226 On the Variation of the Magnetic Needle. 



Mr. Canton first attempted to explain the cause of the diurnal 

 variation. He established by experiment the following principle, 

 viz. that the attractive power of a magnet decreases while the mag- 

 net is heating, and increases while it is cooling. He then assumes 

 that the magnetic parts of the earth in the north, on the east side 

 and on the west side of the magnetic meridian, equally attract the 

 north end of the needle. If then the eastern magnetic parts be 

 heated faster by the sun in the morning than the western parts, the 

 needle will move westward, and the absolute variation will increase ; 

 but when the western magnetic parts are either heating faster or 

 cooling slower than the eastern, the needle will move eastward, or 

 the absolute variation will decrease. This explanation seems to ac- 

 count satisfactorily for the principal motion of the needle as exhibit- 

 ed at London, but it is not obvious how it can account for the slight 

 easterly motion in the morning. Mr. Barlow has adopted this by- 

 pothesis, with some modification. He observes : whHe the sun is 

 between the magnetic east and south, those parts being then most 

 heated, their power will be diminished, and the south end of the 

 needle ought to incline to the west, or the north end to the east, and 

 we ought to expect that the greatest declination eastward should 

 take place when the sun is equally distant between those points ; as 

 the sun approaches nearer the south, the parts to the west of the 

 magnetic meridian, as well as those to the east, become heated, and 

 the eastern deviation ought to decrease and disappear entirely as the 

 sun passes the magnetic meridian, because then the effects on each 

 side of that meridian are equal to each other. 



Beyond this period, the southwestern parts will receive the great- 

 est power of the solar rays, become weakened in their action, and 

 the south end of the needle will deviate to the eastward, or the north 

 end to the westward, and continue increasing in its deviation till the 

 sun becomes S. W. (magnetic,) which happens between one and 

 two o'clock in the afternoon, and its effect will be greater than the 

 morning easterly deviation, because it happens when the sun has a 

 greater altitude, and consequently a more intense action. From 

 this period, the western deviation ought to diminish till the sun be- 

 comes west, (magnetic,) when it ought to cease entirely ; because 

 then the parts on the western side of the needle being equally heat- 

 ed, both to the north and to the south, there can be no tendency in 

 either end to incline from the meridian. 



Now the preceding theory may account for the variation of the 



needle at London, where the declination is about 24° W., but it does 



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