130 



SYDNEY CHAPMAN, ESQ., M.A., D.SC, ON 



Another circumstance connected with, these daily magnetic 

 variations deserves mention. A variable system of electric cur- 

 rents, wherever situated, tends to produce associated currents in 

 neighbouring bodies, and will do so in so far as these are capable 

 of conducting electricity. In the present case the electric 

 currents produced in the upper air, in the manner described, 

 vary regillarly throughout the day, as the rotating earth carries 

 its difierent parts through successive stages of light and darkness. 

 There is consequently a tendency for a daily varying system of 

 currents to flow within the earth itself, and the magnetic varia- 

 tions indicate that this tendency takes effect. There is some 

 flow of current, indeed, within the first few miles beneath the 

 surface, where the oceans and permeable strata are moderately 

 good conductors. The evidence of the magnetic changes is, 

 however, such as to indicate that the bulk of the internal currents 

 flow within the central core of the earth, beyond a depth of 

 about 1000 kilometres. Apart from the layers quite near the 

 surface, the upper 1000 kilometres of the earth's substance 

 seem to be badly conducting : beneath that the conductivity 

 appears to be of about the same order as that of sea water. 



One further important class of magnetic variations remains 

 to be dealt with. Besides the regular daily variations the causes 

 of which ha^^e just been referred to, and besides the slow but 

 powerful secular changes, there are frequent irregular disturbances 

 of a fleeting character. Sometimes they are of small intensity, 

 bat occasionally their magnitude is great and the fluctuations 

 in magnetic force rapid — disturbances of this kind are called 

 magnetic storms. They commonly commence with great sudden- 

 ness, at times breaking out at a time of apparent magnetic 

 calm. The commencement is simultaneous, to within a fraction 

 of a minute, over the whole earth, and their effects are world 

 wide. Auroral phenomena, both boreal and austral, are usually 

 associated with them, and are visible in regions much less 

 removed from the equator than those in w^hich these lights are 

 generally to be seen. Powerful earth currents also accompany 

 magnetic storms, and are sometimes of such intensity and irregu- 

 larity as to produce a temporary derangement in the telegraphic 

 communications of this and other countries. At magnetic 

 observatories, where the magnetic force is continuously registered 

 photographically, the records show rapid and relatively intense 

 fluctuations in the amount and direction of the force. Varia- 

 tions in the direction of the compass, for instance, which as a 



