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



magnetism. But the laws which, on such a theory, connect 

 the size and speed of rotation of a body with its degree of magneti- 

 zation are quite unknown, nor has it been explained why, in 

 the case of the earth, the magnetic axis diverges from, and moves 

 relatively to, the axis of rotation. The great secular changes 

 in the earth's magnetism confront us ^^'ith the fact that the 

 inner history of our planet is by no means concluded, and that 

 within the interior, to the knowledge of which we have so few 

 means of access, profound changes are still being wrought by 

 agents as powerful as they are mysterious. 



The earth's magnetism is also affected, more transiently and, 

 in the long run, to a smaller degree, by causes into the nature of 

 which we have gained more insight. At every station on the 

 earth's surface the magnetic " elements," declination, dip, and 

 intensity of force, show regular variations characteristic of the 

 place, dependent partly on the time of day (or position of the sun), 

 and partly on the position of the moon. These changes are 

 found to have their origin above the earth's surface, in high 

 layers of the atmosphere to which we have as little chance of 

 direct access as we have to great depths mthin the earth. 

 In the case of the variations depending upon the position of the 

 moon, the primary cause seems to consist in a lunar tidal motion 

 of the atmosphere. Living, as we do, at the bottom of the great 

 aerial ocean which envelopes the earth, we are yet 

 quite unconscious of this lunar tide, more unconscious, 

 perhaps, as far as our physical sensations are concerned, than 

 marine organisms at deep sea levels are of the ordinary ocean 

 tides. Nevertheless the lunar atmospheric tide exists, and the 

 air currents associated with it produce minute but ascertainable 

 variations in the height of the barometer (or the pressure of the 

 air), having maxima and niinima each Uxice in a lunar day. 

 These air currents extend up to high regions of the atmosphere, 

 and it is in a layer perhaps 30 or 40 miles above the surface 

 that conditions prove to be favourable to the production, by the 

 air currents, of the magnetic variations in question. The motion 

 of the air, in the presence of the earth's field of magnetic force, 

 has results similar to those proceeding from the motion of the 

 armature between the magnetic poles of a dynamo — there arises 

 a tendency for electric currents to flow. This is the case at all 

 levels, but at the earth's surface, and in other layers, the electrical 

 resistance of the air is too great for the tendency to have 

 much actual effect. There is e^ndence, however, independent 



