On the Thermal Action in the Wire of a Voltaic Circuit. 191 



morphism of stratified rocks which has been referred by geolo- 

 gists to the agency of heated fluids and vapours. (See Rogers, 

 Pa. Report, vol. ii. p. 700. Lyell, Silliman's Journal, (2) vol. 

 xxxix. p. 22.) 



The inclination presents some anomalies that are difficult to 

 explain ; and whether we compare the solar- diurnal or the annual 

 curves at the principal northern and southern stations, the " in- 

 dications of a difference in the mode of operation of the solar 

 influence in the two cases " seem as striking and perplexing as 

 they did to Hudson and Herschel in their examination of the 

 influence of heat on the barometer (Proc. Amer. Phil. Soc. vol. ix. 

 p. 283), and to Sabine in his discussions of the semiannual de- 

 clination curves (St. Helena Obs. 2. cxix.). But the disappear- 

 ance, in the progress of our investigations, of these once seem- 

 ingly insurmountable difficulties — the wonderful coincidence in 

 the general features of the gravitation and magnetic currents — 

 and the a priori probability that all disturbing forces, of whatever 

 character or variety, will tend constantly to a mutual equilibrium 

 — encourage the belief that this apparent paradox may be like- 

 wise susceptible of a simple interpretation which will drive it 

 from its latest lurking-place. 



I can think at this moment of no more probable causes of the 

 want of symmetry here spoken of, than the different distribution 

 of land and water in the two hemispheres, and the influence of 

 powerful alternating land and sea breezes. A long series of 

 connected observations at a number of new stations may perhaps 

 be required before it can be satisfactorily ascertained whether the 

 disturbances thus occasioned are sufficient to account for all the 

 phenomena ; but meanwhile it is interesting to observe the degree 

 of accordance that exists at northern inland stations like Phila- 

 delphia and Toronto, between the curves of vertical force and 

 force of wind, on the one hand, and those of horizontal force and 

 barometric pressure on the other, as well as the agreement that 

 has been pointed out by Dr. Lloyd and Mr- Homer between the 

 annual curves of declination and of temperature. 



XXIV. On the Thermal Action in the Wire of a Voltaic Circuit 

 as indicating that the current-force operates on the surface of a 

 metallic conductor only, and not through its substance. By 

 John James Waterston, Esq. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



WHILE engaged on the subject of electric conduction, I 

 have endeavoured in vain to discover if there is any 

 experimental evidence on record of what seems generally taken 



