1 8 Experimental Researches in Electricity. 



the parapet of the bridge, and dropping from its extremities 

 other wires, having extensive plates of metal attached to them, 

 to complete contact with the water, With this arrangement 

 constant deflections of the galvanometer were procured, but 

 with great irregularity, and they were in succession referred 

 to other causes than that sought for. The different con- 

 dition of the water as to purity on the two sides of the 

 river ; the difference in temperature ; slight differences in 

 the plates and in the holder used ; all produced effects in 

 turn, and nothing satisfactory could be observed. Still, 

 however, although in these experiments sensible effects could 

 not be obtained, it is nevertheless theoretically true that, 

 whenever masses of water are flowing, then electrical currents 

 are formed, and hence it may be inferred that the great 

 oceanic currents, the flow of tidal waves, and of the vast 

 rivers of the old and new continents will, by influencing the 

 intensity of terrestrial magnetism, exercise a perceptible 

 effect on the directions of the iso-dynamic lines, or lines 

 of equal variation on the earth's surface in their imme- 

 diate vicinity. Before leaving this branch of the enquiry, 

 Faraday remarks : <c I hardly dare venture, even in the most 

 hypothetical form, to ask whether the Aurora Borealis and 

 Australis may not be the discharge of electricity thus urged 

 towards the poles of the earth, from whence it is endeavour- 

 ing to return by natural and appointed means above the 

 earth to the equatorial regions. The non-occurrence of it 

 in very high latitudes is not at. all against the supposition ; 

 and it is remarkable that Mr. Fox, who observed the deflec- 

 tions of the magnetic needle at Falmouth, by the Aurora 

 Borealis, gives that direction of it, which perfectly agrees 

 with the present view. He states that all the variations 

 at night were towards the east, and this is what would 

 happen, if electric currents were setting from south to north 

 in the earth, under the needle, or from north to south in 

 space above it." 



