[ 344. ] 



XLVIII. On the Cause of the Diurnal Variations of the Mag- 

 netic Needle. By W. H. Barlow, Esq., M.I.C.E. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IN the Number of your Journal for April, an extract from 

 a letter from M. de la Rive to M. Arago is published, in 

 which the author attributes the diurnal variations of the mag- 

 netic needle and the auroras boreales to the effect of electric 

 currents at the surface of the earth and in the atmosphere. 



In confirmation of this theory, mention is made of a re- 

 markable effect observed by M. Matteucci in the apparatus of 

 the electric telegraph between Ravenna and Pisa during the 

 magnificent aurora on the 17th of last November; and the 

 author concludes by observing that " it would be highly in- 

 teresting and important to profit by those telegraph wires, 

 which are found to have a direction more or less approaching 

 to that of the declination needle, in order to make with them, 

 when they are not in use for ordinary purposes, some obser- 

 vations which would enable us to demonstrate and to measure 

 the electric currents which probably traverse them." 



My object in addressing you is to state, that in the early 

 part of 1847 I was led to undertake extensive observations on 

 this subject, in consequence of the peculiar disturbances occa- 

 sionally visible on the telegraph instruments of the Midland 

 Railway (on which line the telegraph was erected under my 

 superintendence as the company's engineer). 



These disturbances were at first attributed to atmospheric 

 electricity passing to the earth by means of the wires ; but 

 from certain effects observed, I was led to infer that they were 

 due to other causes; and in order to explain these effects, it 

 is necessary to state that the Midland system of telegraphs 

 consists of four principal lines centring in Derby, as follows: — ■ 



1st. From Derby northwards to Leeds. 



2nd. From Derby north-east to Lincoln. 



3rd. From Derby southwards to Rugby. 



4th. From Derby south-west to Birmingham. 



The disturbances on these four telegraphs were observed 

 to occur simultaneously, with rare exceptions; and the direc- 

 tion of the current in the two telegraphs proceeding northerly 

 and north-easterly was always contrary to those proceeding 

 southerly and south-westerly; that is to say, when the deflec- 

 tion was such as to indicate that the current was towards 

 Derby on the first two, it was from Derby on the last two ; 

 and when it changed in one, it changed in all. It was also 

 observed that on the 19th of March 1847 there was an un- 



