268 Prof. Norton on Molecular Physics. 



will be sensibly the sura of the individual actions, and will be 

 perpendicular to the line proceeding from the middle of the 

 magnet. The directive action of the earth will be virtually this. 



This being allowed, it follows, as deduced in the former paper, 

 that, except in high latitudes, the needle will be perpendicular 

 to the lines of equal molecular magnetic intensity ; also that the 

 horizontal directive force exerted by the earth will be propor- 

 tional, or nearly so, at each station to the molecular magnetic 

 intensity; and the vertical force approximately proportional 

 to the difference of these intensities on one side and the other of 

 the lines of equal force. It may be added here that the above 

 conception brings our theory into essential correspondence (from 

 the mechanical point of view) with Gauss's, and thus that the 

 cod elusions of his memoir become deducible from the present 

 physical theory. 



If we conceive the magnetic force of the earth to be wholly 

 due to the direct action of the electric currents circulating from 

 molecule to molecule, the force exerted by each element of the 

 current should be of the same character, and have a similar direc- 

 tion to that in the case just supposed. But since the resultant 

 currents are shifting their position from year to year, it follows 

 that they may differ somewhat from the lines of equal molecular 

 force. In the sequel we shall in general, for greater simplicity 

 and distinctness of conception, regard the magnetic action of the 

 earth as due to the primary currents, developed as before ex- 

 plained. If these give place, either wholly or in part, to mole- 

 cular currents, the results will be essentially the same. 



Distribution of Terrestrial Magnetism. — The inequality in the 

 distribution of the magnetism of the earth, upon the same pa- 

 rallel of latitude, may be supposed to arise from differences of 

 conductibility in different parts of the earth. It is conceivable 

 that such differences may exist as a consequence of the exist- 

 ence of two great systems of continental elevations, and that 

 the magnetic condition of the earth may be represented by sup- 

 posing that two sets of currents, originating in these elevations, 

 are superimposed upon those which are due to the undisturbed 

 condition of the crust of the earth. But there is another con- 

 ception that may be formed of the possible origin of the unequal 

 distribution of the earth's magnetism, which does not involve 

 the supposition of unequal conductibility. It is that the present 

 magnetic state of the earth originated ac a remote period in the 

 history of the earth, when it was still in the process of conden- 

 sation, and its period of rotation was much longer than at pre- 

 sent. It will readily be seen that at every epoch during this 



transition period, in which the period of rotation was the - part 



