Gravity on Magnetic Declination. 189 



phases one hour, the disturbances would assume the following- 

 forms, the change between 7 p.m. and 1 a.m. being scarcely, if 

 at all, perceptible : — 



7 A.M. to 1 P.M. 1 P.M. tO 7 A.M. 



Northern zones . . S.E. S.W. 



Southern zones . . N.E. N.W. 



At the equinoxes the amounts of deflection in the northern and 

 southern magnetic hemispheres should be equal ; at other seasons 

 the shortest lines would suffer the greatest displacement, the 

 deflections being greatest in the northern zones from April to 

 September, when the sun is in the northern signs, and in the 

 southern zones from October to March, when the sun is in the 

 southern signs. My experiments have shown that the compass- 

 needle sympathizes with, and is to some extent controlled by, 

 purely mechanical vibrations ; and if in obedience to such con- 

 trol it should tend to parallelism with the sethereal currents, a 

 westerly disturbance of declination (the declination being always 

 conventionally referred to the north pole of the needle) would 

 correspond either to an equatorial south-easterly deflection of the 

 southern, or a north-westerly deflection of the northern extremity 

 of a half meridian ; and an easterly disturbance to a south-west- 

 erly deflection of the southern, or a north-easterly deflection of 

 the northern extremity. 



Substituting these declination-values for the current-devia- 

 tions to which they correspond, the almost precise accordance 

 of theory and observation in the prominent features of the normal 

 variations of declination may be seen by a reference to the fol- 

 lowing Table : — 



Daily maximum* 



"I Easterly. 

 > Easterly. 

 J Westerly. 



Westerly. 

 Westerly. 

 Easterly. 



Morning 

 Mean. 



Evening 

 Mean. 



Stationary. 



Half-yearly max. April to Sept.f 

 Half-yearly max. Oct. to Mar.f 



Theoretical 



7 a.m. 

 6-8 „ 



] P.M. 

 1 „ 



10 A.M. 



Q3 

 y 4 » 



4 P.M. 



5 „ 



7P.M. tO 1 A.M. 



8 „ „ 3 „ 



Observed [I. to V.] | 





General Sabine, in speaking of the opposition of the annual 

 and semiannual curves (St. Helena Obs. 2. cxix.), says, " These 

 remarkable systematic dissimilarities maybe regarded as sufficient 

 indications of a difference in the mode of operation of the solar 

 influence in the two cases." I am not aware that any attempt 

 has hitherto been made to explain this apparent difference, or to 

 show that it is only apparent, and may result from the action of 



* In the northern zones. t Over the whole earth. 



% The bracketed references are to the numbered quotations from the 

 Girard College discussions. 



