CHAP. XVII.] GRADUATION 389 



and forming the curve in the manner described on p. 387 when 

 graduating in the ordinary way. The extension of the gradua- 

 tion of a chart is carried out on similar principles. 



All graduation should be done in one forenoon, so as to 

 permit the paper to alter as little as possible during the opera- 

 tions. 



The meridians and parallels being ruled at, say, every 10', To take 

 the distance in inches is measured N. or S. from the ^ to tude and 

 the nearest parallel ; 10' of difference of latitude being measured Longitude 

 on the brass scale, the ratio which the fu'st measurement bears from a 

 to the second gives the proportion of 10' corresponding to the ^^^^^ 

 difference of latitude of the A N. or S. of the parallel used, on the 

 The distance of the A in inches should then be measured ^^o^o^iic 



Projec- 



E. or W. of the nearest meridian ; the distance between two tion. 

 adjacent meridians in the latitude of the -^ being also 

 measured on the brass scale. The proportion of 10' represented 

 by the ratio of the two measurements is the difference of 

 longitude of the A E. or W. of the meridian used. 



Besides a comprehensive title in the usual form, in which Memoir of 

 should be given the names of all officers who took part in the tion and 

 survey and the months of the year during which the sound- '^**^®- 

 ings were obtained, every original chart must have a memoir 

 written on it containing the following information, to enable 

 the authorities at home to put the proper value on the 

 work : 



1. The method of construction, with the base used, and how 

 measured. 



2. References to triangulation sheet, and returns of latitudes 

 and meridian distances which may affect the sheet, stating the 

 latitudes, meridian distances, and true bearings as actually 

 observed, together with any correction that may have been 

 applied to them to obtain the finally accepted result, and the 

 number of feet per mile by which the distances by triangulation 

 have been corrected to agree with the accepted positions. 



3. In a graduated chart, the latitudes and longitudes 

 adopted for the graduating points, with the secondary meridians 

 on which they depend. 



4. A table giving the latitudes and longitudes of various 

 points on the chart, as calculated in the triangulation and as 

 taken from the graduation. 



