ON MAGNETIC DISTURBANCES AND EARTH-CURRENTS. 465 



magnetic pole might cause these regular earth-cni'rents to be greatest in 

 the S.W. and N.E. direction. The lagging of the lunar eai'th-currents be- 

 hind 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 portion 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 earth-currents, and 

 may help to account for the lagging behind of the effects due to the sun, 

 so that the fact that the greatest solar effect happens about 2.80 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 sus- 

 pended magnets which are distinguished by the name magnetic dis- 

 turbances. In 1874, Dr. Lloyd said of them : ' The duration and the 

 magnitude of these oscillations are as yet outside the domain of law, 

 and probably depend upon so many operating causes that, like the gusts 

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

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

 to order.' 



Certain facts relating to these disturbances were made out from the 

 series of observations started by Gauss in 1834, and made every five 

 minutes at the same time at a variety of places, at first in Europe, and 

 afterwards in various parts of the world. 



The disturbing 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. 



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

 earth, and the direction of the disturbing force seemed to be constant, 

 yet sometimes there were great differences in the deviations at places 

 not far apart, and from the result of his observations Weber was led to 

 believe that there was a centre of disturbances, which was somewhere in 

 the neighbourhood of St. Petersburg. 



However sudden and unconnected single disturbances may seem to 

 be, they still follow certain laws in their occui-rence. Sabine found that 

 they had daily and yearly variations from their mean values, and that 

 they have an 11-year period, which agrees with the 11-year period 

 of the appearance of spots upon the sun. 



Disturbances are more frequent in summer than in winter, and this 

 applies to each hemisjjhere, and it has been confirmed by various 

 observers that they are also subject to the influence of the moon. 



Lamont says of these disturbances, their cause is a force which is 

 subject to certain laws, but which does not act constantly ; the mean 

 direction and frequency have yet to be discovered. Observations have 

 shown that the magnetic disturbances and electric currents on the earth 

 are so nearly related to one another, that people naturally look upon the 

 electric currents, either in the crust of the earth or in the atmosphere 

 outside it, as the cause of the magnetic disturbances. These currents in 

 the earth have usually been attributed to changes of temperature, because 

 they also are found to be in some way governed by the sun. 



The improved methods of recording observations by photography 

 give the opportunity of discovering the laws of the occurrence of 

 magnetic disturbances, and for some years past photographic records 



1881. H H 



