

Geological Society. 63 



into it not the entire lengths a x , a 2y a 3f '. . ., but only their 

 (positive or negative) excesses over a. We therefore put 



a 1 = a + S u a 2 = a + 8 2 , • • • • a n — a + Ki 



and in like manner 



a\ + a 2 + <^3 + • • - + a» _ , 



n 



Then, more simply, 



Si + S 2 + S 3 + ... + S„ 



a= ; 



n 



and hence the table of corrections of the wire is : — 



From to a ... . a—B 1 



„ a „ 2a a — 8 2 , 



„ 2a „ Za a — S 3 



&c. 



As a peculiar advantage of the method, we might insist 

 especially upon the simplicity of the means. It has only one 

 drawback, that many single placings have to be made, through 

 which possible variations of Temperature might enter disturb- 

 ingly ; but the work can be rapidly performed, since approxi- 

 mate positions are given by the nature of the thing itself. If 

 only a moderately sensitive mirror-galvanometer be employed, 

 the position-errors are of the same order as the errors of read- 

 ing off. With our bridge, 2\ metres long, a tenth of a milli- 

 metre in the placing was perfectly secure. 



Doubtless other methods which have been proposed for cali- 

 brating the thermometer might, on the basis of our principle, 

 be transferred to the galvanic calibration of a wire. The one 

 above described, however, is certainly sufficient for all pur- 

 poses. 



Wiirzburg, Physical Institute, 

 April 10, 1880. 



X. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from vol. ix. p. 386.] 



May 26, 1880.— Eobert Etheridge, Esq., E.E.S., 



President, in the Chair. 



npHE following communications were read : — 



J- 1. " The Pre-Carboniferous Eocks of Charnwood Forest." (Part 



III. Conclusion.) By Eev. E. Hill, M.A., F.G.S., and Prof. T. G. 



Bonney, M.A., F.E.S., F.G.S. 



In their former communications the authors had paid less atten- 



