MAGNETIC INTENSITY. 87 



equator. w This undulating line does not coincide either 

 with the geographical or the magnetic equator. 



The limitation of the zone where the intensity is generally 

 very weak, and in which the horary alterations of the mag- 

 netic needle participate, in accordance with the different 

 seasons of the year, in producing the alternating phenomena 

 observed in both hemispheres 89 . 



In this enumeration 1 have restricted the use of the word 

 pole to the two points of the earth's surface, at which the hori- 

 zontal force disappears, because, as I have already remarked, 

 these points, which are the true magnetic poles, but which 

 by no means coincide with the maxima of intensity, have 

 frequently been confounded in recent times with the four 

 terrestrial points of greatest intensity. w Gauss has also 

 shown that it would be inappropriate to attempt to distinguish 

 the chord which connects the two points, at which the dip of 

 the needle = 90, by the designation of magnetic axis of 

 the earth 91 . The intimate connection which prevails between 

 the objects here enumerated fortunately renders it possible 

 to concentrate, under three points of view, the complicated 

 phenomena of terrestrial magnetism in accordance with the 

 u je manifestations of one active force Intensity, Incli- 

 nation, and Declination. 



Intensity. 



The knowledge of the most important element of terres- 

 trial magnetism, the direct measurement of the intensity of 



88 See Mrs. Somerville's short but lucid description of terrestrial 

 magnetism, based upon Sabine's works (Physical Geography, vol. ii, 

 p. 102). Sir James Ross, who intersected the curve of lowest intensity 

 in his great Antarctic expedition, December, 1839, in 19 S. lat. and 

 29 ' 13' W. long., and who has the great merit of having first deter- 

 mined its position in the southern hemisphere, calls it " the equator of 

 less intensity." See his Voyage to the Southern and Antarctic Regions, 

 vol. i, p. 22. 



89 " Stations of an intermediate character, situated between the 

 northern and southern magnetic hemispheres, partaking, although iu 

 opposite seasons, of those contrary features which separately prevail (in 

 the two hemispheres) throughout the year." Sabine, in the Phil. 

 Transact, for 1847, pt. i, pp. 5357. 



90 The pole of intensity is not the pole of verticity. Phil. Transact* 

 for 1846, pt. iii, p. 255. 



91 Gauss, Allyem... Theorie des Erdmagnetism**, 31. 



