302 INTENSITY OF MAGNETIC FORCE. SECT. XXX. 



points have been ascertained in which that curve cuts 

 the equator; yet Captain Duperry, who crossed it re- 

 peatedly, affirms, from his own observations combined 

 with those of M. Jules de Bosville and of Colonel 

 Sabine, that it crosses the terrestrial equator in two 

 points only, and those diametrically opposite one to the 

 other, and not far from the meridian of Paris. One of 

 these nodes he places in the Atlantic, the other in the 

 Pacific ocean. He finds that the magnetic equator 

 deviates but little from the terrestrial equator in that 

 part of the Pacific where there are only a few scattered 

 islands (6), that as the islands become more frequent 

 the deviation increases, and arrives at a maximum both 

 to the north and south in traversing the African and 

 American continents ; and that the symmetry of the 

 northern and southern segments of this curve is much 

 greater than was imagined. 



The intensity of the magnetic force is different in dif- 

 ferent parts of the earth. If a magnetic needle, freely 

 suspended so as to move horizontally, and at rest in a 

 magnetic meridian, be drawn any number of degrees 

 from that position, it will make a certain number of os- 

 cillations before it resumes its state of rest. The inten- 

 sity of the magnetic force is determined from these os- 

 cillations, in the same manner that the intensity of the 

 gravitating and electrical forces is known from the vibra- 

 tions of the pendulum and the balance of torsion (c) : 

 and in all these cases it is proportional to the squares of 

 the number of oscillations performed in a given time, 

 consequently a comparison of the number of vibrations 

 accomplished by the same needle during the same time 

 in different parts of the earth's surface will determine 

 the variations in the magnetic action. By this method 

 it was discovered that the intensity of the magnetic force 

 increases from the equator toward the poles ; but the 

 foci of the greatest total intensity of the magnetic force 

 seem neither to coincide with the magnetic nor rotatory 

 poles of the earth (d). One of these foci, according to 

 Colonel Sabine's magnetic chart, is situate about the 47 

 south latitude and 140 east longitude, while another of 

 less energy is in 60 south latitude and 235 east longi- 

 tude. The point of least total magnetic intensity on the 



