382 



" boxing the compass," which takes young people so long to become 

 familiar with, will be entirely superseded, and I think the sooner such 

 method becomes obsolete the better it will be for the interests of the 

 mariner, for, together with other advantages, the tedious operation 

 of a " day's work " will be divested of half the usual trouble. 



When giving a course to the " quarter- master," or " man at the 

 wheel," no mistake, so liable to be the case at present, can well 

 occur ; it will merely be necessary to direct him to steer, for instance, 

 " north five points east," or more briefly, " north five east," " south 

 six west," &c. &c. 



I recollect an instance of a vessel steering N.W. by N. -5- N., 

 instead of W. by N. N. during thick weather in the Bristol Channel, 

 thus running into danger from the similarity of sound between the 

 courses alluded to. 



The practical application of the decimal card would not materially 

 affect the charts previously published, which could have printed 

 compasses containing thirty-six points pasted over the others. Such 

 might be sold by any chart-seller. 



III. " On the Theory of the Electric Telegraph." By Pro- 

 fessor WILLIAM THOMSON, F.R.S. Received May 3, 1855. 



The following investigation was commenced in consequence of a 

 letter received by the author from Prof. Stokes, dated Oct. 16, 1854. 

 It is now communicated to the Royal Society, although only in an 

 incomplete form, as it may serve to indicate some important practi- 

 cal applications of the theory, especially in estimating the dimen- 

 sions of telegraph wires and cables required for long distances ; and 

 the author reserves a more complete development and illustration of 

 the mathematical parts of the investigation for a paper on the conduc- 

 tion of Electricity and Heat through solids, which he intends to lay 

 before the Royal Society on another occasion. 



Extract from a letter to Prof. Stokes, dated Largs, Oct. 28, 1854. 



" Let c be the electro -statical capacity per unit of length of the 

 wire ; that is, let c be such that civ is the quantity of electricity 

 required to charge a length / of the wire up to potential v. In a 



