5* COMETS. [Lesson xiz. 



Nor indeed is there any occasion for apprehension 

 that they should at all injure the earth we inhabit. 

 The probability is some millions to one against a 

 cometary and a planetary body corning into con- 

 tact : and even the tail of a comet cannot come 

 near our atmosphere, unless the Comet be at its 

 inferior conjunction very nearly at the time when, 

 it is in a node j circumstances extremely unlikely 

 to happen together. 



Among the various opinions which have been 

 entertained concerning the nature of Comets, that 

 of NRVVTON was, till Jatdy, most generally adopt- 

 ed. According to his hypothesis, Comets consist 

 of a very compact, durable, and solid substance, 

 capable of bearing most exceedingly great degrees 

 of heat and cold without dissolution : they are 

 of anopakenature,shining by reflection of the Sun's 

 light, as the planets do. They move in stated 

 periods in very long elliptical orbits, having the 

 Sun in one of their Foci j in one part of their or- 

 bits they approach extremely near to the Sun, and 

 in another part they are immensely distant from 

 him ; sometimes they come much nearer to the 

 Sun than Mercury's orbit, at other times they are 

 greaily farther from the Sun than Georgium Sidus. 

 Comets differ much in their magnitude, though 

 most of those which have been observed are con- 

 siderably less than the Moon ; but their dimensions 

 are not determined with accuracy ; neither are their 

 periods : for as one person's life is seldom longer 

 than a Comet's period, and very frequently not so 

 long, there is but little probability of his again 

 seeing a Comet, when it returns near the Sun after 



another 



