Eagle River review. [volume] (Eagle River, Wis.) 1890-1927, 1915-02-12

be said of the harbors of Germany.
But these contrivances for harbor Defense while
much more formidable and dangerous to an enemy.
harmless to friendly ships They what
are known as observation mines, and, being con-
nected by wire with the shore, are set off by the
sending of electric spark at moment when
hostile vessel may happen to be within range OF
their tremendous explosive activities.
All the navigable channels of the harbor of
Portsmouth, for example, are at this moment
guarded by an elaborate system of mine fields,
which are protected by rapid-fire guns shore.
At night they are under constant watch, well
as by day. being swept by huge searchlights.
Such mines are hollow spheres Of galvanized
iron three feet in diameter, each containing 500
pounds of guncotton, which is lighter than water.
so that they can float They are anchored a few
feet below the surface in series of lines across
channel, about eighty feet apart in each line.
If an enemys ship were to succeed by good luck
in getting through line without being blown
up, she could hardly pass another.
The area of water surface covered by a mine
field is laid off (by careful survey in checker
board Of imaginary squares. This checkerboard is
reproduced small scale a table in un-
derground casemate on shore, which is part OF
fort. Suppose hostile vessel to approach. Two
telescopes are aimed at her from points on shore
some hundreds of yards apart, their lines OF sight
crossing, of course, at the spot where she floats
The telescopes electrically connected with two
pointers that move on the table. Moving with the
telescopes, the ends of the pointers meet on the
square corresponding to the one where lover the
mine 1eld) the vessel actually is. button mark
ed with the number of that square is touched, and
bang!-she is blown sky-high.
Small steamers especially equipped for the pur-
pose are used in the business of planting these
mines and the work being of such great import
ance the men who undertake it are highly trained.
Theirs is branch of the coast artillery which
does its fighting literally under water and, apart
from the mechanical details of their employment.
they must have fairly expert acquaintance with
electricity and the chemistry of explosives
part Of the preliminary work of establishing
mine field consists in making soundngs. The
depths all over the surveyed area having been
ascertaned, wire ropes are measured of into cor
responding lengths, heavy leaden sinker Tor
anchor) being attached to end of each, and
mine to the other end BY this simple means the
sphere of galvanized iron is made to float just as
many feet below the surface may be desired.
Recently, however an ingenious automatic an
Choir" has been contrived by which, matter
what the depth, exactly the required submergences
for each submarine may be obtained ofhand.
AS yet experience in actual warfare has not
afforded practical demonstration of the usefulness
of such observation mines But OF their destruc-
tive power there be no question. Many ex-
perments have been made with them in the blow
ing up of old huks, and on this side of the water,
not long ago, miniature ship of scat
mine of corresponding size ignited by electric
spark from distance of mile and quarter
From such trials the conclusion has been drawn
that the explosion of mine containing 500 pounds
of guncotton would at least disable the stoutest
battleship within radius of so feet, IL it did not
sink her. In field" defending harbor channel.
each row of galvanized iron spheres is strung
along electric cable, which is attached to the
sinkers Tor anchors) and runs along the bottom.
The rows are connected together by main cleo
trio cable to which each mine is joined by a
branch cable that meets it just above the anchor
Thus every mine in the field" is under direct
control by the apparatus in the casemate on shore
The casemate is underground room, lined
with concrete, and containing all the mechanism
for controlling the mine system. 1t is connected
by telephone and otherwise electrically, with the
two observing stations, in which are mounted the
telescopes already mentioned, for watching hostile
ships.
Sometimes, for the protection of harborE, what
are called "electro-contact" mines used. They
are much smaller in size, and are commonly ar
ranged in groups Of rve or SIX, which are con-