CHAP. V.] 



TELEGRAPHIC LINES. 



609 



further apart in valleys where the wires may sometimes extend to a 

 length 4 or 5 times as great from pole to pole. The height of the poles 

 is from 6 to 12 yards, but is greater when the line has to clear rivers, 

 roads, &c. In towns, the porcelain insulators are placed on wooden 

 uprights fixed to the walls of houses or other buildings, and sometimes 

 on posts above the roofs ; but for many years, it has been found 

 preferable in carrying, wires through crowded cities and thorough- 

 fares to replace them by subterranean ones, which are also made 

 use of in tunnels. 



Each post generally carries several wires, which are fixed at 

 intervals of about 9 to 12 inches, putting them alternately in front 



FIG. 397. Stretching winches for telegraphic lines. 



and behind, so as to counterbalance the effects of traction, which tend 

 to bring down the post. Every now and then along the line (at 

 every kilometre in France) are placed stretchers insulated as before 

 by being suspended from insulators, a band of iron joining the 

 two stretchers conducting the electricity between the two wires 

 (Fig. 397). This stretching of the wires is necessary to prevent 

 them from touching and entangling. 



In England and Germany other methods of stretching the wires 

 are employed, which may be gathered from Figs, 396 and 397 

 without further details. 



At the outset of electric telegraphy, the system of suspension of 



E 11 



