Prof. Norton on Molecular Physics, 277 



that run from S. to N. These general facts being borne in mind, 

 it may be seen that the secular variations of the declination 

 result from the combined operation of two causes, viz. : 



(1) The prevailing annual action of the resultant E. and W. 

 current, or of the resultant N. and S. current, according to the 

 declination of the needle ; except when the declination is easterly, 

 when the two currents will cooperate. 



(2) The varying action of the impulses proceeding from the 

 resultants of the new currents and those previously existing, 

 shifting and changing in intensity from year to year, which run 

 through all the places that lie on the east and west sides of the 

 magnetic meridian of the station. 



Let us conceive the diverse directions of the needle on differ- 

 ent meridians to be represented by a sinuous curve, alternately 

 concave and convex toward the north, to which the needle is 

 perpendicular — a certain point of the concave portion being on 

 the meridian of Philadelphia, and a point of the convex portion 

 on the meridian of London. Now, confining our attention to the 

 first operative cause on the concave part where the declination 

 (E. or W.) is small, it is plain that the N. and S. current should 

 prevail, and therefore the needle have an annual westerly move- 

 ment. But at a point of the ascending curve where the decli- 

 nation (W.) is large, the other current should prevail, and the 

 needle turn toward the east. Both of these cases are represented 

 by the present secular variations at Philadelphia and London. 

 On the higher part of the curve, where the declination (E. 

 or W.) is small, the N. and S. current should prevail again, 

 and the needle be deflected toward the west. The neutral 

 or transition-points in the curve should fall at about equal 

 distances on opposite sides of the point of maximum declina- 

 tion (W.). 



If we follow the curve ascending toward the west from the 

 line of no declination on this continent, both sets of currents will 

 cooperate, and the needle should turn toward the west, as it 

 now does throughout the United States. It appears, then, that 

 throughout Western Europe and the United States the actual 

 progressive movements of the needle are precisely those which 

 should result from the operation of the first cause above men- 

 tioned ; that is, from the direct action of the new currents deve- 

 loped at the station of the needle. 



To understand how an alternation of movement may occur at 

 a given station, we must consider the probable and possible 

 effects of the other general cause. Under the operation of the 

 first cause the present westerly movement at Philadelphia 

 should continue until Philadelphia has magnetically the position 

 of the more westerly of the two neutral points above mentioned. 



Phil. Mag. S. 4. Vol. 31. No. 209. April 1866. U 



