1/2 THE ADDRESSES, LECTURES, ETC., OF 



in asphaltc, producing altogether a flexible cable of 1 inch outer 

 diameter, which is laid along railways or roads at a depth of about 

 3 feet below the ground. 



Great precautions have been adopted to prevent failure of these 

 newly established lines, whilst the ease with which these compara- 

 tively long circuits can be worked by means of every description 

 of instrument, including the telephone, and their perfect immunity 

 from atmospheric disturbances, will lead, I venture to predict, to 

 the gradual substitution of underground wires for suspended line- 

 wires for all the main arteries of the telegraphic system. 



In submarine telegraphy no startling feat of novelty can be 

 reported, although steady progress has recently been made in 

 improving the manufacture of the insulated conductor, in the 

 attainment of an increased rate of transmission through long 

 distances, in the outer protection given to the insulated conductor, 

 and in the vessels and other appliances employed for submerging 

 and repairing deep-sea cables. 



The conductor almost universally adopted in the construction of 

 submarine cables has been a strand of seven copper wires, covered 

 with three thicknesses of gutta-percha, with intervening layers of 

 a fusible resinous compound. In the case of the Direct United 

 States Telegraph Company's cable, the conductor consists of one 

 large central wire of 0'090 in. diameter, surrounded by eleven small 

 copper wires of 0*035 in. diameter. By this construction an in- 

 crease of about 10 -per cent, of conductivity is obtained for a 

 given outer diameter, which increase has been found to exercise an 

 important effect upon the rate of transmission through the cable. 



The careful selection of the insulating material employed has 

 also an important influence upon the rate of transmission through 

 .long cables, as it is found that different kinds of gutta-percha 

 behave very differently in this respect. India-rubber has, it is 

 well known, considerably less inductive capacity than gutta-percha, 

 and appears on this account the preferable material, but its appli- 

 cation to the conductor, without the risk of faults and of gradual 

 changes in the condition of the material, is beset with considerable 

 practical difficulty which has as yet limited its application. 

 Compounds of india-rubber and gutta-percha, with other materials 

 such as shellac, paraffin, and bitumen, have been proposed from 

 time to time with promising results, but it has been impossible 



