_PART I_
RESEARCHES ON THE ACOUSTIC TRANSPARENCY OF THE ATMOSPHERE IN RELATION
TO THE QUESTION OF FOG-SIGNALLING
Introduction—Instruments and Observations—Contradictory
Results from the 19th of May to the 1st of July
inclusive—Solution of Contradictions—Aërial Reflection
and its Causes—Aërial Echoes—Acoustic Clouds—Experimental
Demonstration of Stoppage of Sound by Aërial Reflection
§ 1. _Introduction_
We are now fully equipped for the investigation of an important
practical problem. The cloud produced by the puff of a locomotive can
quench the rays of the noonday sun; it is not, therefore, surprising
that in dense fogs our most powerful coast-lights, including even the
electric light, should become useless to the mariner.
Disastrous shipwrecks are the consequence. During the last ten years
no less than two hundred and seventy-three vessels have been reported
as totally lost on our own coasts in fog or thick weather. The loss,
I believe, has been far greater on the American seaboard, where trade
is more eager, and fogs more frequent, than they are here. No wonder,
then, that earnest efforts should be made to find a substitute for
light in sound-signals, powerful enough to give warning and guidance to
mariners while still at a safe distance from the shore.
Such signals have been established to some extent upon our own coasts,
and to a still greater extent along the coasts of Canada and the United
States. But the evidence as to their value and performance is of the
most conflicting character, and no investigation sufficiently thorough
to clear up the uncertainty has hitherto been made. In fact, while the
_velocity_ of sound has formed the subject of refined and repeated
experiment by the ablest philosophers, the publication of Dr. Derham’s
celebrated paper in the “Philosophical Transactions” for 1708 marks the
latest systematic inquiry into the causes which affect the _intensity_
of sound in the atmosphere.
Jointly with the Elder Brethren of the Trinity House, and as their
scientific adviser, I have recently had the honor of conducting an
inquiry designed to fill the blank here indicated.
One or two brief references will suffice to show the state of the
question when this investigation began. “Derham,” says Sir John
Herschel, “found that fogs and falling rain, but more especially snow,
tend powerfully to obstruct the propagation of sound, and that the same
effect was produced by a coating of fresh-fallen snow on the ground,
though when glazed and hardened at the surface by freezing it had no
such influence.”[59]
In a very clear and able letter, addressed to the President of the
Board of Trade in 1863,[60] Dr. Robinson, of Armagh, thus summarizes
our knowledge of fog-signals: “Nearly all that is known about
fog-signals is to be found in the ‘Report on Lights and Beacons’; and
of it much is little better than conjecture. Its substance is as
follows:
“‘Light is scarcely available for this purpose. Blue lights are used in
the Hooghly; but it is not stated at what distance they are visible in
fog; their glare may be seen further than their flame.[61] It might,
however, be desirable to ascertain how far the electric light, or its
flash, can be traced.[62]
“‘Sound is the only known means really effective; but about it
testimonies are conflicting, and there is scarcely one fact relating to
its use as a signal which can be considered as established. Even the
most important of all, the distance at which it ceases to be heard, is
undecided.
“‘Up to the present time all signal-sounds have been made in air,
though this medium has grave disadvantages: its own currents interfere
with the sound-waves, so that a gun or bell which is heard several
miles _down_ the wind is inaudible more than a few furlongs _up_ it. A
still greater evil is that it is least effective when most needed; for
fog is a powerful damper of sound.’”
Dr. Robinson here expresses the universally-prevalent opinion, and he
then assigns the theoretic cause. “Fog,” he says, “is a mixture of air
and globules of water, and at each of the innumerable surfaces where
these two touch, a portion of the vibration is reflected and lost.[63]
... Snow produces a similar effect, and one still more injurious.”
Reflection being thus considered to take place at the surfaces of
the suspended particles, it followed that the greater the number of
particles, or, in other words, the denser the fog, the more injurious
would be its action upon sound. Hence optic transparency came to be
considered a measure of acoustic transparency. On this point Dr.
Robinson, in the letter referred to, expresses himself thus: “At the
outset, it is obvious that, to make experiments _comparable_, we
must have some measure of the fog’s power of stopping sound, without
attending to which the most anomalous results may be expected. It
seems probable that this will bear some simple relation to its opacity
to light, and that the distance at which a given object, as a flag
or pole, disappears may be taken as the measure.” “Still, clear air”
was regarded in this letter as the best vehicle of sound, the alleged
action of fogs, rain, and snow being ascribed to their rendering the
atmosphere “a discontinuous medium.”
Prior to the investigation now to be described, the views here
enunciated were those universally entertained. That sound is unable
to penetrate fogs was taken to be “a matter of common observation.”
The bells and horns of ships were affirmed “not to be heard so far
in fogs as in clear weather.” In the fogs of London the noise of the
carriage-wheels was reported to be so much diminished that “they seem
to be at a distance where really close by.” My knowledge does not
inform me of the existence of any other source for these opinions
regarding the deadening power of fog than the paper of Derham,
published one hundred and sixty-seven years ago. In consequence of
their _à priori_ probability, his conclusions seem to have been
transmitted unquestioned from generation to generation of scientific
men.
