282 Geological Society : — 



enters, and at another point of which, B, the same current 

 leaves. Multiplying the expression for y- by r 5 , we get for 

 the difference of potentials between C and D the value 



/o * nfa + ^ + rs + r^ + fo+rsXrg + rO' " * w 



But the resistance of the system of five conductors, between the 



points C, D, is the reciprocal of the sum l/(Vj + r 2 ) + l/(r s H- r±) 



+ 1/7*5 of the conductivities of the three arcs which join C, D, 



that is, 



nOi + ^X^+n) 



rfa + r 2 + r B + r 4 ) + (r x + r s )(r 8 + rtf 



and if a current of amount y 6 enter at C and leave at D, the 

 difference of potentials between C and D will be equal to this 

 expression multiplied by y 6 . The product multiplied by 

 ^1/(^1 + r 2) is the difference of potentials between C and A, 

 and multiplied by r 3 /(r 3 + r 4 ) is the difference of potentials 

 between G and B. Hence, the difference of potentials be- 

 tween A and B is the difference of these products, or 



TenQin— r 2 r 3 ) 



^Oi + r 2 + r 3 + r 4 ) + (r x + r 2 )(V 3 + r$ 



the same value as that given in (8) for the difference of 

 potentials between C aud D. Hence the theorem : 



If to a current entering at one point A of a linear system, 

 and leaving at another point B, there correspond a certain 

 difference of potentials between two other points C and D, then 

 to an equal current entering the system at C and leaving at D 

 there will correspond the same difference of potentials between 

 A and B*. 



XXXVI. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 222.] 



May 11, 1887.— Prof. J. W. Judd, E.E.S., President, in the Chair. 



THE following communications were read : — 

 1. "Further Observations on Hyperodapedon Gordoni" By 

 Prof. T. Huxley, LL.D., F.E.S., F.G.S. 



2. " On the Eocks of the Essex Drift." By Eev. A. W. Eowe, 

 MA., F.G.S. 



The rocks of the drift in Essex are of such great variety that it is 



* The theorems just proved have been obtained in different ways by 

 KirchhofF (Pogg. Ann. Bd. lxxii. 1847, and Ges. Abhand. p. 22), and 

 Maxwell (El. and Mag. vol. i., second edition, p. 371) from a consideration 

 of the general theory of a linear system. 



