S^S On the Diurnal Variations of the Magnetic Needle. 



earth connexions ; that is, the points where the wire was con- 

 nected with the earth. I next made simultaneous observations 

 with the galvanometers and a declinometer needle; from which 

 it appeared, taking the mean of numerous observations, that 

 that part of the day in which the currents flow southwards 

 (that is, from 8 or 9 a.m. until the evening), the variation of 

 the declinometer needle is westerly ; and that during the night 

 and early part of the morning (at which time the currents 

 travel northwards) the variation is easterly ; also, that the 

 large disturbances called magnetic storms are simultaneous on 

 both instruments. 



But although there is this resemblance in the general fea- 

 tures of the movements of both needles, the paths described 

 are not similar. The movements of the galvanometer needle 

 are more frequent and rapid than the declinometer, and the 

 deflection frequently changes over from right to left without a 

 corresponding movement of the declinometer. 



The observations thus briefly recorded formed the subject 

 of a paper which was read at the Royal Society on the 17th 

 of June ] 847 ; and I have thought it desirable to make this 

 communication to your Journal on reading M. de la Rive's 

 letter, because it rather curiously happens, that the unusual 

 delay which has arisen in the publication of my paper by 

 the Royal Society is attributable to the fact, that I arrived 

 from these experiments at the same conclusion as M. de 

 la Rive, as to the electric origin of the diurnal variation of 

 the magnetic needle, which 1 considered to be the effect of 

 the alternating electric currents exhibited by the telegraph 

 wires. 



The Royal Society were unwilling to give their sanction to 

 this view of the case, and only consented to the publication of 

 the observations above described on my omitting that portion 

 of the paper. 



The paper is, however, now in the hands of the printers, 

 and will, I hope, be shortly before the public. 



I ought to state in conclusion, that my idea of the origin of 

 the currents differs in one respect from the theory of M. de 

 la Rive ; inasmuch as he considers them to arise in the atmo- 

 sphere, whei'eas I have attributed them to thermo-electric 

 action in the crust of the earth. I speak of course with great 

 deference on a subject of this kind ; but there is an important 

 fact tending to this conclusion which is now well-ascertained, 

 namely, that in the telegraphs which are laid entirely under 

 ground, deflections occur similar to those before described ; 

 while wires suspended in the air exhibit no deflections, unless 

 they are connected with the earth in two places, and then the 



