MAGNETIC INTENSITY. 87 



netic needle participate, in accordance with the different sea- 

 sons of the year, in producing the alternating phenomena 

 observed in both hemispheres.* 



In this enumeration I have restricted the use of the word 

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

 horizontal force disappears, because, as I have already re- 

 marked, 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.! Gauss has also 

 shown that it would be inappropriate to attempt to distin- 

 guish the chord which connects the two points at which the 

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

 of the earth.f The intimate connection which prevails be^ 

 tween the objects here enumerated fortunately renders it pos- 

 sible to concentrate, under three points of view, the compli- 

 cated phenomena of terrestrial magnetism in accordance with 

 the three 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 

 •the terrestrial force, followed somewhat tardily the knowl- 

 edge of the relations of the direction of this force in horizon- 

 tal and vertical planes (declination and inclination). Oscil- 

 lations, from the duration of which the intensity is deduced, 

 were first made an object of experiment toward the close of 

 the 18th century, and yielded matter for an earnest"^ and con- 

 tinuous investigation during the first half of the 19th centu- 

 ry. Graham, in 1723, measured the oscillations of his dip- 

 ping-needle with the view of ascertaining whether they were 

 constant,§ and in order to find the ratio which the force di- 

 recting them bore to gravity. The first attempt to determ- 

 ine the intensity of magnetism at widely different points of 



* " Stations of an intermediate character, situated between the 

 northern and southern magnetic hemispheres, partaking, although in 

 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., p. 53-57. 



t The pole of intensity is not the pole of verticity. Pliil. Transact, 

 for 1846, pt. iii., p. 255. 



X Gauss, Allgem. Theorie des Erdmagnetismus^ § 31. 



§ Phil. Transact., \oL xxxiii., /or 1724-1725, p. 332 ("to try if the 

 dip and vibrations were constant and regular"). 



