On the Magnetic Disturbance on December \Ath } 1862. 473 



to an extent compounded of the time and of the average value of the 

 disturbing force during that time. The magnetic needle, therefore, 

 will now be acted on by the joint influence of these two sets of par- 

 ticles, whereas at the beginning of the disturbance it was only acted 

 on by one of them, namely, the soft-iron ones. The type of force 

 will therefore have changed if the hard-iron particles are ditTerently 

 distributed in our globe from the soft-iron ones ; and if, instead of 

 two, there are many sets of particles, we shall have a very compli- 

 cated effect. 



Now this duality of disturbing forces must be considered when we 

 attempt to ascertain the connexion between such forces and earth- 

 currents, since we are not entitled to suppose that one of these two 

 forces is related to earth-currents in precisely the same manner as 

 the other. A small soft- iron force may be comparable to a large 

 hard-iron one as far as an earth-current is concerned ; or the reverse 

 may be the case. When, however, there is a very rapid change of 

 disturbing force, since this affects the earth through only one set of 

 particles, we shall by its means be better able to trace the bond of 

 connexion between a single type of magnetic force and the corre- 

 sponding earth-current. Rapid changes of force are therefore of 

 peculiar value in such an investigation. 



Bearing this in mind, let us endeavour to connect together the 

 phenomena of earth- currents and magnetic disturbances by two 

 successive hypotheses, one of which, it would seem, must represent 

 the truth unless there be some new link of connexion between mag- 

 netism and electricity with which we are unacquainted. The first 

 hypothesis is that in which earth-currents are supposed to give rise 

 to magnetic disturbances according to the laws by which a current 

 acts upon a magnet. This may be called the theory of direct action. 



By the second hypothesis, earth- currents are supposed to be in- 

 duced, or secondary currents generated in the crust of the earth by 

 those small but rapid changes in terrestrial magnetism which consti- 

 tute disturbances. This may be called the theory of induced action. 



Now, first, on the theory of direct action. Zinc-currents going to 

 the south should correspond with magnetic disturbances decreasing 

 the declination ; and zinc-currents going to the east should corre- 

 spond with an increase of horizontal force ; but we find by the diagrams 

 that while on this hypothesis the disturbance of the horizontal-force 

 needle will be tolerably well accounted for by the east and west cur- 

 rents, the same correspondence does not hold between the north and 

 south currents and the declination-disturbance. But I do not think 

 that this circumstance, rightly viewed, tells either in favour of or 

 against the hypothesis. A glance at the earth-current curves will 

 show that the ordinates of the one bear generally a fixed proportion 

 to those of the other, showing us that the total current has flowed 

 backwards and forwards along one line* ; and though it is equally ap- 

 parent, by a glance at the magnetic curves, that the same type of force 

 has not been preserved throughout the disturbance, yet the explana- 

 tion of the unity of type in the earth-currents may be that these are 

 twisted into a line of motion, owing to the disposition of the conduct- 



* This had heen previously observed in other cases by Mr. C. V. Walker. 



