182 Mb. J. Brxce on the Recent Progress of the 



coupled, of course, with those of temperatiu-e and pressure. General 

 Sabine discussed, arranged, and published the results. 



It is now pretty clearly ascertained that there are four magnetic poles, 

 that is, four points on the earth's surface, where the dipping needle 

 stands in a vertical position, and which (for distinction's sake), we may 

 call poles of verticity. The west pole was fixed in Boothia Felix, in lat. 

 70' 5' 17" N., long. 96° 45' 4S" W. Another pole hes m the north of 

 Siberia, in lat. 82° 3', long. 114" 33' E. In the southern hemisphere 

 the poles of verticity have been placed, by the researches of Ross, in 

 lat. 68° 51', long. 131° 33' E. ; and south of the American continent 

 nearer the pole of the eai-th, namely, in lat. 76° 7', long. 143° 34' W. 

 The latter he was imable exactly to reach, on account of the magnificent 

 barrier of ice, which presented lofty cliffs seawai-d, in front of Victoria 

 Laud, through a distance, nearly east and west, of 1,000 miles. Most 

 persons will remember the striking descriptions which Eoss gives of this 

 wondrous region — the ice covered land with its lofty volcanoes rivalling 

 Mont Blanc in altitude, and its high icy cliffs presenting a crystal 

 barrier to the roll of the antarctic waves. Here, amid volcanoes and 

 glaciers, Hes unapi)roachable the western magnetic pole of the southern 

 hemisi:)here. 



(2.5.) Besides these poles of verticity in both hemispheres, there are also 

 poles of maximum intensity, distinct, and considerably removed from the 

 poles of verticity. The intensity of the force is estimated in the same 

 way as that of the force of gravity in different latitudes, by means 

 of the oscillations of a pendulum — a freely suspended needle is with- 

 drawn from the meridian, and oscillates a certain number of times in 

 resuming its original position. This being done at different points, 

 the intensities at those points are in the ratio of the squares of the 

 numbers of oscillations. Thus, if at two points the number of oscilla- 

 tions are 24 and 25, the intensities at those points are as 576 : 625, or 

 ]-000 : rOS5. These intensities have been estimated with great care 

 iu so many pai'ts of the globe, that lines can be laid down upon a map 

 connecting them. These are the isodynamic Hues, or lines of equal 

 intensity. They form at first regular ellipses around the poles of inten- 

 sity, and change into various forms, chiefly looped cm-ves, hke the figure 

 eight. The position of these " intensity poles" has been accurately 

 determined within the last four years for the northern hemisphere; 

 the western most recently. It is situated near the south-west comer 

 of Hudson's Bay, in lat. 52° 19', long. 92° W. Here, and at Toronto, 

 the inchnation is about 75^°. Another pole of less intensity is in 

 Siberia, about long. 120° E. The total force has clearly two compo- 

 nents — that which produces declination, or variation cast and west, and 



