402 MAGNETISM. 



course may be always at right angles with the di- 

 rection of the magnetic needle of his compass, 

 keeping the north end of the magnet on the right 

 hand side, and, of course, the south end on the 

 left hand side of the vessel j for as the needle 

 points north and south, and the direction is east 

 and west, the intended course of the vessel is 

 exactly perpendicular to the position of the magnet. 

 A little reflection will show how the vessel may be 

 steered in any other direction. 



An artificial magnet fitted up in a proper box, 

 for the purpose of guiding the direction of a 

 traveller, is called a magnetic needle, and the 

 whole together, is called the mariner's compass. 



Although the north pole of the magnet in every 

 part of the world, when suspended, points towards 

 the northern parts, and the south pole towards the 

 southern parts, yet its ends seldom point exactly 

 towards the poles of the earth. The angle in which 

 it deviates from due north and south, is called the 

 angle of declination, or the declination of the mag- 

 netic needle, or the variation of the compass ; and 

 this declination is said to be east or west, according 

 as the north pole of the needle is eastward or west- 

 ward of the astronomical meridian of the place. 



This deviation from the meridian is not the same 

 in all parts of the world, but is different in differ- 

 ent places, and it is even continually varying 

 in the same place. For instance, this declination 

 is not the same in London as at Paris j and the 

 declination in London, or in any other place, is 

 not the same at this time as it was some years ago. 

 This declination from the meridian is even so 

 variable, that it may sometimes be observed to 

 change, even in one or two hours' time. 



