396 COMETS. 



ferent astronomers, of the different Comets that 

 have made their appearances at different times; 

 and from their testimony it appears, that neither 

 their revolutions, nor the periods of their return, 

 have ever been ascertained. If the motions of 

 Comets were regulated and governed by the 

 same laws which regulate and govern the plane- 

 tary system in general, the expectation of those 

 who have anxiously looked for the periodical re- 

 turn of individual Comets, would not have been 

 disappointed; neither would the system of the 

 universe continue to exist, without some of the 

 parts sustaining violent and dreadful shocks, 

 from the violent appulse which those parts would 

 have sustained from the immediate contact 

 between them and Comets. The Comet which 

 has caused the greatest alarm to the inhabitants 

 of the earth, and been supposed to threaten it 

 with a visit, was the one of 1680. Dr. HALLEY, 

 by calculation found, that Nov. 11, 1680, at 

 1 h. 6m. P. M. it was not above one semi- 

 diameter from the earth ; and had the earth, it is 

 said, been at that time in the part of her orbit 

 nearest to the node of the Comet through 

 which it passed, their mutual gravitation must 

 have caused a change in the plane of the orbit 

 of the earth, and in the length of our year : 

 and Dr. Halley adds, that if so large a body, 

 with a motion so rapid, as that of this Comet 

 nearest its perihilion, were to strike against our 



