426 WELLS'S NATUEAL PHILOSOPHY. 



of a needle which would point due north and south at any place, wiU be 

 the true, or terrestrial meridian of that place. 



What is the The deviation of the needle from the true 

 dS''Hon'"o? north and south, or the angle formed by the 

 the needle? magnetic meridian and the terrestrial merid- 

 ian, is called the variation, or declination of the needle. 

 ■What are the I'hcre arc two lines upon the earth's sur- 

 lines of no va- face, along which the needle does not vary, but 



nation J _ ^ o .7 7 



points to the true north and south. These 

 lines are called the eastern and western lines of no varia- 

 tion, and are exceedingly irregular and changeable. 



Tlieir position is as foUou-s: — The western Une of no variation begins ia 

 latitude 60°, to the west of Hudson's Bay, passes in a south direction through 

 the American lakes, to the West Indies and the extreme eastern point of 

 South America. The eastern line of no variation begins on the north in tho 

 "White Sea, makes a great semicircular sweep easterly, until it reaches tho 

 latitude of 71°; it then passes along the Sea of Japan, and goes westward 

 across China and Hindoostan to Bombay ; it then bends east, touches Australia, 

 and goes south. 



In proceeding in either direction, cast or west from the lines of no varia- 

 tion, the declination of the needle gradually increases, and becomes a max- 

 imum at a certain intermediate point between them. On the west of tbe 

 eastern line the declination is west ; on tliB east it is east. 



At Boston, in the United States, the declination of the needle is cbout 5^° 

 west ; in England it is about 24° west ; in Greenland, 50° west ; at St. Peters- 

 burg, 6° west. 



„ . ,. ,. 800. As the directive property of the magnetic needle is 



How IS the di- f f j o 



rective power observed everywhere in all parts of the world, on all seas, on 

 accou'nteTfor^? ^^^^ loftiest summits of mountains, and in the deepest mines, 

 it is evident that there must be a magnetic force which acts 

 at all points of tlie earth's surface, since magnetic needles can no more take 

 up a direction of themselves than a body can acquire motion of itself. To 

 explain these phenomena, the earth itself is considered to be a great magnet, 

 and the points toward which the magnetic needle constantly turns are called 

 the magnetic poles of the earth. These poles, by reason of their attractive 

 influence, give to the needle its directive power. 



__ The two poles of the great terrestrial magnet which are 



magnetic poles situated in the vicinity of the poles of the earth's axis, are 

 situated ? termed respectively the magnetic north pole and the magnetic 



south pole. These contrary poles attract each other, and thus a magnetic 

 needle will turn its south pole to the north, and its north pole to the south. 

 Hence, what we generally call the north pole of a needle is in reaUty its 

 eouth pole, and its south pole is its north pole. 



The exact position of the northern magnetic pole is about 19° from the north 



