( 14 



[Sect. II. 



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^ 



Sect. 





Section IT. 



TE 



EESTEIAL MAGNETISM. 



BY LTEUT.-COLONEL EDWAIII) SABINE, 



or THE ROYAl. ARTILLERY. 



1. The magnetic observations which are at present 

 raakiTJg by naval officers have for their object the deter- 

 mination of the amount and direction of the Eartli^s mag- 

 netic force in different parts of the globe. 



2. The amount of the magnetic force at any point of 

 the Earth's sm^face may either be measured in ahsolntf^ 



m 



or its ratio may be ascertained to the value of the 

 at another station where its absobate measure is 



value 

 force 



9 



already known. 



means have vet been devised for 

 measuring absolute values at sea ; consequently, all de- 

 terminations of the magnetic force on board ships are 

 necessarily of the relative class : these crive the ratio, or 



5 



proper lion, vvhich the force at the geographical position 

 in which the ship is at the time when an observation is 

 m^ade, bears to its value a^ some land station which is in- 

 eluded in the same series of relative observations, but 

 where an absolute determination has also been made. 

 Ships are therefore supplied with instruments for both 

 absolute and relative determinations ; the latter to be used 

 at seaj and on land at times when the ship is in harbour; 

 the former to be used exclusively on land. 



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