Cooke. — On Telegraph Cables. 327 



to bind the copper strand round with a coil of thin wire 

 fastened only at the ends. If a joint should break the thin 

 wire will still convey the current. In spite of all precautious 

 a rupture of the conductor inside the insulator does sometimes 

 occur. The distance of such a break from either end is easily 

 found by observing the amount of the static charge that part 

 of the broken cable will furnish. Of course, the amount of 

 static charge per mile is always known. 



An unusual kind of fault came under my own notice 

 during the laying of a cable in the West Indies from Santa 

 Cruz to Porto Rico. The conductor was broken inside the 

 insulator, which remained perfect, but the ends of the broken 

 pieces were in loose contact. On testing the resistance of the 

 conductor after laying, it was found to be about double cf 

 what it had been previously. This had no effect on the 

 signals, so it was not discovered till the laying was completed. 

 Such a fault is difficult of detection. There would always 

 be the danger in such a case of the loose contact not being 

 maintained, when the signals would suddenly cease. It was 

 necessary to cut the cable in one or two places to localise the 

 fault, and replace the faulty length by a good one. The fault 

 was found some distance out at sea. 



2. The Insulator. — Most telegraph cables are insulated 

 with guttapercha. I have only had personal experience of 

 those insulated with rubber; I propose therefore to confine my 

 remarks to the latter substance as an insulator. Indiarubber, 

 like guttapercha, is a gum that exudes from several tropical 

 plants. The best rubber comes from the forests of the 

 Amazon. The indiarubber- tree resembles the ash in appear- 

 ance. On making holes in the bark a juice like milk comes 

 out. This, on being dried and smoked, becomes the black 

 solid we are familiar with. Though generally black, some of 

 the pure rubber, especially that sold at artists' warehouses, is 

 much lighter in parts, and occasionally approximates to its 

 natural colour — white. The black colour is simply caused by 

 smoke. One of our company in Brazil tried to secure some 

 pure-white rubber by filling a bottle with juice and corking it. 

 In a day or two, however, the cork was forced out of the 

 bottle ; some of the rubber, now solid, was also forced out, 

 and stood some inches above the neck of the bottle. It had 

 acquired a very disagreeable odour. We gather, then, that 

 the smoking is a necessary process. Eubber is collected from 

 a large part of the Amazon Valley from trees most of which 

 are growing wild. Pieces as large as saucers and from ^ in. 

 to 1 in. thick are strung on a switch and conveyed in canoes 

 to the main stream or the larger tributaries of the Amazon. 

 Eiver steamers then convey them to the seaport of Para, where 

 they are transhipped on to ocean-going vessels. 



