SHAPE OF THE EARTH. 7 



now know, from the mean result of numerous 

 measurements, that the flattening amounts to ^-^ 

 or that the half of the earth's axis is 69,275 feet 

 shorter than the half diameter of the equator, of 

 which the length is 20,922,668 feet. 



It is true that the solid part of the earth's surface 

 does not exactly take the curvature of the sea-level, 

 but is in general uneven and sloping. The loftiest 

 mountains, however, do not reach much beyond 

 twenty-eight thousand feet above the sea, a height 

 far less than the amount of flattening at the poles. 

 Besides, most of these lie in the lower latitudes, 

 that is, nearer to the equator than to the poles ; 

 while hitherto, only very little land, and that for 

 the most part very low, has been found within 

 the polar circles. The curvature then of the 

 main land is the same in general, if we do not con- 

 sider its partial unevennesses, as that of the sea ; 

 and, therefore, the form of the whole earth is that 

 of a sphere somewhat flattened at the poles. 



The deviation of particular parts of the land 

 from this form, or the amount of slope at any 

 place, is readily discovered from the angle which 

 the slanting surface makes with that of still 

 water ; for water must, as I have above explained 

 to you, always set horizontally, that is, so that the 

 direction of the earth's attraction (or, rather, of that 

 attraction as changed by the centrifugal force) shall 

 be at right angles (perpendicularly) to its surface. 



