206 University of California Publications in Zoology [Vol. 15 



While one minute of latitude is approximately equal to a nautical 

 mile, owing to the spheroidal shape of the earth, it varies slightly 

 with the latitude : 



At the equator it is 1842.8 meters 



At 30° it is 1847.5 meters 



At 60° it is 1856.9 meters 



At the pole it is 1861.7 meters. 



This variation in the length of one minute of latitude has been 

 responsible for a lack of precision in defining a nautical mile. In 

 order to establish uniformity it is now defined as 1853.248 meters, 

 which is the length of one minute of a great circle of a sphere having 

 the same superficial area as the earth. 



This definition is adopted by the United States Coast and Geodetic 

 Survey, and the value above given was computed from Clarke's values 

 for the equatorial and polar radii, 6,378,206 and 6,356,584 meters. 



