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



We may lay down the following great truths as apparently 

 established by this discussion. 



1. Lines traced upon the. earth's surface such that the magnetic 

 needle is every where perpendicular to their direction, have a 

 very intimate relation to the isogeothermal lines, and a general 

 correspondence of direction with them, indicative of a close phys- 

 ical connection between the distribution of the earth's heat and 

 that of its magnetism. 



2. If we assume that the intensity of the magnetism of each 

 particle is every where proportional to its temperature, and that 

 its magnetic force is of the character of a tangential force, (as it 

 has been defined,) the formula which makes known the temper- 

 ture furnishes by differentiation another which makes known the 

 first set of lines above mentioned ; and which differs from the 

 formula for the isogeothermal lines only in this, that in the one, 



( the displacement of the cold poles answering to an indefinitely 

 small displacement of the situation of the place is taken into ac- 

 count, in the other neglected. 



In what precedes, I have assumed, on the authority of Sir 

 David Brewster, that the isothennal and isogeothermal lines are 

 parallel to each other. If a more extended and minute investi- 

 gation should reveal differences in the direction of these two 

 classes of lines, it may be found that the lines of equal molecular 

 magnetic intensity to which the needle is every where perpen- 

 dicular, correspond more nearly in their direction with the true 

 isogeothermal lines than with the isothermal lines, to which the 

 isogeothermal lines have been assumed to be parallel. 



1 he small differences which appear to exist between the iso- 

 geothermal lines and the lines to which the needle is perpendic- 

 ular, (whether strictly lines of equal molecular magnetic intensity 

 or not, ) must be left for future consideration. I will only remark 

 here, that the differences may prove not to be incompatible with 

 the supposition made at the outset, that the terrestrial molecular 

 magnetic intensity is strictly proportional to the temperature, as 

 they may be attributable to a want of parallelism in the isogeo- 

 thermal lines lying within the circle of sensible action upon the 

 needle, geological differences in the earth's crust, &c. ; and that, 

 if they are incompatible with this supposition, they are not neces- 

 sarily opposed to the undulatory theory of magnetism, since the 

 supposed difference between the waves of heat and magnetism 

 affords reasonable ground for the supposition, that there may be 

 more or less of difference in the detail of the distribution of these 

 two principles over the earth. It is to be observed, moreover, 

 that, as it appears from the foregoing discussion that the differ- 

 ences between the two classes of lines follow a certain law, the 

 molecular magnetic -intensity is not only approximately pr°P° r " 

 tional to the temperature, but must be rigorously connected wita 

 it by some law. 



