20 Mr. Sidney G. Brown [Jan. 30, 



The earth, as we know, is a magnet, but not a very powerful one, 

 and it has been calculated that if it were wholly of iron, it would 



have an intensity of magnetism 17, times greater than it has. 



All the same the magnetism is sufficiently strong to give a good 

 directive action to a pivoted needle. 



The magnetic poles of the earth are not coincident with the geo- 

 graphical poles, hut are situated some distance away. The magnetic 

 pole was discovered by Sir J. C. Ross to be situated in latitude 

 7<» .V North and longitude 96" 46' West in Bootbia Felix, just within 

 the Arctic Circle sonic L,000 miles away from the actual pole. 



With this displacement of the magnetic poles, we have an 

 irregular distribution of the magnetism over the surface of the earth, 

 and thus the magnetic needle does not point truly north and south 

 at many parts of the earth's surface. In London, for instance, it 

 points at an angle of 10° west of the true north. 



This angle is called the deviation or variation of the needle. To 

 enable ships to steer by the compass, magnetic charts have been pre- 

 pared, and the deviation at different places accurately measured. 



These magnetic charts have to be checked and altered from time 

 to time, as the deviation slowly varies from year to year. Thus in 

 London in 1G59 the needle pointed true north, while in 1820 there 

 was an extreme westerly variation of 24^° ; siuce then it has been 

 slowly coining back to something like 16° at the present time. 



On a wooden ship the accuracy of a good modern magnetic 

 compass leaves little to be desired, but on an iron ship the case is 

 quite different. 



The magnetic field of the earth tends to be weakened in the 

 lengthwise direction of the iron ship, because a portion of the 

 magnetism enters the ship, while across the ship the field is stronger, 

 and as it is essential that the magnetism in which the needle lies 

 should be uniform in strength in whatever direction the ship may 

 happen to point, it is important that this stronger field be reduced 

 by some method of magnetic shielding. This is accomplished by 

 fixing a pair of iron globes athwart the ship on the two sides of the 

 compass. The effect of the iron of the ship and the corrections that 

 have to be made to the compass is to reduce the directive force of 

 the earth's magnetism, and thus the compass is rendered slow and 

 sluggish in its action. This is particularly the case on board a 

 battleship. 



In the interior of a submarine the force is still further reduced, 

 so much so as to render the magnetic compass useless for this class 

 of vessel. 



It is quite possible on an iron ship to correct the errors of a 

 compass, but as the ship itself may be a magnet, and its strength a 

 variable quantity, it is important that the navigator should test the 

 readings of his compass at every available opportunity, and particu- 

 larly at the commencement of each voyage. 



