Cooke. — On Telegraph Cables. 337 



completion of laying. We were instructed, however, to allow 

 the telegraph company the use of the cable for that period if 

 it were protected against lightning. No special precautions 

 for this are required while the cable is on the ship or during 

 laying, but in the practical working a new element of danger 

 is introduced. It arises in this manner : The most convenient 

 place for landing a cable is often at a considerable distance 

 from a town in which the telegraph-office is situate, so the 

 cable is continued by a land-line to the office. This land-line 

 may be struck by lightning, when, if the charge of electricity 

 conveyed by it were allowed to enter the cable, it would pro- 

 bably inflict considerable damage. The place where a land- 

 line joins a cable is accordingly fitted with what is termed a 

 lightuing-guard. These guards are of two kinds. The first 

 consists of a short length of thin wire inserted between the 

 land-line and the cable ; this is fused by the heat developed 

 by any powerful charge such as produces lightning. The 

 second kind consists of a brass plate in contact with the end 

 of the land-line ; this plate is insulated from a similar plate 

 in contact with an earth- wire by a thin layer of some material 

 such as paper soaked in melted paraffin. In the latter case a 

 powerful charge of electricity ruptures the paper, and the 

 lightning passes harmlessly to the ground. In either case the 

 lightning is prevented from entering the cable. 



I waited at Santa Cruz the thirty days. A representa- 

 tive of the telegraph company took another test of the cables. 

 I, as representing the manufacturers, took final tests, and, all 

 these tests being satisfactory, we left. 



IV. The Working of the Cable. 



The instruments used by the telegraph-cable companies 

 for signalling on their submarine lines are usually of a dif- 

 ferent character from those used on land-lines. The principal 

 reason for this is that guttapercha and rubber are only indif- 

 ferent substitutes as dielectrics for the dielectric of land-lines 

 — atmospheric air. The consequence is that the passage of 

 the current is retarded, and especially on cables of consider- 

 able length. 



Attempts have been made to discover the velocity of elec- 

 tricity, but very discordant results have been obtained, as 

 retarding influences cannot be got rid of. On land-lines, 

 however long they may be, the transmission of the current 

 is practically instantaneous. On the Ireland-Newfoundland 

 cables, on the other hand, two-tenths of a second must elapse 

 after sending before a sufficient current arrives at the receiving- 

 station to work the most delicate instrument. The longer the 

 cable the less the speed of the current, which varies inversely 

 as the square of the length, so that a cable twice the length of 

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