1881.] on Matjnctlc Distiirbdiire, Aurora', cnul E(trt}i Ciirrmtx. CGI 



stretching from the North Pole to tlic equator and fixed in space, with 

 the earth, a magnetic body, revolving beneath it from west to cost ; 

 then it foHowR, from Farachiy's laws of induced currents, that the 

 revtdution of the earth on its axis would cause a current in the fixed 

 conductor in a direction from the pole to the equator. 



If tlic conductor moved over the surface of the eartli from west to 

 east, and the earth did not revidve, or revolved at a slower rate, then 

 the current in the conductor would be from the equator to the pole. 

 The current depends upon the relative motion of the earth and the 

 wire. 



If, then, we have an insulated wire running north and south, the 

 tides in the earth's crust of which I have spoken will be equivalent 

 to a lagging behind of magnetic matter, and so we may expect in that 

 wire a current of electricity whose general direction would be from 

 the equator to the pole. The position of the wire with reference to 

 the magnetic pole of the earth would modify the direction of these 

 earth currents, and it is quite conceivable that the position of England 

 with regard to the magnetic pole might cause these regular earth 

 currents to be greatest in the south-west and north-east direction. 

 The lagging of the lunar earth-currents behind the position of the 

 moon would also be accounted for by the lagging of the tides behind 

 the moon. 



If this is a true cause for some j^ortion at least of the lunar earth- 

 currents, then the same reasoning applied to the sun may in a smaller 

 degree apply to the case of the regular solar diurnal eartli currents, 

 and may help to account for the lagging behind of the cftects due to the 

 sun, so that the fact that the greatest solar ellect happens about 

 2.30 P.M. may not be entirely due to the fact that that is the hottest 

 part of the day, but may also in part depend upon the tides. 



We have now to consider those more sudden changes of the suspended 

 magnets which are distinguished by the name magnetic disturbances. 



In 1874, Dr. Lh)yd said of them: — " The duration and the mag- 

 nitude of these oscillations are as yet outside the domain of law, and 

 probai)ly depend upon so many operating causes that, like the gusts 

 and lulls of the wind in an atmospheric storm, they will long balHe all 

 attempts to refer them to the actuating forces, or even to reduce them 

 to order." 



Certain facts relating to these disturbances have long been known. 

 From the series of observations started by Gauss in 1834, and made 

 every five minutes at the same times at a variety of places at first in 

 Europe, and afterwards in various parts of the world, the disturb- 

 ing power was found to increase in northern latitudes, also it was 

 made out that the appearance of a disturbance in several places 

 occurred at the same time, but there were great differences in the 

 results at different places. 



In Europe the agreement was very close, and also in America, but 

 the agreement between Europe and America was not so satisfactory. 



The force seemed to originate in a certain point in the interior of 



2 z 2 



