TERRESTRIAL MAGNETISM IS NOT SUPERFICIAL. 101 



44*. Reasons for believing that Terrestrial Mag- 

 netism does not reside, in any important degree, in the 

 earth's surface. 



The first class of reasons are those general ones 

 which are founded on ordinary observation, of the 

 materials of which the earth's surface is composed, and 

 of their non-magnetic property: and upon the general 

 absence of any perceptible change in magnetism de- 

 pending on the change of soil. The materials of a 

 clay-field are not sensibly magnetic, nor are those of 

 a sand-field, nor is there any change of the general 

 terrestrial magnetism in going from one to the other; 

 nor are the granite rocks in one district, or the lime- 

 stone rocks in another, sensibly magnetic. In some 

 places there are ferruginous rocks, specimens of which 

 when brought near to a delicate compass are found 

 to produce sensible disturbance: but the great masses 

 of those rocks on the earth's surface, when examined 

 (by examination of the declination, dip, and horizontal 

 intensity) at corresponding distances in their neighbour- 

 hood, produce no sensible disturbance. 



The second class of reasons consists of those founded 

 on measures of the magnetic elements at different 

 elevations above the earth's surface. One series in- 

 cludes the observations taken on mountain-heights : of 

 these the most valuable are those of Professor James 

 Forbes (Edinburgh Transactions, vol. xiv.), from which 

 it appears that, for a height of 100 feet, horizontal 

 magnetic force is diminished, in Europe, by 3o ^ 0o part, 



