222 Prof W. A. Norton on Terrestrial Magnetism. 



sity cannot be functions of the same element ; for, if they were, 

 when this element remained the same, both these forces would 

 remain the same, and thus both the dip and intensity would be 

 unaltered. The isoclinal and isodynamic lines would therefore 

 coincide. It accords therefore, with the theory under discussion, 

 since, upon this theory, one of these forces is a function of the 

 absolute temperature at the place, and the other of the variation 

 of temperature. Now, while the absolute temperature is the 

 same at two different places, the variation of temperature may 

 be different. 



Magnetic Poles. — Magnetic Equator. 



The true magnetic poles of the earth are the points on its sur- 

 face where the dip of the needle is 90° ; and where, therefore, 

 the horizontal intensity is equal to zero. The existence of such 

 points may be attributed a priori, to either of the two following 

 named causes : 1. The magnetic intensity of the particles of the 

 earth in the immediate vicinity of the place, to whose action the 

 horizontal intensity is due, may be so feeble as to afford no appre- 

 ciable horizontal force in comparison with the vertical force, 

 which, as it depends upon the difference of the molecular inten- 

 sities north and south of the place, has no necessary connection 

 with the absolute molecular intensities. 2. The acting forces 

 may be so related in direction and intensity that their horizontal 

 actions may destroy each other. 



We have already had reason to suppose that the absolute mag- 

 netic intensities of the particles of the earth are very feeble at 

 the poles of maximum cold, which are in the immediate vicinity 

 of the magnetic poles, so called, and perhaps identical with them. 

 But, whether this be true or not, it may easily be shown that 

 the disposition of the forces is such at these points as to make the 

 horizontal intensity disappear, or 

 nearly so. Let AB, fig. 10, repre- Flg " n 10 ' 



sent a great circle passing through 

 p, one of the cold poles. If we 

 suppose that the temperature 

 increases by equal degrees in 

 every direction from j9, or in oth- 

 er words, that the isogeothermal 

 lines are circles traced around p 7 

 the magnetic intensity of the 



two particles m and n, equally distant irum //, is im »aui<* — 

 the acting forces of these particles will have the directions P * 

 and P b, equally inclined to the vertical PC. 

 of every other pair of similarly situated particles. The directive 

 force of the needle at p will therefore be vertical. The supposi- 

 tion that the isogeothermal lines are circles, in the vicinity of the 



same 



Now 



same 



