299 



might be produced by a smaller rate of change through a greater 

 space ; thus an addition of about 0*5 to the specific gravity of a 

 sphere of 1 mile in diameter, or of 0*33 to one of 10 miles diameter, 

 would have nearly the same effect, and it cannot be contended that 

 these are improbably large. 



Should the experiment ever be repeated, it would be desirable to 

 swing the pendulum at one (at least) intermediate station between 

 the top and bottom of the shaft, by which means any error of this 

 kind might be approximately eliminated. In the mean time I think 

 there are hardly sufficient grounds for impugning the correctness of 

 the value of E (5 '67) deduced by the late Francis Baily from his 

 carefully conducted repetition of the Cavendish experiment. 



IV. " On Practical Methods for rapid Signalling by the 

 Electric Telegraph." By Prof. W. THOMSON, F.R.S. 

 Received November 14, 1856. 



I am at present engaged in working out various practical applica- 

 tions of the formulae communicated some time ago in a short article 

 on the "Theory of the Electric Telegraph" (Proceedings, May 17, 

 1855), and I hope to be able very soon to lay the results in full 

 before the Royal Society. In the mean time, as the project of an 

 Atlantic Telegraph is at this moment exciting much interest, I shall 

 explain shortly a telegraphic system to which, in the course of this 

 investigation, I have been led, as likely to give nearly the same 

 rapidity of utterance by a submarine one- wire cable of ordinary 

 lateral dimensions between Ireland and Newfoundland, as is attained 

 on short air or submarine lines by telegraphic systems in actual use. 



Every system of working the electric telegraph must comprehend 

 (1) a plan of operating at one extremity, (2) a plan of observing at 

 the other, and (3) a code of letter-signals. These three parts of the 

 system which I propose will be explained in order, I. for long sub- 

 marine lines, and II. for air or short submarine lines. 



I. Proposed telegraphic system for long submarine lines. 

 1. Plan of operating. This consists in applying a regulated gal- 



