THE MAGNETIC POLES. 59 



Halley is identified with an important epoch in the history 

 of terrestrial magnetism. He assumed that there was in 

 each hemisphere a magnetic pole of greater and lesssr 

 intensity, consequently four points with 90 inclination of 

 the needle, precisely as we now find among liie four points of 

 greatest intensity an analogous inequality in the maximum 

 of intensity for each hemisphere, that is to say, in the 

 rapidity of the oscillations of the needle in the direction of 

 the magnetic meridian. The pole of greatest intensity was 

 situated, according to Halley, in 70 S.L. 120 east of 

 Greenwich, and therefore almost in the meridian of King 

 George's Sound in New Holland (Nuyts Land). 63 Halley's 

 three voyages, which were made in the years 1698, 1699, 

 and 1702, were undertaken with the view of elaborating a 

 theory which must have owed its origin solely to the earlier 

 voyage which he had made seven years before to St. Helena, 

 and to the imperfect observations of variation made by 

 Baffin, Hudson, and Cornelius van Schouten. These were 

 the first expeditions which were equipped by any government 

 for the establishment of a great scientific object that of 

 observing one of the elements of terrestrial force on which 

 the safety of navigation is especially dependent. As Halley 

 penetrated to 52 south of the equator, he was able to 

 construct the first circumstantial variation chart, which 

 affords to the theoretical labours of the 19th century a point 

 of comparison, although certainly not a very remote one, of 

 the advancing movement of the curves of variation. 



Halley's attempt to combine graphically together by lines 

 different points of equal variation was a very happy one, 64 

 since it has given us a comprehensive and clear insight into 

 the connection of the results already accumulated. My iso- 

 thermal lines (that is to say lines of equal heat or mean 

 annual summer and winter temperature), which were early 



63 Edmund Halley, in the Philos. Transact, for 1683, vol. xii, No. 148. 

 p. 216. 



64 Lines of this kind, which he called tractus chalyboeliticos, were 

 marked down upon a chart by Father Christopher Burrus, in Lisbon, 

 and offered by him to the King of Spain for a large sum of money ; 

 these lines being drawn for the purpose of showing and determining 

 longitudes at sea. See Kircher : s Magnes, ed. 2, p. 443. The first varia- 

 tion chart, which was made in 1530, has already been referred to IL. tko 

 text (p. 56). 



