533 



III, 



IHE COMETS. 



THE comets, which Xenocrates and Theon of Alexandria 

 call lif/lit-clouds, and which according to an old Chaldean, 

 belief, Apollonius Myndius considered to "ascend periodi- 

 cally from great distances in long regulated orbits," though 

 subject to the attractive force of the central body, form a 

 peculiar and separate group in the solar regions. They are 

 distinguished from the planets, properly so called, not merely 

 by the eccentricity of their orbits, and what is still more 

 important, their intersection of the planetary orbits ; they 

 also present a variability of figure, a change of outline, which 

 in some instances has been observable during the space of 

 a few hours ; as, for example, in the Comet of 1 744, so accu- 

 rately described by Hensius, and at the last appearance of 

 Halley's Comet in 1835. Before Encke had discovered the 

 existence of interior comets of short periods of revolution, 

 whose orbits were enclosed within those of the planets, 

 dogmatic speculations, founded upon false analogies as to the 

 increasing excentricity, magnitude, and decreasing density in 

 proportion to the distance from the Sun, led to the opinion 

 that planetary bodies of enormous volume would be disco- 

 vered beyond Saturn, revolving in excentric orbits, and 

 " forming an intermediate group between planets and comets, 

 and indeed that the last exterior planet ought to be called 

 a comet, since perhaps its orbit intersected that of Saturn, 



