1907] AND MOUNTAIN FORMATION. 405 



the resultant forces at any point of space due to the acting and acted- 

 upon systems respectively, 6 the angle between their directions, and 

 dv the element of volume occupied by any element of magnetism, 

 we have (Encyclopedia Britannica, article '' Magnetism," p. 230) : 



^=+4^iriri>^'-^^''"- ('3) 



Now unless R or R' is zero, so that the elements of the triple 

 integral vanish, or one of the magnets exerts only an infinitesimal 

 force upon the other, this mutual potential energy is always finite. 

 Accordingly, whenever a new magnet of sensible power is suddenly 

 developed in the field of the earth's magnetism, the magnetic needle 

 necessarily is disturbed, as in earthquakes and volcanic outbursts. 

 The fact that such disturbances are so repeatedly observed, leaves 

 no doubt of the development of temporary magnets in the earth's 

 field of force; and the intensity of the disturbance necessarily is 

 greatest near the place where the second magnet is developed. Hence 

 the disturbance of the needle by such outbreaks indicates the genera- 

 tion of temporary magnets in the earth's crust, and especially in the 

 atmosphere. The dissipation of the charge restores quiescence to 

 the earth's magnetic field, and the needle ceases to tremble. 



§ 22. Terrestrial Magnetism Modified by Irregularities in the 

 Earth's Crust. — In the National Geographic Magazine for Sep- 

 tember, 1907, Dr. L. A. Bauer has a review of the work done by 

 the Galilee in the survey of the North Pacific ocean authorized by 

 the Carnegie Institution. After describing the work already done 

 he adds : 



"An all-sailing vessel, however, does not permit the magnetic survey to 

 be undertaken with the completeness and success demanded, since with such 

 a vessel it is more or less dangerous to investigate the magnetic irregularities 

 almost invariably shown to exist near land masses. The mapping of these 

 irregularities is of the greatest importance to the mariner, as in many cases 

 they are sufficient, if not allowed for, to land a vessel on the rocks." 



The land masses accessible to the navigator are either islands or 

 shores, and thus irregularities of surface like the mountains which 

 we see on land ; and the change in the magnetism near mountains 

 is also well known. It follows therefore that irregularities of sur- 



