46 IN'TRODUCTrON. 



artificial magnet from the middle to the end, so 

 that the south end of the magnet be applied to the 

 north half of the needle, and the contrary. Flat, 

 broad needles seem to be preferable to the thin and 

 light ones ; and hard brass caps, to those of agate, 

 which soon destroy the point of the pin that bears 

 them. 



2. SURVEYING AND PLANNING THE COASTS. 



Without entering upon full instructions on this 

 important subject, we shall only point out, in gene- 

 ral, that mode of proceeding which we have found 

 to be the shortest and most profitable. 



The whole operation of surveying coasts depends 

 on the determination of the distances of the prin- 

 cipal points of a coast, such as the capes, tops of 

 mountains, and the extreme ends of the land seen. 

 In general, observers content themselves with 

 determining the direction of such points by an 

 approximation to the true bearing by the com- 

 pass, and noting their distances by the eye ; but 

 this method will always be rather uncertain, on 

 account of the unequal transparency of the air. 

 Sometimes, however, it is the only means 5 and it is 

 not to be denied, that, by practice, a tolerable de- 

 gree of readiness in such estimations may be ob- 

 tained, especially for objects not far ofTj but the 

 method of triangulation always deserves the pre- 

 ference, when it can be employed. The ship's way 

 along the coast serves as the basis of the triangle. 



