LINE CONSTRUCTION. 
129 
ground are used when a strong resistance is needed. Where 
they have to support tension apparatus, or are placed at a 
sharp angle, or are exposed to strong winds, they are preferred 
to the ordinary posts of the same height, which have a girth of 
only 16 inches at 2 yards from the ground. 
Posts 34 to 40 feet high, and 24 to 26 inches in circum¬ 
ference, are used at large crossings for holding up the wires, 
according as these are more or less numerous. In most cases 
the first-named size is sufficient. Posts even 45 to 65 feet 
high are used, but only in very rare cases, as for example in 
crossing a piece of water, or passing the wires over a building, 
or again, for supporting a very great number of wires when 
the erection of a second line would present some difficulties. 
In making a telegraph line we should try to give, as far as 
possible, a general uniform rectilinear direction to the wires, 
both horizontally and vertically. Changes of direction in the 
route and differences of height in the supports make the 
tension of the wires by the stretchers very low and of little 
use. It is therefore necessary to connect low posts with high 
posts by means of posts of intermediate height. 
Care must be taken that the strain put upon the posts does 
not exceed the limits of elasticity. For a 30-feet post, sunk to 
a depth of 5 feet, and of 10 inches diameter at the level of the 
ground, the maximum strain is 1 cwt., acting at a point 1 yard 
from the top of the post. 
This sufficiently shows the necessity of strengthening the 
supports at the places where wires terminate, or form sharp 
angles. The pulling force of a wire 0T6 inch diameter 
stretched in the ordinary way may amount to 2 cwt. 
Where the wires go into the offices, they usually end at a 
bracket fixed into the masonry by irons of a particular form. 
Where the wires are joined to subterranean or submarine 
lines, two or three posts are grouped together, according to the 
number of the wires, and these posts are so bound together by 
iron ties that they are perfectly firm. At the base of the posts 
there are also fixed wooden feet that serve to spread the strain 
over a greater extent of ground. The whole is made stable by 
supports placed against the foot of the post in greater or less 
number, according to the nature of the soil. In sharp curves 
the posts are kept upright by props that are fixed higher in 
