78 Scientific Intelligence. 



force is made to assume variable relations to the earth in direction 

 and intensity, which gives rise to the 26-day period. Annual. — 

 The variation of the components of the fields of force, combined 

 with the change of position of the three poles among themselves, 

 and the velocity of the earth in its orbit, produces the annual 

 period, also simultaneously appropriate to both hemispheres. 

 Diurnal. — The position of any observing station is constant as 

 referred to the permanent pole, and both the station and this pole 

 glide past the two instantaneous poles each day, producing the 

 diurnal eastward and westward elongations in the position of the 

 needle. 



The distribution of the elongations. — The two instantaneous 

 poles are located in such positions as to confine the active move- 

 ments of the needle to the morning, forenoon, and early afternoon 

 hours, leaving it nearly quiescent during the night. The after- 

 noon elongation occurs at any station during the same hour 

 throughout the year, as the comparatively steady position of the 

 rotation pole requires. The two morning elongations, which 

 fluctuate through two or three hours, follow the change of posi- 

 tion of the translation pole as it moves eastward and westward 

 through an angle of 45°. The principal neutral line, occurring 

 an hour or more before noon, is coincident with the retardation of 

 phase of the rotation pole. That there should be three sharply- 

 marked elongations, and one undefined in time, agrees with the 

 composition of the forces emanating from one quadrant of the 

 Earth. 



The latitudes. — Near the rotation pole the distribution of 

 potential is at a maximum, which diminishes by a function de- 

 pending upon the angular distance from it ; near the translation 

 pole it is at a minimum, and increases with an approach to the 

 permanent poles. The polar latitudes of the Earth therefore feel 

 especially the influence of the translation pole, and have a single, 

 strong maximum and minimum ; the middle latitudes are subject 

 to both the polar distributions ; the equatorial latitudes come at 

 one time under a northern and again under a southern resultant 

 of component actions. 



The opposite elongations of the North and South Hemispheres. 

 — The north permanent pole is surrounded by a magnetic force 

 of one name, and the southern pole by a force of the other name ; 

 the rotation pole and the translation pole have but one kind of 

 magnetism about each of them, whether these forces are of the 

 same sign or not, which, reaching over into the Northern and the 

 Southern Hemispheres, simultaneously produce opposite motions 

 of the respective needles. 



The disturbances. — The magnetic inductive forces can be traced 

 within the surface of the solid earth by laws similar to those em- 

 ployed in discussing the inductions within the atmosphere; and 

 there is every reason to believe that these currents, in conjunction 

 with the free electricity deposited in the atmosphere as an asso- 

 ciated function, due to the air being an heterogeneous conductor, 



