at tlie Magnetic Observatory at Prague. 



429 



This table shows that the least disturbance takes place in 

 the declination from 8 to 10 a.m., and the greatest from 8 to 

 10 p.m., a result which had already appeared from the Milan 

 observations. In the horizontal intensity also the disturbances 

 are more frequent in the evening than in the morning. 



9. If we now take the signs into account, and call an aug- 

 mentation of either element +, and a diminution , the fol- 

 lowing table exhibits the remainders when the one sum is 

 taken from the other. 



Direction of the Changes. 



These numbers confirm for the declination the deduction 

 already derived from the Milan observations, viz. that the 

 declination is increased by the disturbances in the forenoon 

 and middle of the day, and diminished by those occurring in 

 the evening hours. In respect to the horizontal intensity, the 

 negative signs are the prevailing ones, so that, in general, 

 disturbances diminish this element, which is also in corre- 

 spondence with the previous deductions; but it also appears 

 from the numbers, that this occurs in a much higher degree 

 during the hours of the night and morning than in the fore- 

 noon and afternoon. 



10. During the greater disturbances we did not fail to ob- 

 serve for several hours, from 5 minutes to 5 minutes, and to 

 study the march of the phenomena in all the elements with 

 as much exactness as possible. Ten perturbations were thus 

 observed, and the following circumstances were noticed as 

 common to them all. These were before partly indicated by 

 former observations, and appear to give a character of regu- 

 larity to phsenomena which at first sight might be regarded 

 as wholly irregular. 



I. Although the general effect of a disturbance is to weaken, 

 on the whole, the horizontal intensity, considerable augmen- 

 tations of that element do take place, but are of brief dura- 

 tion, and always preceding the diminution. 



11. The horizontal intensity remains weaker for some time 



