TERRESTRIAL MAGNETISM. 



125 



For more than a century after Columbus' discovery, and beyond 

 the time of publication of Gilbert's treatise, it remained unknown 

 that not only does the direction of the compass vary from place 

 to place, but also from time to time. This further important 

 fact was first perceived by Gellibrand, a Gresham professor of 

 mathematics, in 1634, on comparing an observation of declina- 

 tion then made by him, at Deptford, with earlier observations 

 made in 1580 and 1622. The change of declination in the longer 

 interval was about 7°. A new complication was thus intro- 

 duced into the practical utilization of the magnetic properties 

 of the earth, not only in navigation, but also in land and mine 

 surveying. A chart or survey directed by the compass will not 

 remain true, as judged by the compass, for many years. The 

 fact is one of considerable economic importance, because in 

 many countries old maps and surveyors' plans were drawn by 

 compass rather than by astronomical directions. The magni- 

 tude of the change of compass declination, over a long period, 

 is very great. Between 1580 and 1810 the declination at London 

 changed from 11° E. to 24° W., a total variation of 35°. The 

 compass now points about 14° west of north, in London, and is 

 still diminishing, at present at the rate of 1° in six or seven years. 

 These remarkable changes in the direction of the horizontal 

 component of the earth's magnetic force are associated with 

 changes in the magnetic dip, and also (as is now known) in the 

 magnitude as well as direction of the force. They are mani- 

 festations of mysterious and far-reaching changes in the 

 earth itself, but magnetic science has not yet succeeded in 

 either explaining or predicting these secular variations. Their 

 existence, however, renders it necessary for magnetic charts to 

 be revised from time to time, at intervals of a few years, indeed, 

 if they are to remain of value in navigation. 



Halley, the author of the first chart of magnetic declination, 

 aroused interest in the study of the earth's magnetism by the 

 publication (in 1683) of a theory intended to explain the secular 

 changes in the compass declination. He saw that the distribution 

 of declination (at any one time) was too complicated for it to 

 be supposed that the earth was a uniformly magnetized 

 sphere with two poles, even if these were not regarded as 

 coincident with the poles of the axis of rotation. He suggested 

 that there were, on the contrary, four magnetic poles, " near 

 each pole of the equator two," and he tried to explain the peculiar 

 nature of the secular changes of declination by assigning diflerent 



