34 THE SCIENTIFIC PAPERS OF 



for that purpose. This difficulty has been removed by the intro- 

 duction of permanent magnets, which continue the effect produced 

 by the instantaneous action of the line-wire current, until the 

 opposite effect is produced by the succeeding negative current. 

 The vibrating tongue of the instrument is for this purpose 

 balanced midway between the similar poles of a comparatively 

 powerful permanent magnet, being equally attracted by both 

 bat remaining in the proximity of either of them, into the attractive 

 sphere of which it happens to be brought by the instantaneous 

 action of the line- wire current, changing for an instant of time the 

 name of one of the contending poles. A relay on this principle 

 was first exhibited at the Great Exhibition of 1851 by Siemens 

 and Halske. 



The relative dimensions of the inductive coils, and of the coils 

 in the relay, (depending upon the length and other conditions of 

 the cable itself) are points which require very careful attention. 

 The common practical rule, that the resistance of the coils must 

 be increased with the increased length of the conductor, is here 

 entirely at fault, for the electric wave, when once formed, is no 

 longer under the influence of its source, but may be compared to 

 the dying wave of the ocean running up a shallow beach, which 

 would have no power to force its way through a long and narrow 

 tube, but is yet capable of delivering a large quantity of water 

 into an open duct. For an analogous reason the coils of the relay 

 must be composed -of comparatively short and thick wire. The 

 same rule applies to the inductive coils, which must be composed 

 of thick wire in order to produce a quantitative wave. The 

 Cagliari, Malta, and Corfu line is worked by instruments upon 

 this principle, and the results obtained are very satisfactory, the 

 messages being worked through the entire distance of 700 nautical 

 miles (without making Malta a relay station) with ease, and at a 

 sufficient rate. 



This result proves that telegraphic cables not exceeding a 

 thousand miles in length may be worked satisfactorily, and that, 

 consequently, all reasonable doubts may be considered as being 

 removed about the successful operation of a line from London to 

 Calcutta, a result which I sincerely hope to see soon established in 

 fact. 



For distances exceeding a thousand miles, the difficulty of 



