344 LINES OF EQUAL INTENSITY. [SECT. xxx. 



is even supposed to be decreasing. In the higher latitudes 

 the lines of equal magnetic intensity, or those which are 

 imagined to go through all the points on the earth's surface 

 where the magnetic intensity is the same, form a series of 

 ovals round each of the foci of greatest magnetic intensity in 

 each hemisphere, having their major axes in the line of direc- 

 tion joining the two foci. At a greater distance these lines 

 form a figure of 8, each loop of which incloses a focus with its 

 surrounding ovals. Exterior to this figure of 8, the lines of 

 equal intensity do not cross ; but the inflexions which pro- 

 duced the double loop become less and less, and the curves 

 more and more regular, as they approach nearer to the dy- 

 namic equator. The lines in the southern hemisphere have 

 the same general character ; but none of the ovals are of the 

 same size, because the foci in the southern hemisphere are 

 at a less distance from each other than those in the northern, 

 and are otherwise unsymmetrical with regard to them. The 

 moon occasions periodical variations in the intensity, de- 

 pending on her position with regard to the earth. 



In consequence of the attraction and repulsion of the 

 magnetic poles, the declination needle or compass, or indeed 

 any piece of iron free to move in every direction, only 

 remains in equilibrio when in a magnetic meridian, that is, 

 when in a plane passing through the magnetic poles of the 

 earth. In some places the magnetic meridians coincide 

 with the geographical meridians ; and in these the declina- 

 tion needle points to the true north and south ; but, if it be 

 carried successively to places having different longitudes, it 

 will deviate, sometimes to the east and sometimes to the 

 west of the true north. Imaginary lines on the globe, 

 passing through all places where the needle points due 

 north and south, are lines of no variation ; and lines passing 

 through all places where the magnet deviates from the 

 geographical meridian by an equal quantity are called lines 

 of equal variation. They are extremely irregular, and form 

 two closed systems, that is, they surround two points, one in 



