24 



TERIIESTRIAL MAGNETISM. 



[Sect. II. 



vicinity of the instrument, and particularly so at stcitions 

 where the rocks are of igneous character, such as traps, 

 basalts, granites, &c. As a precautionary measure, ther.. 

 U raagnetical instruments should always he used on 



fore 



stands which raise them 3 or 4 feet above the ground ; 

 and those stations are to he preferred of which the <>eo- 

 logical character is sedimentary or alluvial. Stations of 

 igneous character, though less eligible for obtaining results 

 which show the correct magnetical elements corresponding^ 

 to the geographical position of the station, may nevertlie- 



be serviceable as stations of comparison between the 





IS essen- 



land and sea instruments ; but for this purpose it 



tial that the different instruments to be compared should 



1 



.*v V 



be used precisely on one and the same spot, in wdueh cu.. 

 the local attraction may be supposed to be a constant 

 quantity. And if the station be one frequently resorted 

 to by vessels from which magnetic observations are made, 

 it is desirable that tlie spot should be suscepti'^^e of a 

 definite and well-recognisable description. 



At sea, from the quantity of iron which a ship contains, 

 it is scarcely possible that its influence on the instruments 

 should be altogether avoided ; but from the circumstance 

 the' the greater part of the most influential iron is in 

 fixed positions in the ship, it has been proved by sufficient 

 experience, that by a proper selection of the place in 

 vihich a magnetic instrument is used on board ship, and 

 by a certain process of observation (repeated whejiever 

 the ship has undergone any consideriible changes of geo- 

 graphical position), the influence of her iron is susceptible 

 of a sufficiently annroximate calculation, and ot uemg 



'7 ap 



eliminated accordini>i 



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Sect 



111 



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this f^^' 

 If .houia be 



it- 



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iietlc obserr; 

 Ills sliip- has 



take care 



th 



tbe sljip is 

 himself that 

 jicrienced ii 

 id' - may 

 ^'■, >al' tlieii 



tbi instrume^ 



auaoestatic:: 

 relati\e d 



fertile g*-^ -■* 

 ^" pilar fbr ■ 



^tions 



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