280 Geological Society : — 



independent of suddennesses, if the effective ohmic resistance 

 ivere constant. But this supposition is not true ; and that it 

 is very effectively untrue for copper wires of a millim. dia- 

 meter or more, and times of change in the primary less than 

 3 Jq of a second, we see readily by looking to the diffusional 

 curve and the time-number, ^o of a second for curve 10, 

 corresponding to the diffusivity of copper for electric currents 

 (which is 131 square centimetres per second) given and ex- 

 plained in my short article on a Five-fold Analogy, in the 

 British Association Report for Manchester, 1888 (to be found 

 also in the Electrical Journals, and ' Nature'). 



[Postscript, February 23, 1890. — The thermal analogy, 

 which is very simple for the case of electric currents in 

 parallel straight lines, has, as soon as I have considered it, 

 shown me the fallacy pervading the text ; and has led me to 

 make the corrections in the insertion and footnotes enclosed 

 in brackets [ ], all of which are of the date of this postscript. 

 Preliminary experiments with the suspended coil instead of 

 the steel magnet, in a ballistic galvanometer now nearly com- 

 pleted by Mr. Tanakadate, have already disproved the large 

 differences which I expected between the time-integrals of 

 the secondary currents on the break, and on the make ; and as 

 far as they have yet gone are consistent with the perfect 

 equality which I now find proved by theory.] 



XXX. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 135.] 



December 18, 1889.— W. T. Blanford, LL.D., F.R.S., President, 

 in tne Chair. 



rpiLE following communications were read : — 



-*- 1. " On the Occurrence of the Genus Girvanella, and remarks 



on the Oolitic Structure." By E. Wethered, Esq., F.G.S. 



2. " On the Relation of the Westleton Beds or ' Pebbly Sands ' of 

 Suffolk to those of Norfolk, and on their extension inland, with some 

 observations on the Period of the final Elevation and Denudation of 

 the Weald and of the Thames Valley."— Part II. By Prof. Joseph 

 Prestwich, M.A., D.C.L., F.R.S., F.G.S. 



The author having, in the first part of this paper *, discussed 

 the relationship of the Westleton Beds to the Crag Series and to 



* Phil. Mag. [5] xxviii. p. 142. 



