Chase.] 102 f^P"'' 



modifications were sufficient to override the slight easterly tendency 

 at 6 to 12 p. M., and to advance the phases one hour, the disturb- 

 ances would assume the following forms, the change between 7 P. M. 

 and 1am. 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 " . . 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 deflec- 

 tions 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 control, it should tend to parallelism 

 with the aethereal 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 southeasterly 

 deflection of the southern, or a northwesterly deflection of the north- 

 ern extremity of a half-meridian; and an easterly disturbance to a 

 southwesterly deflection of the southern, or a northeasterly deflection 

 of the northern extremity. 



Substituting these declination values for the current deviations to 

 which they correspond, the Silmost precise accordance of theory and 

 observation in the prominent features of the normal variations of de- 

 clination may be seen by a reference to the following table : 



Gen. Sabine, in speaking of the opposition of the annual and 

 semi-annual curves (St. Hel. Obs., 2, cxix), says "these remarkable 

 systematic dissimilarities, may be 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 



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



X The bracketed references are to the numbered quotations from the 

 Girard Collesre discussions. 



