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mets. It is more probable that comets, like planets, ' 

 have their regular periods : although they frequently es- 

 rape our observations, as not revolving but in a long 

 term of years. 



They are distinguished from other stars by a large 

 train of light, which is always opposite to the sun, and 

 grows fainter and fainter the farther it is from the body 

 of the comet. When a conret moves from the sun it is 

 said to be bearded, because that light is seen before it. 

 When it moves towards the sun, the train follows it, 

 and is called its tail. When the comet and sun are op- 

 posite, (the earth being between them) the train is hid 

 behind the body of the comet, except a little that ap- 

 pears round it, and is termed its hair. 



Sir Isaac Newton has proved, that the heat of the 

 suu to the comet in December, 1 680, was to his heat 

 with us at midsummer, as 28000 to one : and that the 

 heat of the body of the comet was near 2000 times 

 greater than that of red hot iron. 



After having acquired so immense a heat, it must be 

 a long time in cooling. Sir Isaac computes, that a 

 globe of red hot iron 200 times as large as the earth, 

 would scarce be cool in 50,000 years. If then the comet 

 be supposed to cool a hundred times as fast as red hot 

 iron, yet since its heat was two thousand times greater, 

 supposing it of the bigness of the earth, it would not be 

 cool in a million of years. 



Comets seem to be a peculiar kind of planets, which 

 move in very oblique orbits, and persevere in their mo- 

 tions, even against the course and direction of the other 

 planets. Their tails are doubtless vapours emitted by 

 the comet when heated by the sun. Yet they do not 

 ascend swiftly fi om it, and then presently disappear ; 

 but are permanent columns of exhalations, gathered from 

 * the comet by a gentle motion, and in a great space of 

 time, which then move with it through the celestial 

 regions. 



One great use of comets probably is, to give moisture 



