THE ELECTEIC TELEGRAPH. 



piles. To show the strength of the wires thus extended, a rope 

 was, for experiment, hung to the centre of the wire of largest 

 span, and a soldier climbed up it, the weight of his body pro- 

 ducing but a slight curvature. The common deflection arising 

 from the weight of a wire of a furlong span does not exceed 

 eighteen inches. 



Dr. O'Shaughnessy's plan of underground communication, 

 when such a mode of laying down the wires is desirable, is very 

 economical. The copper wires coated with gutta percha, instead 

 of being inserted in iron tubes, are inlaid in wooden sleepers, well 

 saturated with arsenic, to protect them from the white ants, and 

 they are then laid in a trench about two feet deep. An under- 

 ground system of two wires may thus be laid down for 35/. the mile. 



The plan adopted for joining the lengths of the thick galvanised 

 wire is to have the two ends turned, so as to link into one another, 

 which are then introduced into a mould, like a bullet-mould, and 

 an ingot of zinc being cast over them, they form a most substantial 

 joint, and perfect metallic connection.* 



It appears from reports received in May, 1854, that at that 

 date a telegraphic line was in full operation from Calcutta to 

 Agra, a distance of 800 miles, and it was then expected that the 

 entire line to Bombay, a distance of 1500 miles, would soon be 

 completed and put in operation. 



This line is reported to have been completed and brought into 

 operation since the preceding paragraphs were in type. 



101. To produce the effects, whatever these may be, by which 

 the telegraphic messages are expressed, it is necessary that the 

 electric current shall have a certain intensity. Kow, the intensity 

 of the current transmitted by a given voltaic battery along a 

 given line of wire will decrease, other things being the same, in 

 the same proportion as the length of the wire increases. Thus, if 

 the wire be continued for ten miles, the current will have twice 

 the intensity which it would have if the wire had been extended 

 to a distance of twenty miles. 



It is evident, therefore, that the wire may be continued to such 

 a length that the current will no longer have sufficient intensity 

 to produce at the station to which the despatch is transmitted 

 those effects by which the language of the despatch is signified. 



102. The intensity of the current transmitted by a given 



* Year-Book of Facts, 1853, p. 150. 

 174 



