304 CHANGES OF MAGNETIC INTENSITY. SECT. XXX. 



ceived simultaneously over widely extended regions; 

 while others of less magnitude and duration occur more 

 frequently, and are, equally witty the greater, not amena- 

 ble to any known laws. 



The dip is subject to a secular variation, and according 

 to Colonel Sabine has been decreasing in northern lati- 

 tudes for the last fifty years at the rate of three minutes 

 annually, and is probably owing to the secular motion of 

 the magnetic equator. There are disturbances also in 

 the dip of a periodic nature, and others very transient, 

 which M. Kreil attributes to weak shocks of earth- 

 quakes, having observed that the greatest vertical dis- 

 turbances have almost always coincided with consider- 

 able earthquakes even when they occurred in remote 

 regions. 



The magnetic intensity is subject to various changes. 

 M. Hansteen has found that it has been decreasing an- 

 nually at Christiana, London, and Paris at the rate of 

 its 235th, 725th, and 1020th parts respectively, which 

 he attributes to the motion of the Siberian magnetic 

 pole. The moon increases the onagnetic intensity in 

 our hemisphere : but her influence differs with her dif- 

 ference of position in the heavens. The times of vibra- 

 tion of the needle are less when the moon has south 

 declination than when she has north, and they are less 

 when she is in perigee than in apogee. It is still doubtful 

 whether magnetic intensity varies with the height above 

 the earth or not. 



The diurnal variation in the horizontal intensity ob- 

 served by M. Hansteen at Christiana is probably owing 

 to the sun's influence : indeed the whole of the magnetic 

 disturbances have been ascribed to that cause ; and he 

 has even found a general resemblance between the iso- 

 thermal lines and the lines of equal dip on the surface 

 of the earth : yet in the present state of our knowledge 

 the magnetic phenomena can only be regarded as the 

 effects of a combination of causes whose separate action 

 is still unknown. 



The inventor of the mariner's compass, like most of 

 the early benefactors of mankind, is unknown. It is 

 even doubted which nation first made use of magnetic 

 polarity to determine positions on the surface of the globe. 



