The telegraph manual: a complete history and description of the semaphoric, electric and magnetic telegraphs of Europe, Asia, Africa, and America, ancient and modern.

AMERICAN INSULATORS. 545
shaped iron cap marked 1, from three to four Fig. 38.
pounds in weight, projecting an inch below
the lower edge of the glass, protecting it from
being broken; this is then fitted with much
care to the top of the pole, marked 3, and is <f
covered with paint or varnish. The line wire
is fastened to the top of the cap by the pro-
jecting iron points, and the whole of the iron
cap is thus in the circuit, as the iron wire is _
not insulated. To prevent the deposit of =
moisture, the glass is covered by a varnish of
gum-lac dissolved in alcohol, and the rim-like -
form of the glass is to cause any moisture to - c=
be carried to the edge and there drop off.
The insulator represented by fig. 39 has been in use for some
years on the Boston Fire Alarm telegraph, and has proved to be
a success. The cast iron cap is represented by the black line
in the section. This is lined throughout with glass, by the
operation of blowing, or with porcelain. The shank is then
introduced with a hot mass of glass or any fused or semi-fused
material, by which it is firmly fixed in its place. This is represented by the shaded portion. Fig. 39.
Between the lower edge of the
cap and shank, in the section,
there are four inches of glass sur- | IS!
face. The re-entering angle of
the lower part of the cap protects jli j
the glass within from missiles, and /!
-s calculated, in a storm.of wind /jf/' i f
and rain, to drive the latter down- i I
ward and thus preserve the insulation. The wires pass over the
top of the insulator. The shank,
which should be longer than is krepresented, screws into a bracket
or the ridge-pole of a house.
The insulator represented by fig. 40 is called Batchelder's
hard rubber insulator. Hard india-rubber has been used for
the insulation of telegraph wires for several years past, and
has served successfully during the heat of summer and the
extreme cold of winter. The seasons have not affected it.
This substance does not soften at a lower temperature than
300~ F.; it is much stronger than glass, it does not absorb
moisture, nor does the dew collect upon its surface as readily
as upon glass or porcelain. The figure represents the insulator
35