168 Prof. J. H. Poynting. Electric Current and [Feb. 12, 



he considers this only an approximation to the true normal gradient, 

 and that the readings of the Coal-mines and Artesian -well experiments 

 are, owing to the causes he enumerates, still too high. He also dis- 

 cusses the question whether or not the gradient changes with the 

 depth. His own reduction of the observations gave no result, but he 

 points out that in all probability the circulation of water arising 

 from the extreme tension of its vapour is stayed at a certain depth ; 

 while as it is known experimentally that the conductivity of iron 

 diminishes rapidly as the temperature increases, this may possibly in 

 a different degree a.pply to rocks. If, therefore, there is any change, 

 these indications would be in favour of a more rapid gradient. 



Taking all these conditions into consideration, the author inquires 

 whether a gradient of 45 feet per degree may not be nearer the 

 true normal than even the one of 48 feet obtained by the observa- 

 tions. 



III. " On the Connexion between Electric Current and the 

 Electric and Magnetic Inductions in the surrounding 

 Field." By J. H. Poynting, M.A., late Fellow of Triuity 

 College, Cambridge, Professor of Physics, Mason College, 

 Birmingham. Communicated by Lord Rayleigh, M.A., 

 D.C.L., F.R.S. Received January 31, 1885. 



(Abstract.) 



This paper describes a hypothesis as to the connexion between 

 current in conductors and the transfer of electric and magnetic in- 

 ductions in the surrounding field. The hypothesis is suggested by the 

 mode of transfer of energy in the electromagnetic field, resulting from 

 Maxwell's equations investigated in a former paper (" Phil. Trans.," 

 vol. 175, pp. 343 — 361, 1884). It was there shown that according 

 to Maxwell's electromagnetic theory the energy which is dissipated in 

 the circuit is transferred through the medium, always moving perpen- 

 dicularly to the plane containing the lines of electric and magnetic 

 intensity, and that it comes into the conductor from the surrounding 

 insulator, not flowing along the wire. 



Symbolising the nature of the induction by unit tubes drawn in the 

 direction of the induction in the usual way, i.e., so that there is unit 

 quantity of induction over every section of a tube, the electric induc- 

 tion is equal to K X electric intensity 4k, and the magnetic induction 

 is equal to /jl X magnetic intensity. The electric induction is the same 

 quantity as Maxwell's " displacement." The hypothesis now made 



