NAVIGATION. 71 



through the point of the sky defined as the 

 celestial pole ( 1 1 above) is the meridian of 

 that place. 



(4.) A horizontal plane is a plane perpendicular 

 to the plumb-line or vertical ; or it may be defined 

 as a plane surface of mercury, or water, or other 

 liquid, in a basin large enough to give a middle 

 portion of liquid surface, not sensibly disturbed 

 by the capillaVy action which curves the liquid 

 near the sides of the vessel ; yet not so large as to 

 show any sensible influence from the curvature of 

 the earth. Either a plummet or a basin of liquid 

 is practically used for finding horizontal planes or 

 horizontal lines. 



(5.) The altitude of any object, terrestrial or 

 celestial, as seen from any point of view, is the 

 angle between a line drawn to the object and 

 a horizontal line in the same vertical plane with 

 it ; or it is the angle between the line going 

 to the object and the nearest horizontal line ; 

 or, as it is sometimes put, it is the inclination to 

 the horizontal plane of a line directed to the 

 object. 



