Geological Society. 667 



of the wave-length along '0158 cm. platinum two measure- 

 ments were taken. In the first the bridge was placed at the 

 end of the region of possible maximum intensity giving as 

 great a wave-length as possible. In the second the measure- 

 ment was taken with the bridge at the other end of the 

 uncertain region in which a maximum of demagnetization was 

 obtained. Thus the two readings given are the two extremes 

 of the possible true position of the bridge. The positions 

 of the extremes were based on two entirely different sets of 

 readings between the taking of which the wire of the 

 secondary was rearranged. The position midway between 

 the two is given: — 



Wave- length. Proportional Wave-length. 



•015 cm. copper 651 100 



•0158 cm. platinum ^y^l^S^ 99'63 



Thus the velocity of electric waves along platinum wire of 

 diameter "0158 cm. was less than the velocity along copper 

 wire of the same diameter by about '4 per cent., the wave- 

 length along - 015 cm. copper being 651 cm., and the distance 

 apart of the parallel wires being 7*8 cm. The possible error 

 in this is necessarily great ; for a more exact determination 

 the average of a number of such results must be taken. 

 This, however, is of the order that might be expected from 

 the results of the calculations of Sommerfeld and Mie. 



1 hope in a subsequent paper to compare these experimental 

 results with those obtained by a theoretical treatment of the 

 subject. 



My thanks are due to Prof. J. J. Thomson for suggestions 

 and advice relative to this work. 

 Cavendish Laboratory. 



LXVII. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 600.] 



November 21st, 1901 (cont.).— J. J. H. Teall, Esq., M.A., F.R.S., 

 President, in the Chair. 



The following communications were read : — 



2 'The Geology of Mynydd-y-Garn (Anglesey).' By Charles 

 A. Matley, Esq., B.Sc, E.G.S. 



Mynydd-y-Garn, a hill of less than 600 feet elevation, stands 

 above the village of Llanfair-y'nghornwy in North-west Anglesey. 

 The mass of the hill is an inlier of sericitic and chloritic phyllites 

 (Garn Phyllites), surmounted by a massive conglomerate (Garn 

 Conglomerate), and surrounded by black slates and shales of 



