NAVIGATION. 73 



times "horizon" is used to designate the actual 

 line of earth or sea which is seen in line with the 

 sky, that is to say, the boundary of the visible 

 portion of the surface of earth or sea ; sometimes, 

 again, " the horizon " means the boundary line of 

 the ideal celestial sphere, separating the visible 

 part of it from the part eclipsed by the earth or 

 sea. This little ambiguity does no harm. When 

 we speak of the distance of the horizon, an ex- 

 pression frequently used in navigation, horizon has 

 its terrestrial signification. When we speak of 

 the distance of a star from the horizon, it is the 

 heavenly horizon that we mean. 



45. (10.) A nautical or geographical mile is 

 the length of one minute of longitude at the 

 equator, and contains 6086 feet or 1014 fathoms. 

 This is very nearly the average length of a minute 

 of latitude, as the approximately elliptic quadrant 

 from the equator to either pole is very nearly 

 equal in length to the quadrant of the equator. 

 At the equator the length of a minute of latitude 

 is less by 1/150, and at the pole it is greater by 

 i 150 than the minute of longitude at the equator. 



