278 Prof. Norton on Molecular Physics. 



But the needle will not, in fact, remain stationary when this po- 

 sition has been reached ; this could not be the case unless the 

 effects of the varying resultants of the new and old currents 

 should exactly counterbalance each other. In reality those on 

 the east side should preponderate over those on the west side, 

 because they will be more displaced and the currents of im- 

 pulses proceeding from the same number of points will corre- 

 spond more nearly in direction on the east than on the west 

 side. The tendency of the second general cause should then 

 be to give the needle at Philadelphia a motion toward the east 

 in the magnetic position in which it would otherwise remain 

 stationary. 



If we now revert to London as a type-station for Western 

 Europe, the present easterly movement of the needle should 

 continue until the magnetic position of the more easterly of the 

 two neutral points so-called is reached. But at this position 

 the resultant currents at places lying to the west should, in 

 the existing condition of the currents of the eastern continent, 

 preponderate over those lying to the east, and the easterly 

 movement should therefore continue. The continued operation 

 of the second general cause may thus keep up an easterly move- 

 ment until the needle attains to a certain easterly declination. 

 But the direct tendency to a westerly movement that increases 

 as the easterly declination becomes greater, must ultimately 

 prevail, and the needle begin to* turn toward the west. 



It is obvious that the general result is the same as if the 

 whole system of currents were gradually transferred to the west ; 

 or the representative sinuous curve had such a motion, its folds 

 at the same time changing more or less. Or rather, to obtain 

 a comprehensive view of the entire process, we should conceive 

 of a system of such representative curves traversing the earth's 

 surface at various distances from the equator, and suppose the 

 whole system to be carried bodily toward the west. 



To explain completely the secular variations, especially of the 

 horizontal force, we must take into account another cause in ope- 

 ration not yet mentioned. It is that the resultant currents at 

 any station may either be increasing or decreasing in intensity 

 from year to year, for the reason that the annual diminution of 

 intensity of currents already existing may be over-compensated 

 by the new currents, or the reverse*. During the period of 

 over-compensation or of increasing intensity, the period of the 

 secular change of declination should increase, and decrease in the 



* A tendency to a diminution of the horizontal force may arise from two 

 causes, viz. a gradual decline of existing currents, and an increase in the 

 ecliptic photospheric currents developed by the impact of the auroral mat- 

 er received from the sun. 



