136 
THE TELEGRAPH. 
been adopted, in which the wires are fixed to the roofs of 
houses. The “Metropolitan Telegraph Company ,” now, like 
the rest, absorbed by the Post Office, set up its lines on iron 
posts that supported a single wire, beneath which was suspen¬ 
ded a light cable composed of a number of insulated wires. 
This system has been modified, and where such cables are 
maintained by the English authority they are arranged as 
follows :— 
Only the very best materials are used in their construction. 
The supports are iron uprights of a height varying according 
to the requirements. They are attached to feet fixed on the 
ridges of the roofs. 
All the posts used in this kind of work must be strengthened 
by stays in every direction. The conductor consists of a 
cable formed of three strands of No. 16 wire. This renders 
it less liable to break from its own weight, and it makes less 
noise than would be caused by the vibrations of a solid wire. 
Where it is exposed to smoke or gaseous emanations it is covered 
with tarred tape by which the destructive effects on the wire 
are much diminished. The insulators are of the form shown 
in the annexed drawing (figs. 78 and 791, and the result is a 
diminution of the friction that necessarily occurs in the long 
spans given to the wires. This form of insulators also reduces 
the risk of the wire breaking under the strain. As far as 
possible highways must be crossed at a right angle and not 
diagonally, for the shorter the crossing wires the less is the 
danger of breakage. 
When posts cannot be fixed to the roofs, chimneys are used 
as points of attachment; but in that case they must be care¬ 
fully selected with regard to their stability. They are then 
surrounded by an iron band on which the supports are fixed. 
The inhabitants commonly complain of the noise caused by 
the vibration of the wires, and this more especially happens 
when the wire is too tightly stretched. In winter also the 
vibrations are increased by the effect of contraction. It has 
been attempted to obviate these inconveniences by wrapping 
the insulators with chamois leather, and with satisfactory 
results. The best way, however, of doing away with the noise 
is to use a piece of chain, about a yard long, on each side of 
he point of attachment. The wire is securely fixed and 
