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CHAPTER XVIII. 



ON THE GENERAL APPLICATION OF THE LAWS OF TERRES- 

 TRIAL MAGNETISM. 



Notwithstanding the existence of magnetism has been so long 

 known, and turned to account in navigation and general survey- 

 ing, and has been the subject of numerous experiments and 

 theories, the application of this principle, beyond that of the 

 above, has been of a nature purely local, confined to mere toys, and 

 never has been duly considered in the magnitude of its effects 

 and universal character. A detail of all the various useful applica- 

 tion of which this universal principle is susceptible, would occupy 

 volumes, as it embraces all phenomena now classed under the 

 head of gravitation ; it has an intimate connexion w 7 ith our pro- 

 ceedings, and we are always under the immediate dominion of 

 its laws, which laws are found permanent, consistent, intel- 

 ligible, and discoverable with only a moderate degree of research. 

 Hence, since it is certain that such a principle exists, it is mani- 

 festly of the utmost importance to study its laws and to know 

 how to apply them, were it for- no other reason than to enable 

 us in all w r e undertake to have at least the laws on our side, so 

 as to avoid struggling in vain against difficulties opposed to us 

 by natural causes. 



It is the variation in the density of the enveloping magnetic 

 fluid which is the cause of pendulums of equal length varying in 

 the number of their oscillations in a given time in different 

 latitudes ; and in order to make them keep correct time, it is 

 necessary to vary the lengths respectively according to the 

 variation in the intensity of the radial attraction in each latitude. 

 We find also that the zones of equal density in the atmosphere 

 are not at equal heights in different latitudes, and therefore in 

 making barometrical measurements the formulae should be made 

 to correspond to the variation in each latitude. 



The polarity of terrestrial magnetism is the mariner's faithful 

 guide wdien enveloped in darkness on the boundless ocean ; and 

 he knows that if masses of iron should be placed near the mag- 

 netic needle it becomes disturbed and rendered useless. The 



