4 HISTORY OF 



two opposite forces, circulates round its great 

 centre of heat and motion. 



In this manner, therefore, is the harmony of 

 our planetary system preserved. The sun, in the 

 midst, gives heat, and light, and circular motion 

 to the planets which surround it: Mercury, 

 Venus, the Earth, Mars, Jupiter, Saturn, and 

 the Georgium Sidus, perform their constant cir- 

 cuits at different distances, each taking up a 

 time to complete its revolutions proportioned to 

 the greatness of the circle which it is to describe. 

 The lesser planets also, which are attendants 

 upon some of the greater, are subject to the same 

 laws ; they circulate with the same exactness j 

 and are, in the same manner, influenced by their 

 respective centres of motion. 



Besides those bodies which make a part of our 

 peculiar system, and which may be said to reside 

 within its great circumference, there are others, 

 that frequently come among us from the most 

 distant tracts of space, and that seem like dan- 

 gerous intruders upon the beautiful simplicity of 

 nature. These are comets, whose appearance 

 was once so terrible to mankind, and the theory 

 of which is so little understood: but we know 

 that their number is much greater than that of 

 the planets ; and that, like these, they roll in or- 

 bits, in some measure obedient to solar influence. 

 Astronomers have endeavoured to calculate the 

 returning periods of many of them ; but expe- 

 rience has not, as yet, confirmed the veracity of 

 their investigations. Indeed, who can tell, when 

 those wanderers have made their excursions into 



