PART III. EARTH CURRENTS AND EARTH MAGNETISM. CHAP. I. 795 



% ' 



The currents that will be induced at the beginning of such a storm will of course, everywhere in 

 rather lower latitudes, have a direction E W, as will instantly be seen from the formulae for i fa I'Q, 

 which, according to equations (5) and (6) on p. 759 may be written in the form 



>f) === 2s n A ns Pains = O 



i m = - S, E n A ns P 9ns 



where A ns is a certain function of n, p s , s, Q and t, of which the analytical expression is easily found 

 by equation (6). At the beginning of the perturbation, therefore, one would expect to find a deflection 

 in the E W curve which is uniform in direction that quickly increases to a maximum, and again 

 quickly decreases towards 0. 



If the E W and N S curves answered to the earth-current components in the magnetic E W 

 and N S, the latter of these two should not exhibit a similar condition. 



We have seen however, that simultaneously with the commencement of such a perturbation, one 

 or more rather locally circumscribed polar systems of precipitation are formed. 



A system such as this, however as we remember in medium latitudes will act throughout most 

 strongly in the N S component. As* the polar systems of precipitation, which we have ordinarily seen 

 to be of a briefer nature, so that the deflections in the magnetic curves increase to a maximum only to 

 decrease again immediately afterwards, the earth-current curves answering to them will as a rule be in 

 the form of twofold undulations. 



We should expect, therefore, that at the beginning of the perturbation deflections of such a nature 

 might sometimes be found, especially in the comparatively high latitudes here under consideration. 



If we now look at the examples of such storms given in our material from the three southern 

 stations, we see, for instance at Pare St. Maur, on the nth January, 1894, a very characteristic example 

 of a case of this kind. 



In the E W curve we find at first a uniformly-directed deflection, while in the N S curve the 

 deflection has the character of twofold waves. We see that the first earth-current impulse in this latter 

 component must undoubtedly, at any rate to a very great extent, produce the "starting impulse" that 

 appears so distinctly in the Z)-curve. 



For the rest, fairly strong polar systems of precipitation are acting here all through the further 

 course. 



We thus see here that the character of the deflections at first in the E W and N S curves is 

 rather different. 



As it will very often be difficult or impossible to separate the deflections that are due to equa- 

 :orial perturbations from those that are due to simultaneous polar systems of precipitation, when con- 

 sidering the magnetic curves, it will of course be so to a still greater extent if we were to try to 

 separate the deflections in the earth-current curves that were due to the variations in these two systems. 



As, however, we have seen a distinct example of the great difference between the character of 

 he deflections in the N S and the E W curves, where two such systems are acting, it seems likely 

 hat this might frequently have something to say; in other words, the difference in the two earth-current 

 :urves, that we have before pointed out and ascribed chiefly to local causes, might to some extent, 

 jossibly a very great extent, be caused by the different induction-effect of simultaneously occurring 

 )olar and equatorial perturbation-systems. 



If we look at the other examples that we have of equatorial storms, we find everywhere these 

 ;ame conditions confirmed. 



From Germany and England we have two examples of such storms, on the i6th and 2oth 

 Dctober, 1883. 



