4 Prof. W. A. Norton on Terrestrial Magnetism. 



it is the main design of the present article to exhibit, and apply, 



although suggested by these views, is not necessarily dependent 



upon them. The quantitative results arrived at, simply establish 

 the existence of the forces supposed and of the relations conceiv- 

 ed to subsist between them and the temperature of the earth. Dif- 

 ferent views may be entertained of the physical origin of these 

 forces ; or, we may rest upon the forces themselves as so many 

 primary properties of matter. 



The mechanical theory of the magnetism of the earth, of 

 which I propose to give an exposition, is based upon the follow- 

 ing fundamental principles. These were obtained inferentially 

 from the physical theory of terrestrial magnetism which has been 

 briefly explained : but for our present purpose, they may be re- 

 garded as mere assumptions, to be tested by the conclusions and 

 results to which they lead. 



1. Every particle of matter at the earth's surface, and to a cer- 

 tain depth below the surface, is the centre of a magnetic force 

 exerted tangentiaily to the circumference of every vertical circle 

 that may be conceived to be tra- 

 ced around it. Thus, if A, fig. 1, 

 be a particle of the earth's mass 

 at or near the surface, P a particle 

 of a magnetic needle, and BPC 

 a circle traced in a vertical plane 

 around A as a centre and passing 

 through P, P will be urged by a 

 force whose line of direction is 

 the tangent mPn. Whether there 



Fig. 1. 



are probably tangential forces ly- 

 ing also in oblique planes, I do not 

 here consider. If there are such forces it appears from the results 

 of the investigation that they may be disregarded in the present 

 inquiry. According to the views which have been offered of the 

 probable physical nature of magnetism, the tangential forces here 

 supposed are due to the transversal vibrations of the ethereal 

 waves of magnetism propagated from the point A, and originated 

 by certain vibratory movements of the particle at A. 



2. The direction of this force will be different according as it 

 solicits the north or south end of the needle ; and it is always 

 such, that to the north of the acting particle the north end of the 

 needle is urged downwards and the south end upwards, and that 

 to the south of the same particle the north end is urged upwards 

 and the south end downward. Thus, in fig. 1, if P be to the 

 north of A and P' to the south of it, at P the north end of a 

 magnetic needle will be solicited to move in the direction F** 

 and the south end in the direction Pm ; and at P' the north end 

 will be solicited in the direction P'm', and the south end in the 



