THE LAYING OF SUBMARINE CABLES. 181 



and C. It is the same as the double pattern with B and D 

 short-circuited together. The cable is connected to A as 

 before, C to the land-line or instrument, and B to earth direct. 

 The best earth to use is the cable sheath, which is connected 

 to the base of the guard as shown. As in the double form 

 lightning is checked by the self-induction of the coils from 

 passing the same way as the signalling currents, namely, from 

 A to C, and the easiest path open to it is by the shunt gap A to 

 B, thus discharging direct to earth and protecting the line and 

 instrument. These guards are manufactured by the well- 

 known makers of cable appliances, Messrs. Muirhead & Co. 



Final Tests. — After the cable is laid tests are taken to 

 ascertain if the cable passes the specification and also to obtain 

 the mean temperature of the ocean bed in which the cable is 

 laid. It is usual to conduct these tests at the cable houses and 

 to use battery powers of 100 to 150 volts, as during submersion. 

 The first test for insulation is from 30 minutes to one hour, with 

 each current according to the length of cable, and when com- 

 pleted the cable is generally used for trafiic at once, and the tests 

 continued at such intervals during the .SO days' guarantee as 

 the line can be spared. The copper resistance is frequently 

 measured during this period, the observed results being cor- 

 rected for the temperature of the bridge and compared with the 

 factory tests at the standard temperature of 75°F., from which 

 it is easy to deduce the mean temperature of the cable as laid. 

 As instances of this determination on shallow and deep-water 

 cables the following examples of final tests on the cables 

 between Bonny and Brass (Niger Coast Protectorate) and 

 between Bonny and the Island of Principe, may be of interest. 

 Shallow Water Cable. — (Bonny-Brass section) : — 



Length between cable houses 68-27 nauts 



Distance over water 66 72 „ 



Percentage slack 232 



Maximum depth 8 fathoms 



Mean depth 6 „ 



Observed resistance of line 645-55 ohms 



Corrected for temperature 647'01 „ 



Ptesistance per knot 9-477 „ 



Do. at standard temperature (75°F.) 9-396 „ 



