82 THJi WORLD. 



the United States, the meridian of Washington is very often used. 

 Navigators are accustomed to carry with them Chronometers, 

 or very accurate time-keepers, which are set to Greenwich time, 

 and give,' at any moment, by simple inspection, the precise time 

 which is then indicated by the clock at Greenwich. On a clear 

 day, the true time on ship-board, or the exact instant of apparent 

 noon, is ascertained by means of the quadrant, figured below. 

 This is an arc of a circle, embracing something more than one- 

 eighth of the whole circle, but it is graduated into 90, for the 

 degrees are only half the length they would be, if the angles were 

 measured without being twice reflected. 



A is called the index glass ; it is a plane quicksilvered glass 

 reflector, placed, by means of adjusting screws, truly perpendicular 

 to the plane of the quadrant, and attached to the brass index arm 

 A B, thife index turns on a pin directly under A. C is called the 



