XIX.] THE FIGURE OF THE EARTH. 325 



example, a canal has to be cut, an allowance must be made 

 for curvature in order that the depth of water in the canal 

 may be the same throughout. A convincing experiment to 

 prove the rotundity of the earth was made by Mr. Wallace, 

 in 1870, in the Bedford Levei. Three signals, each thirteen 

 feet four inches above water-levei, were erected at distances 

 of three miles apart. On looking through a telescope, 

 adjusted, in such a manner, that the line of sight touched 

 the tops of the first and last poles, it was found that the 

 middle signal was upwards of five feet above the line. 

 This rise was of course due to the convexity of the earth's 

 surface. 



Such evidence as that which has been adduced in this 

 chapter, proves conclusively that the earth has a curved 

 surface, and that the curvature is that of a globular 

 body. Very delicate operations have enabled men to detei- 

 mine the figure of the earth with the greatest accuracy, and 

 have shown that this figure is not exactly that of a true 

 sphere. The sphere is, in fact, a little flattened in the neigh- 

 bourhood of the poles, so that, using a popular comparison, 

 it may be likened to the shape of an orange : only it must 

 be remembered that the earth's flattening is very much Icis 

 proportionally than that of the orange. In consequence of 

 this flattening, a line running round the globe through the 

 north and south poles is not exactly a circle, but is an 

 ellipse, or something like a circle which has been slightly 

 squeezed at opposite points. Fig. 102 is such an ellipse, 

 but the extent of flattening is exaggerated prodigiously. 

 The polar diameter, or the line which passes through the 

 earth's centre, from pole to pole, is found to measure 7899-5 

 miles ; but the equatorial diameter, or the line which passes 

 through the earth's centre, from point to point on tiie 

 equator, is not the same in all directions. For the equator, 



