TERRESTRIAL MAGNETISM. 185 



The astronomer, after the comet has disappeared from his view, 

 begins his chief employment, and resting on the laws of gravita- 

 tion, calculates from the observations the elements of its true 

 path, and is thus enabled to predict its future course. And in 

 like manner the magnetician proposes to himself as the object of 

 his research, as far as the different and in some respects less 

 favourable circumstances permit, — the study of the fundamental 

 causes which produce the phaenomena, their magnitude and their 

 mode of operation, — the subjection of the observations, as far as 

 they extend, to those elementary principles, — and the anticipa- 

 tion, with some approximation at least, of their effects, in those 

 regions where observation has not yet penetrated. It is at least 

 well to keep in view this higher object, and to endeavour to pre- 

 pare the way for it, even though the great imperfection of the 

 data may render its attainment impossible at present. 



It is not my purpose here to notice the earlier fruitless attempts • 

 to explain the enigma of these phaenomena by hypotheses ha- 

 ving no physical foundation. A physical foundation can only be 

 allowed to such attempts as have considered the earth as a real 

 magnet, and have employed in the calculation only the demon- 

 strated mode of action of a magnet operating at a distance. All 

 attempts of this nature hithert omade have this in common; — 

 that instead of first examining what the conditions, whether simple 

 or complex, of this great magnet must be to satisfy the phaeno- 

 mena, certain determinate and simple conditions were presup- 

 posed, and the subject of inquiry has been the accordance or non- 

 accordance of the phaenomena -R-ith these presupposed condi- 

 tions. We see here a repetition of what has often occurx'ed in 

 the early history of astronomy and of other sciences. 



The simplest hypothesis of this kind is that which supposes 

 a very small magnet in the centre of the earth ; or rather (as it 

 is not hkely that any one ever beheved in the actual existence of 

 such a magnet) supposes magnetism to be so distributed in the 

 earth, that its collective action at and beyond the surface is equi- 

 valent to the action of an imaginaryinfinitelysmall magnet; much 

 as gravitation towards a homogeneous sphere is equivalent to the 

 attraction of a sphere of equal mass condensed in its central 

 point. In the supposed case, the magnetic poles are the two points 

 where the prolonged axis of the little central magnet intersects 

 the earth's surface ; where the magnetic needle is vertical and 

 the intensity is also greatest. In the great circle midway be- 



