ASTRONOMY. 433 



upon its disc; but Hcrschell has not been able to 

 see them ; consequently, the time of rotation 

 round its axis is not decidedly known. Venus also 

 appears with phases, and transits sometimes take 

 place, which are of very great importance in astro- 

 nomy. 



The Earth which we inhabit is a globular body, 

 as may be proved from a variety of circumstances, 

 the chief of which we shall here enumerate. It is 

 always observed by mariners, that as they sail 

 from any high objects, such as mountains, steeples, 

 &c. they first begin to lose sight of the lower part 

 of those objects, and then gradually of the higher 

 parts ; also, persons on shore first discover the 

 upper parts of the masts of approaching vessels. 

 This could not be the case if the earth were a 

 plane, but is very easily accounted for on the sup- 

 position of its being a sphere, as will be easily 

 understood by examining Fig. 4. Also various 

 navigators have sailed completely round the earth, 

 by continuing in the same direction, at last coming 

 to the same place from which they set out. 



The earth, however, is not a perfect sphere, but 

 a spheroid, having its equatorial diameter longer 

 than the polar diameter, or axis. It is, consequently, 

 flattest at the poles, and more protuberant at the 

 equator. The diameter at the equator is 7893 

 English miles; that at the poles is 7928 miles. 

 The surface of the earth is much diversified with 

 mountains and vallies, land and water. The high- 

 est mountains in it are the Andes in South America, 

 and the Himala mountains in the East Indies, some 

 of which are about four miles in perpendicular 

 altitude. About two-thirds of the globe is covered 

 with water. 



VOL. I. PF 



