon the motion of the earth and the other dense bodies of our 

 system, we are compelled to admit it to be so minute, that millions 

 of years must elapse before it becomes sensible. But, however 

 long the time required for the development of its effects, we have 

 here a cause which eventually must put a stop to all the great 

 movements which we observe in the heavens. The earth and the 

 planets must gradually be retarded by the resistance of the medium 

 in which they move : their gravity towards the sun must thus 

 preponderate over the centrifugal force ; and they will gyrate in 

 nearer and nearer orbits, and in continually diminished periods, 

 until at length they reach the central body, and then all motion 

 will be at an end. 



This termination to our system cannot fail to suggest many 

 and grave reflections. Indeed there are few subjects which force 

 so strongly upon the mind the conviction of a ruling Power, as 

 those which indicate the finite nature of all created things which 

 mark the period when the now order of nature did not exist, and 

 predict the time when it shall cease which point, in short, to a 

 beginning and an end. But while our thoughts are thus arrested 

 by the probable termination of our system, the astronomer bids us 



