346 HORARY VARIATION. [SECT, xxx, 



from them as from the oscillations of the barometer. In 

 the southern hemisphere, the end of the needle that points 

 to the south accomplishes the same horary variations, but 

 exactly in the contrary direction. Their extent is greatest 

 when the sun is in the southern signs, and it decreases to- 

 wards the equator. 



It was supposed that as the horary variations in declina- 

 tion are in different directions in the two hemispheres, 

 though always the same in each, and, as they decrease in 

 angular value from the middle latitudes towards the equator, 

 that there would be some line on the earth in which there 

 would be no horary variation. Colonel Sabine has proved 

 that no such line exists ; on the contrary, that there seems 

 to be a line encircling the earth, either coinciding with, or 

 very near, the line of least magnetic intensity in which the 

 phenomena of both hemispheres are combined, each pre- 

 dominating alternately at opposite seasons. At St. Helena, 

 which is nearly on the line of least intensity, the horary 

 motion of the north end of the magnet, at the same hours, 

 corresponds in direction, during one half of the year, with 

 the movement in the northern hemisphere, and, in the other 

 half of the year, the direction corresponds with that in the 

 southern hemisphere, the passage from one to the other 

 being at the equinoxes. In March and April, September 

 and October, the diurnal variation at the usual hours par- 

 takes, more or less on different days, of the characteristics 

 of both seasons, while in May, June, July, and August, the 

 magnet reaches its eastern extreme; and in November, 

 December, January, and February, it reaches its western 

 extreme nearly at the same hours. The line of least mag- 

 netic intensity is, probably, the line of separation between 

 the northern and southern magnetic hemispheres ; for it is 

 that line which appears to separate the opposite phenomena 

 of the diurnal changes. 



Vast magnetic disturbances, or storms, occur at irregular 

 periods, and extend contemporaneously over wide areas 



