Prof. W. A. Norton on Terrestrial Magnetism. 5 



direction P'n'. This amounts to saying that the magnetic force 

 of A in its action upon the north end of the needle is directed 

 tangentially in the circle from right to left, as shown by the 

 arrow, and in its action upon the south end of the needle is 

 directed from left to right. 



Upon the undulatory theory of magnetism these differences of 

 action are attributable to ethereal waves whose transversal forces 

 of vibration lie in opposite directions, and to certain differences 

 in the magnetic states of the two ends of the needle. 



3. The intensity of the magnetic force of a particle of the 

 earth, at a given distance, is assumed to be approximately propor- 

 tional to its temperature, or amount of sensible heat. This as- 

 sumption was made under the idea that the sun was the source, at 

 the same time of waves of heat, light and magnetism, and that 

 the molecular forces of vibration due to the different kinds of 

 waves would probably vary according to the same law in passing 

 from one point to another on the earth's surface. 



The magnetic force of a particle at the earth's surface, and for 

 a certain depth below the surface, will have a certain mean inten- 

 sity about which the actual intensity will vary during the day 

 and year, by an amount decreasing with the depth. Beyond a 

 certain depth, the magnetic intensity, like the temperature, will 

 remain the same throughout the year, and will have a value 

 greater than the surface mean in proportion as we descend lower. 

 Lines conceived to be traced on the earth's surface connecting 

 the points where the annual mean magnetic intensity of the par- 

 ticles near the surface is the same, will, according to the present 



fi 



and 



Let 



then, AB, CD, EF, fig. 2, represent 

 portions of three isogeothermal lines, 

 regarded as parallel to each other, 

 and GPH an arc of a great circle 

 crossing these lines perpendicularly. 



Fig. 2. 



m 



pect 



me action ot the particle m upon 

 the north end of a magnetic needle 



will be perpendicular to mP and directed obliquely downward. 

 The action of the particle n will be perpendicular to nP and also 

 directed obliquely downward. The magnetic forces of the parti- 

 cles r, s, will be respectively perpendicular to rV and sV and di- 

 rected obliquely upward. Now it is evident that while one effect 

 of the action of m will be to urge the north end of the needle to- 

 ward C, the particle n will have an equal tendency to urge it to- 

 ward D. In like manner, the components of the forces of r and 

 s, which solicit the north end of the needle in the directions PC 





