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LECTURE XLVI. 



ON GEOGRAPHV. 



From the consideration of the stars, the sun, and the planets in general, 

 we are now to descend to that of the earth, the particular planet which we 

 inhabit, and which we can examine more minutely than the other parts of 

 the solar system. Its external form, its divisions, whether astronomical or 

 natural, its most remarkable features, and its internal structure, will require 

 to be separately investigated. 



The general curvature of the earth's surface is easily observable in the dis- 

 appearance of distant objects, and in particular, when the view is limited by 

 the sea, the surface of which, from the common property of a fluid, becomes 

 naturally smooth and horizontal: for it is well known that the sails and.rig- 

 ging of a ship come into view long before her hull, and that each part is the 

 sooner seen as the eye is more elevated. On shore, the frequent inequalities 

 of the soird parts of the earth usually cause the prospect to be bounded by 

 some irregular prominence, as a hill, a tree, or a building' so that the 

 general curvature is the less observable. 



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The surface of a lake or sea must be always perpendicular to the direction 

 of a plumb line, which may be considered as the direction of the force of 

 gravity; and by means either of a plumb line or of a spirit level, we may as- 

 certain the angular situation of any part of the earth's Surface with respect to 

 a fixed star passing the meridian; by going a little further north or south, 

 and repeating the observation on the star, we may find the difference of the 

 inclination of the surfaces at both points; of course, supposing the earth a 

 sphere, this difference in latitude will be the angle subtended at its centre by 

 the given portion of the surface, whence the whole circumference may be 

 determined; and on these principles the earliest measurements of the earth 



