FIGURE OF THE EARTH. 155 



the constant direction of its major axis towards the Earth ; 

 the accumulation of matter on that half of the Moon which 

 is turned to us determines the relation of the periods of 

 rotation and revolution, and doubtless reaches back to the 

 earliest epoch in the history of the satellite. The mathe- 

 matical figure of the Earth " is that which it would have 

 if its surface were covered by water in a state of repose :" 

 it is to this imaginary surface, which is unaffected by the 

 accidents and inequalities of the true physical surface, that 

 all geodesical measurements of degrees are referred. ( 127 ) 

 The general figure of the Earth is determined when we know 

 the magnitude of the equatorial axis and the compression 

 at the poles ; but to obtain a complete representation of 

 its form, measurements in two directions, perpendicular to 

 each other, are required. 



Eleven measurements of degrees (or determinations of the 

 curvature of the Earth's surface in different parts), of which 

 nine belong to the present century, have made known to us 

 the magnitude of our globe, which Pliny ( 128 ) long since 

 termed a " point in the immeasurable Universe.'" If these 

 measurements are not always accordant in giving equal 

 curvatures for the different meridians under the same parallel 

 of latitude, this very circumstance rather testifies in favour 

 of the accuracy of the instruments and methods employed, 

 and of the fidelity of the partial results. The conclusion 

 respecting the figure of a planet, from the increase of the 

 attracting force in going from the equator to the poles, is 

 dependent on the distribution of density in its interior. In 

 his immortal work, " Philosophise Naturalis Principia/' 

 Newton, incited probably by Gassings discovery of the ellip- 

 ticity of Jupiter ( 129 ) made previous to 1666, determined 



