428 WELLS'S NATURAL PHlLOSOPnY. 



ing, that the points upon the earth's surface where the greatest degree of cold 

 is experienced, or where tlie yearly mean of the thermometer la lowest, 

 coincides with the location of the magnetic poles. 



What is the ^^^' B^side the variation from the true 

 uo"!r''of^'"the north and south, the magnetic needle is sub- 

 needie? jgg^ to a dlumal Variation. This movement, 



or variation, commences about sevenin the morning, when 

 the north end of the needle begins to deviate toward the 

 west ; it reaches its maximum deviation about two o'clock 

 in the afternoon, when it begins to return slowly to its 

 original position. 



The magnetic needle is subject also to an annual movement, and a move- 

 ment different in the winter months from that noticed in the summer months. 



wiiat is the ^^^^ daily, monthly, and yearly variations 

 of the^triodiMi ^f *^® needle are supposed to be occasioned by 

 the needle? "^ Variations in the temperature of the earth's 

 surface, depending upon the changes in the 

 position and action of the sun. 



Observations made for a great number of years seem to show that the en- 

 tire magnetic condition of the earth is subject to a periodical change, but 

 neither the cause or the laws of this change are as yet understood. 



For most practical operations, as in navigation and sur- 

 veying, the deviation of the magnetic needle from the true 

 north and south, is carefully taken into account, and a rule 

 of corrections applied. A knowledge of the amount of vari- 

 ation, east or west, for different localities upon the earth's 

 surface, may be obtained from tables accuratel} arranged 

 for this purpose. 



The variation of the magnetic needle from tof vnie north and south, is said 

 tc have been first noticed by Columbus in his Irst voyage of discovery. It 

 was also observed by his sailors, who were alarmed at the fact, and urged it 

 as a reason why he should turn back. 



When was the Thc compass is clainaed to have been dis- 

 ^™e?ed? "^'^ covered by the Chinese : it wa.-3, however, 

 known in Europe, and used in the Mediterra- 

 nean, in the thirteenth century. The compasses of that 

 time were merely pieces of loadstone fixed to a cork, which 

 floated on the surface of water. 



802. The resemblance between magnetism and ekcirlcity is very striking, 

 and there are good reasons for believing that both are but modifications of 



