118 THIRD REPORT — lSo3. 



earth is not the cause of its magnetism, yet it is highly pro- 

 bable, from Mr. Faraday's experiments *, that, magnetism ex- 

 isting in the earth independently of it, electrical currents may 

 be produced, not only by the earth's rotation, but by the motion 

 of the waters on its surface, and even by that of the atmosphere ; 

 so that the direction and intensity of the magnetic forces would 

 be modified by the influence of these currents. 



This subject is at present involved in obscurity : still, consi- 

 dering how many have been the discoveries made within a few 

 years, — all bearing more or less directly upon it, though none 

 afford a complete explanation of the phaenomena, — it does not 

 appear unreasonable to expect that we are not far removed from 

 a point where a few steps shall place us beyond the mist in which 

 we are now enveloped. 



Mr. Fox, having observed effects attributable to the electri- 

 city of metalliferous veins, appears disposed to refer some of 

 the phaenomena of terrestrial magnetism to electrical cur- 

 rents existing in these veins -f ; but although we should not be 

 jvarranted in denying the existence of these currents, indepen- 

 dently of the wires made use of in Mr. Fox's experiments, or 

 even their influence on the needle, yet I think we should be 

 cautious in drawing conclusions from these experiments J. 



II. Intensity of the Terrestrial Magnetic Force. 



I have as yet said little on the intensity of the terrestrial mag- 

 netic forces. Graham, after having discovered the daily varia- 

 tion of the needle, suspected that the force which urges it varies 

 not only in direction, but also in intensity. He made a great 

 variety of observations with a dipping needle, but drew no ge- 

 neral conclusion from his results. Indeed, with the instruments 

 then in use, he was not likely to determine that which has al- 

 most escaped detection with instruments of more accurate con- 

 struction, for the diurnal variation of the whole magnetic force 

 may perhaps still be considered doubtful. Later observations, 

 particvilarly those of Professor Hansteen, have shown that the 

 time of vibration of a horizontal needle varies during the day, 

 from which it was inferred that the horizontal force also varies. 

 Professor Hansteen, by this means, found that the horizontal 

 intensity of terrestrial magnetism has a diurnal variation, de- 



• 



Philosophical Transactions, 1832, p. 176. f ^l>'^- 1830, .p 407. 



X Mr. Henwood informs me that he has repeated the experiments of Mr. 

 Fox ill from forty to fifty places not before experimented on, and that he pro- 

 poses greatly extending them. i\s far as he can yet see, he considers that his 

 results go to confirm Mr. Fox's deductions, — I suppose with regard to the elec- 

 tricity of metalliferous veins. 



