338 Tr ansae tions. — Misce llaneous . 



another has only one-fourth the speed. Worse than this is 

 the fact that when a current arrives it does so gradually : a 

 small gradually increasing current is what is really received, 

 so that with any rapid signalling the several signals run into 

 one another, and a great difficulty is experienced in reading 

 them. The reason of this retardation is twofold — first, a 

 part of the charge enters the insulator ; and, secondly, the 

 cable being surrounded by water, which is a conductor of 

 electricity, the current induces another current in the water 

 near it, and these currents act on each other. The fraction 

 of time named — two-tenths of a second — as requisite for the 

 front part of a current to traverse the Atlantic is not to be 

 looked on as inconsiderable in telegraphy. Expert telegraph 

 clerks have succeeded in sending forty words a minute, while 

 an automatic sender can send one hundred words in that 

 time. Each word may be reckoned on an average to consist 

 of five letters, and each letter of two signals, which would 

 make four hundred signals a minute in the first case and one 

 thousand a minute in the second case. 



As at first only a small fraction of the current arrives at 

 the distant end of the cable, it is obvious that very sensitive 

 instruments — that is, those worked with very feeble currents 

 — are most suitable for cable work. So it was that Sir Wil- 

 liam Thomson's invention of the mirror galvanometer made 

 Atlantic telegraphy practicable. Hence, too, it is seen that 

 such instruments as the Morse and the Sounder, which are 

 in use at the present time in our telegraph-offices, are not 

 suitable for long cables, they being far less sensitive than gal- 

 vanometers. On the cable from Santa Cruz to St. Thomas, 

 however, only some forty miles long, I obtained clearer signals 

 with a Morse instrument than with a galvanometer. It is 

 only where a cable is of considerable length that difficulties as 

 to speed of working have to be taken into account. In a cable 

 that can be just conveniently worked with a galvanometer, 

 not a twelfth part of the signalling could be got through in 

 the same time with a Morse instrument. 



The use of the mirror galvanometer for cables is now 

 superseded by that of another instrument known as the 

 "siphon recorder." This, as its name implies, is a recording 

 instrument, and, like the Morse, is an electro-magnetic one — 

 that is to say, the current on arrival temporarily converts a 

 piece of soft iron into a magnet, which attracts suitable ''ap- 

 paratus, so that a mark is left on paper. Unlike the Morse, 

 it is almost as sensitive to feeble currents as the mirror in- 

 strument. The invention of this instrument is also due to 

 Sir William Thomson. The use of the siphon recorder neces- 

 sitates the cable being fitted with a large condenser at each 

 end similar to those used in laying cables. This arrangement 



