70 POPULAR LECTURES AND ADDRESSES. 



centrifugal force would lie almost entirely in a belt 

 round the equator, leaving a great island of dry 

 land round each pole. 



43. In elementary books on geography, as- 

 tronomy, and navigation, terrestrial latitudes and 

 longitudes, and meridians, and horizontal planes, 

 and verticals and altitudes, are commonly de- 

 fined on the supposition that the earth is an exact 

 sphere. I prefer definitions of a more practical 

 kind, which, be the figure of the earth what it may, 

 shall designate in each case the thing found when 

 the element in question is determined in practice 

 by actual observation. 



(i.) A vertical in any place is the direction of 

 the plumb line there, when the plummet hangs at 

 rest. The zenith is the point of sky vertically 

 overhead, or the point in which the vertical 

 produced upwards, cuts the sky. 



(2.) Any plane through a vertical is called a 

 vertical plane. The prime vertical is a vertical 

 plane perpendicular to the meridian, that is to say, 

 it is an east and west vertical plane. 



(3.) The vertical plane in any place passing 



