■>x+- - 



I - 



— ■ H* 



i ^ 



4 



i 

 t 



]' 



r 



Sect, 11.] 



TERRESTRIAL MAGNETISM. 



no 



US prefixed, when the north end of the needle inclines 

 below the liorizon ; and is counted negative, or has the 

 minus sign prefixed, wlien the north end of the needle 

 inclines above the horizon, Sometiints, instead of the 

 signs + and — , the term 



C 



o 



s 



1 are used 



•) 



D 



ep 



A 



o • 



IS 



of the needle dips below the lionzori, and South In 

 clination or Dip is when the south end of the needle dip 

 below the horizon. Thus an Inclination of — 30 

 equivalent to 30° South Dip. 



9, The Declination is measured by the azinuith com- 

 pass, an iDstrument too well known to naval officers to 

 require any 'ascription here, or any directions for tlie 

 niethv)d of observing with it either on land or at sea. As 

 now made, under the superintendence of Captain Johnson. 

 R.N., and on the plan recommended by tlie Committee 

 for the Improvement of Ships' Compasses, the azimuth 

 compa£: in the hands of a careful observer, attentive to 

 tlie practical rules published by the Admiralty for ascer- 



taining the deviations of the compa/-" caused by the iron 

 of a ship, will give results, both at sea and on land^ which 

 leave little to be desired. The use of the dipping needle, 

 which measures the inclination, not being so generally 

 familiar to naval officers, full directions for its employment 

 are eiven in Annendix No. 3. 



Local Attraction. 

 10. It has been found that the results of maimetic 



servations, whether of the declination, inclination, or of the 

 intensity of the magnetic force, are liable to be influenced 

 by local attraction proceeding from the rocks or soil in the 



