*06 



MAGNETISM. 



A magnetical needle constructed for the pur- 

 pose of showing this property is called a dipping 

 needle. 



If the geographical poles of the earth (that is, 

 the ends of its axis) coincided with its magnetic 

 poles, or even if the magnetic poles were con- 

 stantly at the same distance from them, the incli- 

 nation of the needle, as well as its declination, 

 would always be the same ; and hence, by obser- 

 ving the direction of the magnetic needle in any 

 particular place, the latitude and longitude of that 

 place might be ascertained ; but this is not the 

 case, for the magnetic poles of the earth do not 

 coincide with its real poles, and they are also con- 

 stantly shifting their situations ; hence the mag- 

 netic needle changes continually and irregularly, 

 not only in its horizontal direction, but likewise in 

 its inclination, according as it is removed from one 

 place to another, and also while it remains in the 

 same place. 



This change of the dip in the same place, how- 

 ever, is very small. In London, about 1576, the 

 north pole of the dipping needle stood 71° 50" 

 below the horizon ; and in 1775, it stood at 72° 

 3', the whole change of inclination, during so 

 many years, amounting to less than a quarter of 

 a degree. 



The Magnetic Touch, or communicated Magnetism. 



When a piece of iron comes sufficiently near to 

 a magnet, it becomes itself a magnet ; and this 

 magnetism is more easily communicated to, but, 

 at the same time, more easily lost, by soft iron 

 than by steel. 



