NA VIGA TION. 67 



of the ship. On account of the great exactness 

 attainable by it, this process is valuable when 

 greater accuracy is desired than can be obtained 

 by the use of the azimuth compass, and when three 

 objects or landmarks are available. It is also of 

 great value as a means for determining the error 

 of the compass. It is continually used in nautical 

 surveys ; also frequently in ordinary navigation. 

 The sextant is also used for finding the distance of 

 the ship from some object of known magnitude, as 

 for example a lighthouse tower, or another ship. 

 Suppose, for example, the height of the tower from 

 its base, or a conspicuous mark near its base, to its 

 top to be known to be 100 feet. This at a dis- 

 tance of a nautical mile (6086 feet), will subtend an 

 angle a little less than 1/60 of the radian. Taking 

 the radian as 57'3, dividing this by 6086, and 

 multiplying by 60, to reduce to minutes, we get 

 5 6'* 5 as the angle, subtended by 100 feet, seen 

 at a distance of a nautical mile. Hence we have 

 the rule : Multiply the magnitude of the object in 

 feet by '565, and divide by the angle which it 

 subtends ; the result will be the distance in miles. 



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