SECT. XXX. AND ELECTRICITY. 311 



dundant quantity of positive electricity to another, or 

 deprive another of its electricity, the one gaining at the 

 expense of the other ; but there is no instance of a body 

 possessing only one kind of polarity. With this excep- 

 tion, there is such perfect correspondence between the 

 theories of magnetic attractions and repulsions and elec- 

 tric forces in conducting bodies, that they not only are 

 the same in principle, but are determined by the same 

 formulae. Experiment concurs with theory in proving 

 the identity of these two unseen influences. Hence if 

 the electrical phenomena be due to a modification of the 

 ethereal medium, the magnetic phenomena must be 

 owing to an analogous cause, and therefore, notwithstand- 

 ing the high authority of M. Poisson, they must also be 

 attributed to the redundancy and defect of only one fluid. 



With reference to the subject of this chapter I have 

 received the following information from Colonel Sabine, 

 one of the best authorities in this branch of science. 



The passage marked (A) confounds under the com- 

 mon term of " magnetic pole," two things which are 

 alike distinct in conception and different in reality. 

 These are, 1st the localities on the globe where the 

 needle is vertical, or the horizontal force ; and 2d 

 the localities where the magnetic forces acting on the 

 surface of the globe have a maximum intensity, around 

 which the isodynamic lines on the surface arrange them- 

 selves in curves, and in departing from which in every 

 direction (on the surface) the intensity of the force is 

 found to decrease. 



The progress of terrestrial magnetism has been greatly 

 impeded by mistakes arising from the different under- 

 standings which different people have of what is meant 

 by the term magnetic pole. It is the more important 

 to have clear ideas and a correct knowledge of facts in 

 this matter, because the facts of science are not such as 

 in any respect to justify a confusion of terms ; not one 

 of the localities where the intensity of the force is a 

 maximum coincides with a position where the dip is 

 90 ; nor does a dip of 90 anywhere coincide with a 

 position where the force is a maximum. 



There is in each hemisphere one locality where the 



