128 THIRD REPORT — 1883. 



consequence of a symmetrical constitution of that surface. But 

 even if such symmeti-y did exist, the action of all the currents 

 at different stations on the surface could scarcely be referred 

 to the same two points as centres of force ; and without this 

 symmetry, it would be absurd to expect it. The hypothesis, 

 therefore, of only two poles, as explanatory of the phagnomena, 

 must be rejected ; and if we are to refer these phaenomena to 

 centres of action, we must, besides two principal ones, admit 

 the existence of others depending, upon local causes. 



It has been said that if we refer the magnetism of the earth 

 to another body, we only remove the difficulty, and gain little 

 by the supposition*. It, however, appears to me, that if we 

 could show that the magnetism of the earth is due to the action 

 of the sun, independent of its heat, — which, however, I think 

 the more probable cause, — the problem would be reduced to 

 the same class as that of accounting for the light of the sun, the 

 heating and chemical properties of its rays : we only know the 

 fects, and are not likely to know more. 



If difficulties meet us at every step when we attempt to ex- 

 plain the general phaenomena of terrestrial magnetism, these 

 difficulties become absolutely insurmountable when we come to 

 the cause of their progressive changes. Hei'e, at least, we 

 must for the present be satisfied with endeavouring to discover 

 whether these changes are governed by any general laws : 

 should they be so, their cause may possibly be discovered. 

 Diligent and careful observation is the only means by which 

 we can hope to attain this end, and indeed is that on which we 

 must principally rely for gaining a more correct knowledge of 

 all the phaenomena, and of their causes ; and, consequently, im- 

 provements in the methods of observation, and in the instru- 

 ments to be employed, become of the highest importance. 



This Report has already so far exceeded the limits within 

 which I wished to have confined it, that I must restrict the re- 

 marks on this part of the subject to a few points. 



In the observations of Humboldt, in those of M. Rossel, of 

 Captain Sabine, and of Captain Foster, the terrestrial magnetic 

 intensity had been determined by the vibrations of a dipping- 

 needle in the plane of the magnetic meridian ; but as there is 

 by this means, in consequence of the friction upon the axis, a 

 difficulty in obtaining a sufficient number of vibrations to ensure 

 accuracy, and a dipping instrument is besides ill adapted for 

 carriage, Professor Hanste^n proposed to determine the same 

 by means of a small needle suspended horizontally by a few 



• Hansteen's Inquiries cmicerntjig the Magnethm of the Earth. 



