XII. OX EARTH CURRENTS, AND THEIR CONNEXION WITH 

 THE DIURNAL CHANGES OF THE HORIZONTAL 

 MAGNETIC NEEDLE. 



Transactions of the Royal Irish Academy. Vol. XXIV. 



1. WHEN the discovery of Oersted had made known the con- 

 nexion which subsists between magnetism and current electricity, 

 the idea occurred to many that the magnetism of the earth or, 

 at least, its diurnal fluctuation, was the result of electric currents 

 traversing its crust. This idea gained much force from the fact, 

 soon after discovered by Seebeck, that electric currents are 

 generated when heat is applied to a circuit composed of different 

 metals ; and it was supposed that the phenomena were thus 

 traceable to the thermal agency of the sun, operating in succes- 

 sion upon the conducting substances of which the earth's crust 

 is composed. 



The most explicit statement, and chief support of this hypo- 

 thesis, is contained in a memoir by Professor Christie, published in 

 the Philosophical Transactions for 1827. In this memoir it is 

 maintained that the phenomena of the diurnal variation of the 

 horizonal magnetic needle correspond with those which are pro- 

 duced experimentally, by the application of heat to a globe 

 composed of two metals ; and the author was even led by his 

 experiments to anticipate the fact, at that time unknown, that at 

 opposite sides of the equator the poles of the needle, having the 

 same name as the latitude, are always deflected in the same 

 direction. 



I propose, in the present memoir, to examine this theory apart 

 from the hypothesis as to the origin of the currents ; and to show, 



