SUN AND THE EARTH'S MAGNETIC FIELD — FLEMING 



183 



chart to show lines along which the compass direction differed from 

 the true north by the same angle, that is, lines of equal magnetic 

 declination or isogonics. Corresponding isoclinic and isodynamic 

 charts show lines of equal dipping or inclination and of equal hori- 

 zontal, vertical, or total magnetic force acting on the compass. World 

 charts of these magnetic contours for different epochs show marked 

 changes in their trends — changes caused by secular variation. Secu- 

 lar variations show apparent small-order dependence in their progress 

 on the sunspot cycle. 



Figure 3. 



-Isomagnetic chart of lines of equal ilecliiiatioii or isogonics, epoch 

 1930. (After U. S. Hydrographic Office.) 



The second important time change is the 24-hour daily or solar 

 variation. This variation takes place chiefly during daylight and 

 changes in a more or less regidar maimer in magnitude and character 

 with geographic position, with the seasons of the year, and with solar 

 cycles ; it was first observed ° and defined in 1722. It is repeated 

 from day to day and is most clearly seen on records obtained on 

 magnetically quiet days. As dawn approaches each day the north 

 end of every compass needle in the Northern Hemisphere shifts 

 slightly toward the east, attaining a maximum eastward elongation 



s Experiments in the presence of the King at Louveaii, Thailand, in 1082 showed the 

 compass direction to be different on seven different days ; probaldy tliese were made at 

 different times of day and thus were really the flrst observed indications of diurnal 

 variation. 



