SECT, xxx.] DYNAMIC EQUATOR. 343 



secular motion in the magnetic equator. There are also 

 disturbances of a periodic nature, and others very transient, 

 which M. Kreil ascribes to weak shocks of earthquakes, 

 having observed that the greatest vertical disturbances 

 have almost always coincided with considerable earth- 

 quakes, even when they occurred in remote regions. 



The intensity of the magnetic force is different in 

 different parts of the earth, and varies as the square of 

 the number of vibrations which the declination needle 

 makes in a given time in different latitudes. It has been 

 thus discovered, that there are four points on the globe in 

 which the magnetic force is a maximum, two in each 

 hemisphere, which neither coincide with the magnetic 

 poles nor with the poles of the earth's rotation. One of 

 these foci of maximum intensity is in North America, to 

 the south-west of Hudson's Bay ; the other is in 120 east 

 longitude, in Northern Siberia. In the southern hemi- 

 sphere, one focus is in the South Atlantic, in 20 south 

 latitude, and 324 57' east longitude; and the other in 

 60 19' south latitude, and 131 20' east longitude. The 

 greatest and least intensities are in the southern hemi- 

 sphere : taking one as the unit at the magnetic equator in 

 Northern Peru, they are as 2'07l to 0706, the former deter- 

 mined by Sir James Ross, the latter by M. Erman. 



On account of that unequal distribution of force, the 

 decrease in the magnetic power, from the four foci towards 

 the equator, is so irregular that the dynamical equator, 

 which is the line supposed to pass through that point in 

 each meridian in which the magnetic intensity is least, is a 

 very irregular curve encircling the earth. It is of unequal 

 intensity in its different parts, and neither coincides with the 

 magnetic equator nor with the equator of the earth ; but it 

 separates the forces which attract the north end of a magnet 

 from those that attract the south end. The intensity at 

 St. Helena, which is situate near the line of least dynamic 

 force, is less than in any part of the globe yet known, and 



