435 



LECTURE XLVI. 



ON GEOGRAPHY. 



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 

 disappearance 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 rigging 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 fre- 

 quent inequalities of the solid parts of the earth usually cause the prospect 

 to be bounded by some irregular prominence, as a hill, a tree, or a build- 

 ing, so that the general curvature is the less observable. 



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

 tion 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 

 ascertain 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 differ- 

 ence 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 circum- 

 ference may be determined ; and on these principles the earliest measure- 

 ments of the earth were conducted. The first of these, which can be con- 

 sidered as accurate, was executed by Picart* in France, towards the end of 

 the seventeenth century. 



But the spherical form is only an approximation to the truth ; it was 

 calculated by Newton, and ascertained experimentally by the French Aca- 

 demicians, sent to the equator and to the polar circle, that, in order to 

 represent the earth, the sphere must be flattened at the poles, and promi<r 

 nent at the equator. We may therefore consider the earth as an oblate 

 elliptic spheroid ; the curvature being greater, and consequently every 

 degree shorter, at the equator, than nearer the poles. If the density of the 

 earth were uniform throughout, its ellipticity, or the difference of the 

 length of its diameters, would be -3-^5- of the whole ; on the other hand, if 

 it consisted of matter of inconsiderable density, attracted by an infinite force 

 in the centre, the ellipticity would be only ^-y ; and whatever may be 

 the internal structure of the earth, its form must be between these limits, 

 * Hist, et Mem. vii. I. 46. 

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