SECT, xxxv.] EARTH MAGNETIC BY INDUCTION. 377 



magnetic properties. The action on the galvanometer is 

 much greater when a cylinder of soft iron is inserted into 

 the helix ; and the same results follow the simple introduc- 

 tion of the iron cylinder into, or removal out of, the helix. 

 These effects arise from the iron being made a temporary 

 magnet by the inductive action of terrestrial magnetism ; 

 for a piece of iron, such as a poker, becomes a magnet for 

 the time, when placed in the line of the magnetic dip. 



Dr. Faraday observes that, whatever the temperature of 

 magnetic metals may be, they and all their compounds still 

 retain a certain degree of power to become magnetic by in- 

 duction. The deep-seated magnetic contents of the globe, 

 therefore, though they probably do not of themselves con- 

 stitute a central magnet, are just in the condition to act as 

 a soft iron core to the currents around them or other in- 

 ducing action, and probably are highly important in this 

 respect. 



M. Biot has formed a theory of terrestrial magnetism upon 

 the observations of M. de Humboldt as data. Assuming 

 that the action of two opposite magnetic poles of the earth 

 upon any point is inversely as the squares of the distances, 

 he obtains a general expression for the direction of the 

 magnetic needle, depending upon the distance between the 

 north and south magnetic poles ; so that, if one of these 

 quantities varies, the corresponding variation of the other 

 will be known. By making the distance between the poles 

 vary, and comparing the resulting direction of the needle 

 with the observations of M. de Humboldt, he found that, 

 the nearer the poles are supposed to approach to one another, 

 the more the computed and observed results agree; and 

 when the poles were assumed to coincide, or nearly so, the 

 difference between theory and observation is the least 

 possible. It is evident, therefore, that the earth does not 

 act as if it were a permanently magnetic body, the distin- 

 guishing characteristic of which is, to have two poles at a 

 distance from one another. Mr. Barlow has investigated 



