THE "DIP" OF THE MAGNETIC NEEDLE IN NEW BRUNSWICK. 105 



ARTICLE II. 



* 



THE " DIP " OF THE MAGNETIC NEEDLE IN 

 NEW BRUNSWICK. 



By Prof. A. Wilmer Dfff. 



Read December 6, 1898. 



Nearly everyone knows the use of a magnetic needle for rinding 

 the direction of North. Those who are more familiar with the use of 

 such a compass know that the needle does not point to the true North, 

 but only approximately so ; that, in fact, the difference between the 

 true North, and the magnetic north as indicated by the compass, 

 differs by an angle which surveyors call the "variation" of the needle. 

 The angle is different at different places, and even at any one place it 

 is always gradually changing, although the change is a very slow one 

 and requires months to be perceptible, unless the measurements be 

 made with very great accuracy. 



A casual glance at a compass needle is sufficient to show anyone 

 that the needle is only free to swing in a horizontal plane, owing to 

 the fact that the point of support of the needle is higher than its 

 centre of gravity. Now what would happen if the needle was sup- 

 ported so exactly at its centre of gravity that it could swing in a 

 vertical plane ass well as in a horizontal plane 1 It would be found to 

 come to rest at an inclination to the horizontal. The angle it would 

 then make with the horizontal is called the " dip " of the magnetic 

 needle, or, briefly, •' the magnetic dip." The dip, like the variation, 

 is different at different places, and if measured some months apart at 

 any one place it would be found to have changed appreciably. 



