The 



316 MAGNETISM 



Fig. 238 those for equal dip or isoclinic lines for 1876. 

 figures on the right of Fig. 238 arc the tangents of the dip. 



Several careful magnetic surveys of limited areas on the earth's 

 surface have been recently made. Thus, Thor, Kiuker 



made a survey over Great Britain and Ireland in 1891-92,* finding 

 various local deviations due, apparently, to the presence of 

 magnetised rocks. Some of these deviations could be 



140 160 180 100 140 120 IOO BO GO 4O 2O O SO 4O O O IOO ?O I4O 



Fio. 



to the known presence of such rocks, and from others the Mih- 

 terranean existence of magnetised matter ua> intern d. 



Sketch of Gauss's theory of terrestrial magnetism. 

 In 1839 Gauss published a paper on the (n in nil Theory of 

 Terrestrial J/^^v/<7/-v?//,f in which lie investigate! a formula \\hich 

 should represent the earth's field at every point of its surface. 

 We shall give only a brief account of the principles ot the th< 

 The original memoir is well worthy of stiulv. not onlv tor the 

 subject-matter, but for the admirable introduction which it gi- 

 to the use of spherical harmonics. 



Let us assume that the earth is a sphere and that its \\\u- 

 field is due in part to magnetisation within the surface, in part 

 to magnetisation without the surface, and in part to electric 

 currents which lie wholly within or wholly without the surface. 

 These electric currents may be replaced by magnetic shells (p. ~ - 

 so that the field may be regarded as due to a definite distribution 

 of magnetism. This distribution will have a potential given by 



V= / , where dm is an element of magnetism and r its distance 

 from the point considered, and V will have a single definite value 



* Phil. Trans., A., vol. clxxxviii. (1896). 

 f Werlie, v. p. 119 ; Taylor's Scientific Memoire, ii. p. 

 n, p. 223, a brief abstract is given. 



184. In Lloyd's 



