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CHANNEL DEMARCATION.
277
upon sound as a warning
medium. This is accom-
plished in various ways,
principally by foghorns and
syrens on the shore, and by
bed - buoys and whistling
buoys on the water. Of
foghorns, it need siniply
be said that they consist
of a brass trumpet through
which a strong blast of
oompressed air or steam is
expelled at definite inter-
vals. They are raucous and
unmusioal in the extreme.
The syren gives out a note
of high frequency due to
the impulsion of air or
steam through a series of
holes in a rapidly revolving
disc.
Of bell-buoys there
are several different ar-
rangements. A fixed bell
may be struck by pendant
clappers, or by a set of balls
rolling freely in horizontal
grooves or cylinders.
Wheu the water is smooth,
as is commonly the case in
foggy weather, neither of
these appliances can be
counted upon to emit sig-
nals, depending as they do
upon the swaying action of
waves. In that event, an
automatic lever apparatirs,
worked by the agency of
gas, as in the Pintsch
system, has been found use-
ful. The gas forces up a
diaphragm until it works a
lever which closes the inlet
valve and opens the outlet,
simultaneously actuating
Fis. 247. —Courtenay Whistling Buoy.