304 DESCRIPTION OF GARNETT'S 



the Chart, and that the given parallel of latitude shall cut the 

 course on the Index, either in degrees or points of the com- 

 pass; extending naturally towards the North or South as the 

 course directs, but indifferently whether to the right or left, 

 as either may occasionally represent the East or West (the 

 difference of longitude being equal on either side,) so that the 

 centre C of the Index must be placed on the right hand of the 

 centre of the Chart, when the course is towards the equator, and 

 on the left hand when from it, as will readily appear in the 

 practice. 



The solutions of all the cases being on a more correct prin- 

 ciple than the common method by middle latitude, will be found 

 more accurate — particularly where the difference of latitude 

 is great. 



Should the distance exceed the extent of the Index, or the 

 whole difference of longitude on the Chart, the work must be 

 repeated from the last found latitude, until the difference of 

 longitude and latitude corresponding to the whole distance 

 is obtained. 



The following examples of the different Cases of Sailing by 

 this Chart, will make its use sufficiently easy. 



CASE I. 



Given the Latitudes and Longitudes of two places; to find the 

 Course and Distance behvecn them. 



EXAMPLE. 



Suppose the Latitudes of the two places to be 49° 10' N, 

 and 5 3° 20' N. respectively, and their difference of Longitude 

 6° 10'; required the course and distance between them. 



1st. Lay the centre C of the Index on the meridian which 

 is about half the given difference of longitude, or 3 degrees on 

 the left side of the Central Meridian, and in the parallel of lati- 

 tude 49° 10'. 



2nd. Extend the distance line of the Index to the parallel 

 of 53° 20' of latitude, on the meridian of 3° 10' cliff, of long, 

 nt the right side of the Central Meridian, — and the distance 



