74 W. A. Norton on Molecular Physics. 
should continue until Philadelphia has magnetically the position 
of the more westerly of the two neutral points above mentioned. 
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 
becanrse they will be more displaced, and the currents of impul- 
ses proceeding from the same number of points will correspond _ 
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 stationar 
If we now revert to London, as a type-station for “Western 
Europe, the present easter] 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, pre ~ 
ponderate over those lying to the east, and the easterly move- — 
ment should therefore continue. The continued operation of | 
the second general cause may thus keep ap 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 pre- _ | 
vail, and the needle begin to turn towa 
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 o . 
the earth’s surface, at various distances from the equator, a 
suppose the whole system to be carried bodily toward the west. 
‘lo explain completely the secular variations, especially of oe : : 
horizontal force, we must take into account another cause in Op- 
from year to y or the reason that the annual diminution of 
intensity of currents already existing may be over-compensa 
ted by the new currents, or the.reverse.* ng the period of 
over-compensation, or of i een the riod of the. 
increase, a the 
secular pre cee of declination — 
ink: 
oo ee Sich hts (ace ace ag er een 
pear 
