Sect II.] 



TERRESTIIIAL MAGNEi 



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and have a i^roper distribution over the surface of the 

 globe, and if the formulae are carried to a sutrkMent num- 

 ber of terms, it may be expected that the elements 

 comnuted from them will have the same degree of exact- 



ness as the maps i, jm which their coefficients are taken. 



It 



may 



be natural 





this stane to inquiri; 





prospect exists of being able to complete a work of this 



magnitude within a reasonable time 



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; and to this question 

 more satisfactory reply can be returned than may per- 

 haps be generally anticipated. Nearly three-fourths of 

 the surface of the globe being covered by the ocean, it is 

 chiefly by naval surveys that the materials for sucli a, 

 work can be collected. By the zealous and unwearying 

 assiduity of British officers, acting under the sanction and 

 ^- the approval of the Lords Commissioners of th 



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Admiralty, and in some instances in expeditions specially 

 a])pointed for the purpose, magnetic observations designed 

 expressly for the object above-mentioned, and conducted 



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upon a uniform system, ha\e been extended, since theii 

 commencement in 1839^ over nearly all the accessible 

 parts of the ocean. 



Of these surveys, the results of some have been already 

 deduced and published in the Philosophical Transactions, 

 the expense of publication having been borne conjointly 

 by the Government and by the Royal Society ; the results 



of others are undergoing the process of calculation and 



arrringement for publication ; and in a third class, more 

 .:...._rous than either of the other two, the observations 



themselves are still in progress. Tn the class first referred 

 to, viz. surveys in which the observations have been com- 

 pieted and the results deduced and published^ may be 



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