PART I. ON MAGNETIC STORMS. CHAP. II. 



I0 7 



can be approximately given. The time when the perturbation is at its height can also be determined 

 with tolerable accuracy; but as in so many other cases, that of its cessation is difficult to decide. 



In the table below is given the hour at which the perturbation commences, as also the time at 

 which the horizontal component of the perturbing force has its highest value, and the magnitude of its 

 maximal strength, and further the time at which the perturbing force has sunk to about five per cent, 

 of its maximal amount, this hour being given as the time when the perturbation ceases. This deter- 

 mination cannot lay claim to any great accuracy, and is therefore found by an estimate. 



TABLE XVI. 



It appears from the Table, as also directly from the curves, that at the northern stations the per- 

 turbation occupies a peculiar position in relation to the other stations. 



The times of the commencement and of the maximum of the perturbation, it will be seen, are 

 very different at our four Norwegian stations. At Axeleen the perturbation commences about a quarter 

 of an hour, and at Dyrafjord and Kaafjord about eight minutes, later than at Matotchkin Schar, although 

 the distance between the stations is only from 900 to 1800 kilometres. It should also be mentioned in 

 this connection, that at the arctic stations the curves exhibit great variableness from place to place. 



In marked contrast to this, we find that at all the other stations scattered over the northern 

 hemisphere, the perturbation commences simultaneously. The slight differences in time, which do not 

 exceed three minutes, need not imply an actual difference in time, but may be ascribed to inaccuracy 

 in determining the time on the magnetograms. The hour for the maximum is also the same for wide 

 districts of the earth ; and the form of the curve is repeated almost without change from station to station, 



