186 



The Variation of the Needle. 



supplemented by thrwe ar the Island nf Sr. Helena, and at Ilobartown 

 and Toronto, as the result of years of cautious and critical observation, 

 afforded proofs that the earth's magnetism is governed by certain 

 general laws, while the observations of the amateur astronomer 

 Schwabe, covering a period of forty years, proved the magnetism of 

 the earth to be, to a certain extent, an induced condition, dependent 

 upon the variations in the energy of the sun. But the correspondence 

 of the periods of unusual solar energy with simultaneous magnetic dis- 

 turbances upon the earth's surface, and indeed all the laborious years 

 of work at the great magnetic observatories only teach ns that each 

 locality has its own law of variation of the magnetic force — declina- 

 tion, inclination and intensity — expressible by algebraic formula?, 

 and that the time of maximum aud minimum movement in declina- 

 tion, etc., and the amount of movement at one station (save only in 

 the case of " magnetic storms ") differs more or less from that at some 

 other point upon the world's surface ; proving that local observation 

 is the only method by which the local co-efficients of the magnetic 

 force can be determined. 



The magnetic storms that traverse the earth at times, disturbing 

 almost simultaneously the terrestrial force at every magnetic observa- 

 tory are cosmical phenomena synchronous with similar solar disturb- 

 ances, a possible jarring and quaking of the universe, abnormal and 

 only indirectly related to the particular class of phenomena under dis- 

 cussion, which is the more permanent resultant known as the declina- 

 tion, or horizontal variation. 



The chief service of the magnetic observatories, to field observers, is 

 that their constant series of observations afford a knowledge of the 

 times of maximum and minimum declination at localities nearest the 

 new points of observation, indicating to the explorer the most appro- 

 priate period for observation, and thus economizing much of the labor 

 of field work, while by recording the occurrence of magnetic storms, 

 they render it possible to avoid the introduction of such abnormal dis- 



This will explain why, notwithstanding the existence of superb 

 magnetic observatories in different parts of the world, the greatest 

 scientific importance is attached to all local observations of the direc- 



mere theoretic locations for the isogonic lines, but positive observa- 

 tions, showing the exact location of these lines in a manner to clear 

 up and simplify all practical questions dependent upon such data. 

 The location of the theoretical isogonic lines, as shown upon declin- 



