NAVIGATION. 



75 



is very approximately the same round a meridian 

 or round any geodetic line, 1 as round the equator, 

 is 21,600 miles. 



The accompanying diagram represents any 

 section through the earth's centre. HH' are two 



points of the terrestrial or sea horizon, as seen 

 from a point P, at a height of 1/81 of the earth's 

 diameter, that is to say, a height of nearly 85 miles. 

 PH, the distance of the horizon, is 1/9 of the earth's 

 diameter or 762*6 miles. The angle LPH is the 



1 If a line on a given surface be such that a part of it, on each side 

 of any point of it whatever, is the shortest distance on the surface 

 between the two ends of this part, then it is a geodetic line. 



