Evening star., 1896-03-07
the distance from the source of sound be-
ing the same

the distance from the source Of sound be-
ing the same.
Some Interesting Cases.
There are six steam fog whistles on the
coast of Maine; these have been frequently
heard at a distance of twenty miles, and
as frequently cannot be heard at the dis
tance of two miles, and this with no per-
ceptible difference in the state of the at
mosphere.
1.-THE signal is often heard at great dis
tance in one direction, while IN another 1t
VIII be scarcely audible at the distance of a
mile. This is not the effect of wind. as
the signal is frequently heard much farther
against the wind than with it; for example,
the whistle on Cape Elizabeth can always
be distinctly heard in Portland. distance
OF nine miles. during heavy northeast
snow storm, the wind blowing a gale d.
rectiy from Portland toward the whistle.
1.-THE most perplexing difficulty, however
arises from the fact that the signal often
appears to be surrounded by a belt, vary
ing In radius from one to one and one-half
miles. from which the sound appears to be
entirely absent. Thus, in moving directly
from station, the sound is audible for
the distance of a mile, is then lost for
about the same distance, after which it is
again distinctly heard for a long time
rms action is common to all ear-sgnals,
and has been at times observed at all the
stations, at one of which the signal is sit
uated on bare rock twenty miles from
the main land, with no surrounding objects
to affect the sound.
Running by Echo.
steamers are constantly running among
the Islands on the coast of Maine during
the summer. This is the season of thick
and persistent fog. When pilots can hardly
see the length Of their vessels. they keep
up constant noise with their fog signals.
The open sea gives back no sound But
the near or remote vicinity of cliffs, bluffs,
or even high shores. is indicated by the
strength of the echo received back from
them. In fact. running by echo is recog-
nized as one of the necessities of the navi-
cation of those waters
This method is also used to some extent
by steamers on the great rivers, and is
practiced on the great lakes to some ex-
tent, notably at a certain bluff jutting out
into Lake Superior.
IF the law Of these alterations in audi
bility can be evolved and some method dis
covered for their correction, as the vara-
tions of the compass are corrected, then
sound may be depended upon as a more
definite and accurate aid to navigation.
Until then the mariner will do we'll when
he does not get the expected sound of a fog
signal. to assume that he may not hear a
warning that is faithfully given, and then
to heave his lead, and resort to the other
means used by the careful navigator to
make sure of his position.
As to the Use of Echo.
"a fog-signaling device now under con-
sderation by the light house board is the
Invention of Mr.. De la Tore of Baltimore.
1t is what may be called an echo-maker, and
consists of faring funnel screwed on the
nluzzle of rifle. 1t is operated by firing the
rifle in the direction of the supposed oh
stacle, such as rock. iceberg, another
ship, or 6. cliff. If the obstacle Is there
the beam of sound projected through
the funnel strikes the obstacle and re-
bounds; and as the echo is more or less per-
feet in proportion as the obstacle is more
or less parallel to the ship ii ON which the
gun is fired, and it is near or remote, the
position of the obstacle may thus be in
ferred. The board reported that De la
Torres method was firing 81 blank cartridge
from a rifle in the presence of objects as
small as spar-buoy and as large fort
and catching the return sound echo.
lie claims that sharp sound projected
at or nearly at an object, and only when
so directed. will in every case return some
OF the sound sent, So that theoretically
there will always be echo, and the diff
ference in the time between the sound
sent and the echo will Indicate the remote
ness of the object. The board found that
return sound could b., heard from the
side of fort half mile off, from passing
steamers quarter mile off if broa adside-to
from bluffs and sails of vessels about tie
same distance, and from spar-buoys 200
yards away.
Further Experiments.
'The board further states that the sound
from the different kinds of masses is diff
ferent in most cases, and that the ear
could he educated to detect quite a rarge
of different obiecss. as The echo from a
sail was different from the echo from a
buoy or bluff. If two objects were near
the line of projection at different distar, es
an echo would be received from each. The
horizontal limit Of the return of sound
seemed to he about two points ON each
side of the axis Of projection.
IF Mr.. De Ia Tore should see fit to con-
struct his instrument for hearing feeble
echoes, the board indicated that 1t would
recommend that 1T ,e fitted soon to some
vessel of the North Atlantic station, and
that further and, If possible, exhaustive ex-
periments ought to be made to practically
determine the use Of the echo as means
to discover obstacles to navigation. 1t was
also stated that steam whistles could be
heard much farther than the echo; but it
was said that where the OBSTACLE could not
make the sound as in The case of an ice
berg, the echo would be OF the greatest use,
and experiments looking to its atiliza ton
are demanded by :he conditions of naylga-
ton in time of fog