330 Astronomy. [Lecture SI . 



Again, the earth is proved to be nearly sphe- 

 rical in this manner : its roundness permits us to 

 see only a very little extent of its surface : for 

 upon a level space, for instance, a calm sea, the 

 eye elevated six feet above it, cannot perceive an 

 object placed upon it at a distance greater than 

 2551 fathoms; that is, it cannot discern more 

 than the extent of a circle of 5114 fathoms dia- 

 meter. But the circumference of this circle ap- 

 pears to touch the heavens, and the plane of this 

 circle extended to the starry heavens is what is 

 called the horizon. If the observer were placed 

 in the centre T (PL XXIX. fig. 122.) of the 

 earth, the horizon H H would divide the sphere 

 into two parts ; but being placed at the surface a, 

 the superior and visible hemisphere h Z ?i is 

 smaller than the inferior h N h, which is invisible. 

 It may yet be observed, that the radius of the 

 earth T a being infinitely small, compared with 

 the imaginary radius of the starry heavens T H 

 or T Z, the difference between the two horizons, 

 with respect to them, is not perceptible. When 

 the first, therefore, is called the rational, and the 

 other the sensible horizon, which are the names 

 by which they are distinguished, it must be with 

 reference to nearer objects. 



Many other proofs might be adduced to show 

 that the earth is nearly spherical ; nor are the 

 little unevenn esses on its surface, arising from 

 hills and valleys, any material objection ; since 



