MAGNETISM AND THE COMPASS. 665 



Magnetic Poles, the needle is governed thereby, the nearest Pole being 

 always predominant over that more remote. This view of the earth's 

 Magnetism was supported by the results of the labours of Professor 

 Hansteen, one of the chief promoters of the science, as shown in his most 

 valuable work (Magnetismus der Erde, Christiania, 1817). Having col- 

 lected all the observations of value that had been made on the Variation 

 of the needle, he proved that there w eve four points of convergence among 

 the lines of Variation ; viz., a weaker and a stronger point, in the vicinity 

 of each Pole of the globe. This, combined with the result of Sir D. 

 Brewster's inquiries, will certainly lead to the view of the connection 

 between the heat of the earth and its Magnetism. Professor Hansteen 

 considered that the strongest Poles, N. S., lie almost diametrically opposite 

 to each other, and the same is true of the weaker Poles n. s. These four 

 Poles he found to have a regular motioyi obliquely ; the two Northern 

 ones, N. n., from West to East, and the two Southern ones, S. s., from 

 East to West. The following he found to be their periods of revolution, 

 and their positions in 1830 : — 



Time of Revolution round 

 Latitude. Long, from Greenwich, eacli Pole of the Earth. 



PoleN. ... 69°30'N 87° 19' W 1,740 Years. 



Poles. ... 68°40' S 131' 47' E 4,609 



Pole». ... 85°6'N 14n7' E 860 „ 



Poles. ... 78"29'S 137^5' W 1,304 „ 



From calculations based upon subsequent observations he slightly 

 varied these positions and periods ; but he showed very clearly that the 

 changes in the Variation and Dip of the needle, in both Hemispheres, 

 may be well explained by their motion. 



Kecently acquired knowledge enables us to understand the arrangement 

 of these Poles to be as follows. In each Hemisphere there is one 

 Magnetic Pole, where the magnetic needle would take up a vertical 

 position. The Northern Magnetic Pole is to the N.N.W. of Hudson's Bay, 

 in about lat. 70° N., long. 96° W. ; the Southern Magnetic Pole is to the 

 South of Tasmania, in about lat. 73f= S., long. 147i° E. These Poles 

 must not be considered as mere points on the earth's surface, but rather 

 as limited areas ; at the Northern Pole, the dipping-needle has been 

 found by actual observation to be vertical over a region of some 50 miles 

 square.* 



Besides the Magnetic Poles, the directive power of the magnetic needle 

 is also affected by the Magnetic Force. This is not evenly distributed 

 over the earth's surface, and the points of maximum intensity do not 

 coincide with the Magnetic Poles. In the Northern Hemisphere there are 

 two foci of maximum Force, the more powerful in about lat. 52° N., 

 long. 92° W., near the great American lakes, the weaker in Siberia, in 

 lat. 65° N., long. 115° E. It is also believed that two similar foci, of 



• " The Magnetism of the Earth. A Lecture on the Distribution and Direction of 

 the Earth's Magnetic Force at the Present Time : the Changes in its Elements, and 

 on our Knowledge of the Causes." By Captain F. J. Evans, C.B., F.R.S., in the "Pro- 

 oeedingB of the Royal Geographical Society," No. iii., vol. xxii., 1878, pages ISS-— 216. 



N. A. 0. 85 



