to study our system. That sent by England in 1872, of
which Sir Frederick Arrow was chairman, and Captain Webb, R.N., recorder,
reported so favorably on it that since then "twenty-two sirens have been
placed at the most salient lighthouses on the British coasts, and sixteen
on lightships moored in position where a guiding signal is of the greatest
service to passing navigation."

The trumpet, siren, and whistle are capable of such arrangement that the
length of blast and interval, and the succession of alternation, are such
as to identify the location of each, so that the mariner can determine his
position by the sounds.

In this country there were in operation in July, 1883, sixty-six
fog-signals operated by steam or hot air, and the number is to be
increased in answer to the urgent demands of commerce.

_Use of Natural Orifices._--There are, in various parts of the world,
several sound-signals made by utilizing natural orifices in cliffs through
which the waves drive the air with such force and velocity as to produce
the sound required. One of the most noted is that on one of the Farallon
Islands, forty miles off the harbor of San Francisco, which was
constructed by Gen. Hartmann Bache, of the United States Engineers, in
1858-59, and of which the following is his own description:

"Advantage was taken of the presence of the working party on the island to
make the experiment, long since contemplated, of attaching a whistle as a
fog-signal to the orifice of a subterranean passage opening out upon the
ocean, through which the air is violently driven by the beating of the
waves. The first attempt failed, the masonry raised upon the rock to which
it was attached being blown up by the great violence of the wind-current.
A modified plan with a safety-valve attached was then adopted, which it is
hoped will prove permanent. ... The nature of this work called for 1,000
bricks and four barrels of cement."

Prof. Henry says of this:

"On the apex of this hole he erected a chimney which terminated in a tube
surmounted by a locomotive-whistle. By this arrangement a loud sound was
produced as often as the wave entered the mouth of the indentation. The
penetrating power of the sound from this arrangement would not be great if
it depended merely on the hydrostatic pressure of the waves, since this
under favorable circumstances would not be more than that of a column of
water twenty feet high, giving a pressure of about ten pounds