CHAP. XVII.] GRADUATION 385 



distance, according to scale, to the nearest even minute of 

 latitude (as 1', 5', 10', etc., as convenient). 



At H and E lay off short perpendiculars to the meridians, 

 and on these measure the distances H C, E D, the lengths of 

 departure, according to scale, to the nearest even minutes of 

 longitude that may be convenient. 



In liigh latitudes and large scales, if the even meridian 

 required is many miles distant, error will be introduced by 

 this latter operation. It will only be correct for short dis- 

 tances, as the curve of the parallel, on which we ought to 



measure this departure, will not coincide with the perpen- 

 dicular to the meridian for more than a mile or two in such 

 a case. 



We have now C and D, two stations on even meridians and 

 even parallels, which we shall take as our points for gradua- 

 tion. Tliis is exactly the case when we wish to graduate the 

 sheet first, so that henceforward the methods are identical. 

 In the case of after-graduation, when these even points have 

 been obtained, we can rub out on our chart all lines already 

 ruled, to prevent confusion, and we will take a new figure for 

 the similar purpose of facilitating comprehension. 



In Fig. 77 let C and D be the positions for graduation. 

 Calculate spherically, as before, the bearings of C and D from 

 one another, and lay off the meridians C N, D M. 



From C and D lay off the perpendiculars C H, D F, and 



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