48 INTRODUCTION. 



After some time is elapsed, for instance, one or 

 two hours, according to the quickness of the ship*s 

 going, and the apparent change of place of the 

 objects, the same operation is repeated in all its 

 parts; in which, however, if the appearance of the 

 coast has not much changed, the same profile may 

 be used. The rapidity with which a number of 

 points may be determined, by measuring the angles 

 with a sextant, and the accuracy with which the 

 situation of the end points, and, consequently, of 

 those lying between, may be determined by re- 

 peated observations, constitute the essential advan- 

 tage of this mode of proceeding, which the follow- 

 ing remarks may serve to make more complete. 



1. As, especially near the coasts, the currents are 

 often considerable, it is of the greatest importance 

 to determine the ship's place as far as possible, by 

 astronomical observations, which, when the weather 

 i3 clear, may be done for the longitude at least, by 

 repeated solar altitudes with help of the chrono- 

 meter. Sometimes, too, the latitude can be deter- 

 mined for some other time, besides noon, either 

 by Douwe*s well-known method, for which Mendo- 

 za's tables afford the best calculations, or by a cul- 

 mination of the moon, or in the evening and morn- 

 ing twilight with stars ; this will likewise afford a 

 means of correcting the ship's reckoning. 



2. Take careful note of the changes in the ex- 

 ternal appearance of the coasts, and the relative 

 alterations in the positions of the more remarkable 



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