PART i. ON MAGNETIC STORMS. CFIAP. U. 



not exceeding the value 2.5 -/ in magnitude. In the north, it is almost imperceptible at Pawlowsk. 

 Even at Tiflis, where the sensibility is very great (e v = 2.55 y), the deflections in the vertical curve 

 may best be characterised as small vibrations about the mean line; while at the same time, the horizon- 

 tal component has values going up to 24 y. The directions of the vertical components are indicated on 

 the charts by dotted lines, as they are too small to allow of their size being marked. 



It would appear from the above that we here have a perturbation of a very characteristic and 

 peculiar kind, a species of perturbation with which we shall very often meet. As a rule, however, it 

 will appear together with other phenomena, which disturb its regular development; but here we seem 

 to have the perturbation almost alone, and on a quiet day. 



It will often happen that during a perturbation that is powerful at the equator, great storms will 

 occur in the north, of which the effect makes its way southwards, but is weakened towards the equator. 

 Here too, there is an indication of conditions such as these, of which we shall later on have several 

 examples. At Sitka, for instance, a sudden change in the curves occurs between n and 12.30. It is 

 another phenomenon altogether that here makes its appearance, and which has its focus in the polar 

 regions, its effect being almost imperceptible in the vicinity of the equator. It is fairly distinct at the 

 Norwegian stations, and its effect may also be traced in Central Europe. On the chart for 12 o'clock, 

 this current direction represents the total force resolved into one that should answer to the equatorial 

 current; the other component, which answers to the polar current, will then be directed towards the 

 south-west, answering to a current towards the north-west. 



While we allow this perturbation to serve as a typical example of these perturbations, the positive 

 equatorial perturbations may be more fully characterised as follows. 



The perturbation appears with greatest strength in the regions round the equator. It is true that 

 for a short time the deflections may be greater at the poles than at the equator; but the force does 

 not remain constant for so long a time. The conditions at the poles are frequently characterised as an 

 oscillation about the mean line, of a somewhat local character. 



The perturbing force in southern latitudes, and more especially in the neighbourhood of the 

 equator, is directed northwards in the direction of the magnetic meridian. 



The perturbations appear simultaneously all round the equator, and with a similar course, but not 

 always with the same strength. 



The curves for the horizontal intensity, where the perturbation mainly shows itself, present a charac- 

 teristically serrated appearance. The serrations may very frequently be recognised all over the earth, and 

 in such case occur simultaneously. 



TABLE XI. 

 The Perturbing Forces on the 26th January, 1903. 



(') As we have only the close of the perturbation, the choice of normal lines is somewhat difficult. 

 Birkeland, The Norwegian Aurora Polaris Expedition, 1902 1903. 



