TABLE IV. 



In the Introduction to this volume, I have stated that I should 

 combine, in this Table, the currents and magnetic variations ; but, as 

 I found it would become necessary to place the variations with the 

 magnetic results, and these could not be introduced in the present 

 volume, I therefore preferred separating the tables, rather than re- 

 peating a part of them, thereby avoiding an extra expense at a slight 

 inconvenience to the navigator, who can refer to them in the next 

 volume, where the actual observations are given, and the variations 

 divested of local attraction of the ship. The local attraction of a vessel 

 is so intimately connected with correct navigation, that it seems almost 

 incredible how much it has been disregarded. The endeavors to 

 rectify it, by patent compasses and appliances to overcome the effect of 

 the subtle fluid, have all proved failures. The deviation, caused by 

 the iron, should always be practically ascertained by swinging the 

 vessel before leaving port, noting the bearings of some very distant 

 object, or that of a compass on shore, where an observer may be 

 stationed, to take the opposite bearing of the observer on board ship. 

 The differences found in these sets of observations, will be the correc- 

 tions, to be applied to the compass on board. When the latitude is 

 changed from North to South, the corrections are to be applied with a 

 contrary sign. 



The Table of Currents will readily point out to the navigator the set 

 of the current, its force, and how determined. These, combined with 

 the variation at the position, with the direction of the ship's head, 

 will enable him to compare his own results with those recorded. 



C. W. 



