SUK AND THE EARTH'S MAGNETIC FIELD — FLEMING 181 



The principal magnetic poles are distant 1,200 miles or more from 

 the geographic poles. The north magnetic pole, visited in 1831 by 

 Ross and in 1903 by Amundsen, is in Boothia Peninsula in north 

 Canada (latitude 70.°5 north, longitude 95.°5 west). The south 

 magnetic pole, reached in 1909 by E. David, Douglas Mawson, and A, 

 Mackay, of Shackleton's British Antarctic Expedition of 1907-09, is 

 in South Victoria Land of the Antarctic Continent (latitude 72. °4 

 south, longitude 155. °3 east). Thus the line joining the magnetic 

 poles is not a diameter of the earth but passes at a distance of some 

 750 miles from its center. It is to be noted that the equivalent axis 

 of the uniform magnetization intercepts the northern hemisphere in 

 latitude 78. °5 north and longitude 69. °1 west. Thus these so-called 

 "geomagnetic" poles are considerably removed from the actual mag- 

 netic poles as observed. 



Measurements to determine the earth's magnetic field at any point 

 and time must include observations of three magnetic elements, 

 namely: (1) Magnetic declination or direction, the angle between the 

 true astronomical north-south meridional plane and the vertical 

 plane through the magnetic north-south direction as defined by the 

 compass; (2) magnetic inclination or dip, the angle through which a 

 magnet entirely free to move would dip below the horizon in the 

 magnetic north-south meridional plane; and (3) the total magnetic 

 force, acting in the magnetic meridional plane or its horizontal com- 

 ponent or its vertical component. 



Painstaking and patient recording and analyses of the complex 

 geomagnetic variations through days, years, and decades at observa- 

 tories and on magnetic surveys on land and sea have disclosed cer- 

 tain systematic features and irregular variations of these, all of 

 which may be designated as time changes of the geomagnetic field. 

 The more pronounced systematic and periodic features are the secu- 

 lar, daily or solar diurnal, lunar-day, and annual variations. 



Secular variation is a progressive change, that is, a slow age-long 

 variation; it was first noted in compass direction — the so-called varia- 

 tion of the mariner — by Gellibrand in 1634 who announced quaintly 

 that "variation is accompanied with a variation." It takes important 

 part in navigation at sea by magnetic direction as indicated by the 

 compass — a use which stimulated and maintained interest in deter- 

 mining its value from the time of Columbus. Before the invention 

 of chronometers it was thought that geographic position might be 

 derived from knowledge of changes from place to place in magnetic 

 declination and inclination. This led to the first systematic oceanic 

 survey by the astronomer Edmund Halley in the Atlantic Ocean 

 during 1698-1700 on the pink ^ Paramour. Halley's was the first 



* A pink is an old-style, narrow-sterned sailing vessel. 



