So POPULAR LECTURES AND ADDRESSES. 



of the disturbance in the directions of verticals at 

 different parts of the earth's surface, produced by 

 the attraction of mountains and continents, and 

 the defect of attraction of great depths of the sea, 

 and by unknown variations of density in the solid 

 earth below the bottom of the sea, and below the 

 visible surface of dry land. But they are nearly 

 enough so for all the purposes of practical 

 navigation ; and therefore lines of equal latitude on 

 the earth's surface are habitually called circles of 

 latitude, or parallels of latitude. 



According to the same supposition of symmetry 

 round an axis, the meridian plane of any locality 

 would pass through the earth's axis of rotation, 

 and it would be the meridian also of every other 

 place on the line in which it cuts the earth's 

 surface. This result of the imagined symmetry is 

 nearly enough true in reality for navigation, and ac- 

 cordingly in navigation it is allowable and usual to 

 regard lines, in which the earth's surface is cut by 

 planes through its axis, as lines of equal longitude ; 

 and farther, these lines are often called meridians, 

 or terrestrial meridians, there being a habitual 



