42 ON MAGNETISM. 



SECTION IV. 



ON TERRESTRIAL MAGNETISM, AS ACTING IN THE HORI- 

 ZONTAL PLANE AT EACH PLACE OF OBSERVATION. 



22. Definition of Local Magnetic Meridian and 

 Magnetic Variation or Declination; instruments and 

 methods for ascertaining roughly the Declination: 

 Azimuth Compass; observation of Suns Azimuth, Va- 

 riation Theodolite; Declination Charts. 



The ' Local Magnetic Meridian ' is the direction, on 

 the horizontal plane of any place of observation, which 

 is taken by the compass-needle. The ' Magnetic Varia- 

 tion' (a very bad term, still commonly employed by 

 nautical men, but for which, among men of science, the 

 term ' Magnetic Declination ' is usually substituted) is 

 the angle made by the Local Magnetic Meridian with 

 the Astronomical Meridian : it requires to have append- 

 ed to it the word " east " or " west " as applied to the 

 north point of the needle. Thus the Magnetic Decli- 

 nation at Greenwich at the present time is about 20 

 West: meaning that the north point of the magnetic 

 needle points west of the astronomical north meridian 

 by 20 nearly. 



