( 465 ) 



IV.— MAGNETIC VARIATION. 



(464.) Among the many modern changes which have affected the science 

 of navigation, none have been more important than the different relation 

 which the Compass now bears to the ship as compared with its place in 

 former times. 



Eude instruments, unadjusted, with errors unsuspected, and under in- 

 fluences destructive to their accuracy, have given place to what may be, in 

 some cases, over-estimation of this primary aid to the seaman. In its very 

 nature the Compass is imperfect and incompetent to show, at sea, the 

 minute quantities which are now disputed over. The consequence is, that 

 it is made, like the topic we have just discussed, Ocean Currents, the 

 scapegoat for many errors of seamanship and judgment, which a more 

 intimate knowledge and therefore greater mistrust and induced caution 

 would have avoided. 



(465.) Our present task deals with the geographic distribution of Mag- 

 netism, not with those local effects caused by the ship or its relation to 

 outer circumstances, but to its position in the Atlantic. The other points, 

 most important in themselves, will be discussed later on. 



The features of the earth's Magnetism, as related to the ship, are the 

 Declination, Inclination or Dip, and hitensity. The Dip and Intensity are 

 very important elements in the adjustment of the Compass in its passage 

 through the varied magnetic conditions which attend an over-sea voyage 

 across the Equator, but they have but little influence on the directive power 

 of the needle in the latitudes usually traversed in commercial pursues. 

 The Magnetic Equator does not correspond with the Geographical Equator, 

 any more than the Magnetic Poles do with the Geographical Poles, but 

 crosses it after the manner of the Ecliptic. On the Magnetic Equator the 

 needle has no Dip, but if it be carried towards the North and South Poles, 

 so will the corresponding ends become depressed from the horizontal, till 

 over the Magnetic Poles it would hang vertical. 



The Declination or Variation is one of the most important elements in 

 navigation, and its correct estimation and application most essential to the 

 safe conduct of a ship. 



(466.) The reasons why the Compass is now placed in so much higher 

 consideration than it was in former years are manifest. The great increase . 

 of the use of iron both for shipbuilding as well as in the fabric of wooden 

 ships, and the consequent vastly increased influence that the ship has upon 

 her Compasses, has been one chief reason w hy attention is so imperatively 

 demanded. Again, since the universal use oi steam, the course of a vessel, 

 in passing directly from one point to another, requires to be much more ■ 

 accurately laid than was thought necessary when wooden sailing ships 

 only were used. 



Another reason, which has arisen in the course of years, is that caused 

 by the Secular Variation. The Compasses appearing on charts founded 



