204 ANNUAL EEPORT SMITHSONIAN INSTITUTION^ 191.3. 



authors have unwittingly credited Halley with a discovery which, 

 in the absence at the time of any observation whatsoever respecting 

 the strength of the earth's magnetic force, he could not possibly 

 have made. The real merit and purport of Halley's deduction has 

 thereby been obscured. The observation material at Halley's dis- 

 posal, before he himself enriched the material during his voyages, 

 consisted of some miscellaneous observations of the compass direction 

 and a few values of the magnetic dip. As has been said, there were 

 no observations of the magnetic force, for the art of measuring this 

 element had not yet become known. 



Scrutinizing carefully his scanty observation material, Halley no- 

 ticed that the direction of the compass needle did not change from 

 place to place in the simple way it would if, for example, the earth 

 had two magnetic poles diametrically opposite each other. In the 

 latter case the needle would set itself tangent to the great circle pass- 

 ing through the magnetic poles and the place of observation. If, 

 then, the compass direction were laiown at two places sufficiently 

 far apart the points of intersection of the two great circles drawn 

 respectively tangent to these compass directions would be the two 

 diametrically opposite magnetic poles. It is such points of inter- 

 section — " points of convergence," as Hansteen later called them — 

 which Halley had in mind as " Magnetic Poles." He was the first 

 to perceive clearly the fact — abundantly verified since — that the 

 various points of convergence as found from successive pairs of 

 compass directions, in the manner just described, do not fall together 

 as they should on the basis of a simple or regular magnetization of 

 the earth. However, it appeared to Halle}'^, and the same conclusion 

 was reached over 100 years later by the illustrious Norwegian mag- 

 netician, Hansteen, that the several points of convergence grouped 

 themselves in a general way about two main centers — 



near each Pole of the Equator Two, and that in those parts of the World which 

 lie near adjacent to any one of those JMagnetical Poles the Needle is goveru'd 

 thereby, the nearest Pole being always predominant over the more remote. 



It will not be well to lay greater stress upon this deduction nor 

 upon those in his 1692 paper, where he seeks to account for the exist- 

 ence of his four " Magnetic Poles " and for the secular variation than 

 to say that Halley drew the best possible conclusions the material at 

 his disposal j^ermitted. In fact, his conclusions were not materially 

 improved upon until a century and a half later, when a much more 

 complete knowledge of the distribution of the earth's magnetism 

 had been gained and when the various mathematical attempts which 

 had been made to compute the magnetic elements on the basis of 

 more or less intricate hypotheses as to the earth's magnetization, 

 had been found to be inadequate. Some later investigators, indeed, 



