50 INTRODUCTION. 



from objects already determined, perform the im- 

 portant service, that the place of the ship itself 

 may, on a subsequent measurement of angles, 

 be deduced from them with great accuracy. This 

 advantage is of peculiar value, on occasions when, 

 after irregularly tacking or lying-to during the 

 night, it is required to renew the connection with 

 the points of the day before, and to determine the 

 place of the ship, which is often carried away by 

 currents. 



The first thing to be done in entering the data 

 thus obtained in a chart, is the division of the 

 geographical net to which they are to be trans- 

 ferred : for this purpose, it is best, in all cases, to 

 use Mercator's projection, in which both the meri- 

 dians and parallels of latitude are parallel strait 

 lines, intersecting each other at right angles. The 

 scale on which the chart is to be made is arbitrary, 

 and is partly determined by the accuracy of the 

 measurements, and the number of details to be in- 

 serted ; but, in most cases, it is to be taken so that 

 the space of one minute of a degree may be so 

 large, as to be very perceptible on the paper, so 

 that a degree of longitude may be half a foot or 

 more ; an extent which must be increased in the 

 planning of creeks, harbours &c. ; but in copying, 

 the chart may be easily reduced to a smaller scale. 

 First, therefore, let a scale of a degree of longitude 

 be made, and, by this, mark the division of the 

 degrees of longitude on the horizontal edge of the 



