325 



PHYSIOGRAPHY. 



[chap. 



is not exactly a circle, but is slightly elliptical ; its longer 

 diameter measuring about two miles more than its shorter 

 diameter. The average equatorial diameter is about 7926-5 

 miles : in other words the equatorial exceeds the polar 

 diameter by about twenty-seven miles. The proportion 

 of twenty-seven miles to 7,926 miles is very nearly that of 

 I to 294, and hence the earth is said to have an ellipticity 

 of^i^. 



These variations from the shape of a true sphere are so 

 extremely slight, in comparison with the great magnitude of 



Fig. 102.— Difference between polar and enuatorial diameters of the earth. 



the earth, that, speaking roughly, the earth may for practical 

 purposes be called sphere ; and it may be regarded as having 

 the shape represented by our ordinary globes. In fact, the 

 departure from the spherical shape is too slight to affect a 

 model of this kind, unless it is of unusual magnitude.^ 



In order to represent any country by drawing its outline 

 upon a globe, or upon a map, it is necessary, in the first 

 place, to have some means of fixing the position of places 

 upon the surface of the earth. The means by which this 

 is accomplished may be easily understood. Suppose that it 



'^ In a globe of 2 ft. 6in. diameter, for example, the difference of the 

 polar and equatorial diameters would be very little more than T^jth of 

 ail inch. 



