282 FERGUSON'S LECTURES. 



LET. Wie are but very small beings when compared with 

 ^1^5' our earthly globe, and the globe itself is but a dimension- 

 less point compared with the magnitude of the starry 

 The opera heavens. Whether the earth be at rest and the heaven 

 turns round it, or the heaven be at rest, and the earth 

 turns round, the appearance to us will be exactly the 

 same. And because the heaven is so immensely large, 

 in comparison of the earth, we see one half of the 

 heaven as well from the earth's surface, as we could do 

 from its center, if the limits of our view were not inter- 

 cepted by hills. 



Circles of We may imagine as many circles described upon the 



the sphere. ,, , , . . . , , - 



earth as we please ; and we may imagine the plane of 



any circle described upon the earth to be continued, un- 

 til it marks a circle in the concave sphere of the heaven. 

 The horizon is either sensible or rational. The sensible 



The hori- horizon is that circle, which a man standing upon a 

 large place, observes to terminate his view all around 

 where the heaven and earth seem to meet. The plane 

 of our sensible horizon continued to the heaven, di- 

 vides it into two hemispheres ; one visible to us, the 

 the other hid by the convexity of the earth. 



The plane of the rational horizon, is supposed parallel 

 to the plane of the sensible ; to pass through the center 

 of the earth, and to be continued through the heavens. 

 And although the plane of the sensible horizon 

 touches the earth in the place of the observer, yet 

 this plane, and that of the rational horizon, will seem to 

 coincide in the heaven, because the whole earth is but a 

 point compared to the sphere of the heaven. 



The earth being a spherical body, the horizon or 

 limit of our view must change as we change our place. 



Poles. The poles of the earth are those two points on its sur- 



face in which its axis terminates. The one is called the 

 north pole, and the other the south pole. 



The poles of the heaven, are those two points in which 

 the earth's axis produced terminates in the heaven : so 