§ 2. _Instruments and Observations_
On the 19th of May, 1873, this inquiry began. The South Foreland,
near Dover, was chosen as the signal-station, steam-power having
been already established there to work two powerful magneto-electro
lights. The observations for the most part were made afloat, one of
the yachts of the Trinity Corporation being usually employed for this
purpose. Two stations had been established, the one at the top, the
other at the bottom, of the South Foreland Cliff; and at each of them
trumpets, air-whistles, and steam-whistles of great size were mounted.
The whistles first employed were of English manufacture. To these was
afterward added a large United States whistle, and also a Canadian
whistle, of great reputed power.
On the 8th of October another instrument, which has played a specially
important part in these observations, was introduced. This was a
steam-siren, constructed and patented by Mr. Brown of New York, and
introduced by Prof. Henry into the lighthouse system of the United
States. As an example of international courtesy worthy of imitation, I
refer with pleasure to the fact that when informed by Major Elliot of
the United States Army that our experiments had begun, the Lighthouse
Board at Washington, of their own spontaneous kindness, forwarded to
us for trial a very noble instrument of this description, which was
immediately mounted at the South Foreland.
In the steam-siren, as in the ordinary one, described in Chapter II., a
fixed disk and a rotating disk are employed, but radial slits are used
instead of circular apertures. One disk is fixed vertically across the
throat of a conical trumpet 16-1/2 feet long, 5 inches in diameter
where the disk crosses it, and gradually opening out till at the other
extremity it reaches a diameter of 2 feet 3 inches. Behind the fixed
disk is the rotating one, which is driven by separate mechanism. The
trumpet is connected with a boiler. In our experiments steam of 70
lbs. pressure was for the most part employed. Just as in the ordinary
siren, when the radial slits of the two disks coincide, and then only,
a strong puff of steam escapes. Sound-waves of great intensity are
thus sent through the air, the pitch of the note depending on the
velocity of rotation. (A drawing of the steam-siren constitutes our
frontispiece.)
To the siren, trumpets, and whistles were added three guns—an
18-pounder, a 5-1/2-inch howitzer, and a 13-inch mortar. In our summer
experiments all three were fired; but the howitzer having shown itself
superior to the other guns it was chosen in our autumn experiments
as not only a fair but a favorable representative of this form of
signal. The charges fired were for the most part those now employed
at Holyhead, Lundy Island, and the Kish light-vessel; namely, 3 lbs.
of powder. Gongs and bells were not included in this inquiry, because
previous observations had clearly proved their inferiority to the
trumpets and whistles.
On the 19th of May the instruments tested were:
On the top of the cliff:
_a._ Two brass trumpets or horns, 11 feet 2 inches long, 2 inches in
diameter at the mouth-piece, and opening out at the other end to a
diameter of 22-1/2 inches. They were provided with vibrating steel
reeds 9 inches long, 2 inches wide, and 1/4 inch thick, and were
sounded by air of 18 lbs. pressure.
_b._ A whistle, shaped like that of a locomotive, 6 inches in diameter,
also sounded by air of 18 lbs. pressure.
_c._ A steam-whistle, 12 inches in diameter, attached to a boiler, and
sounded by steam of 64 lbs. pressure.
At the bottom of the cliff:
_d._ Two trumpets or horns, of the same size and arrangement as those
above, and sounded by air of the same pressure. They were mounted
vertically on the reservoir of compressed air; but within about two
feet of their extremities they were bent at a right angle, so as to
present their mouths to the sea.
_e._ A 6-inch air-whistle, similar to the one above, and sounded by the
same means.
The upper instruments were 235 feet above high-water mark, the lower
ones 40 feet. A vertical distance of 195 feet, therefore, separated the
instruments. A shaft, provided with a series of twelve ladders, led
from the one to the other.
Numerous comparative experiments made at the outset gave a slight
advantage to the upper instruments. They, therefore, were for the most
part employed throughout the subsequent inquiry.
Our first observations were a preliminary discipline rather than an
organized effort at discovery. On May 19th the maximum distance reached
by the sound was about three and a half miles.[64] The wind, however,
was high and the sea rough, so that local noises interfered to some
extent with our appreciation of the sound.
Mariners express the strength of the wind by a series of numbers
extending from 0 = calm to 12 = a hurricane, a little practice in
common producing a remarkable unanimity between different observers as
regards the force of the wind. Its force on May 19th was 6, and it blew
at right angles to the direction of the sound.
The same instruments on May 20th covered a greater range of sound;
but not much greater, though the disturbance due to local noises was
absent. At 4 miles’ distance in the axes of the horns they were barely
heard, the air at the time being calm, the sea smooth, and all other
circumstances exactly those which have been hitherto regarded as most
favorable to the transmission of sound. We crept a little further
away, and by stretched attention managed to hear at intervals, at a
distance of 6 miles, the faintest hum of the horns. A little further
out we again halted; but though local noises were absent, and though we
listened intently, we heard nothing.
This position, clearly beyond the range of whistles and trumpets, was
expressly chosen with the view of making what might be considered a
decisive comparative experiment between horns and guns as instruments
for fog-signalling. The distinct report of the 12 o’clock gun fired at
Dover on the 19th suggested this comparison, and through the prompt
courtesy of General Sir A. Horsford we were enabled to carry it out.
At 12.30 precisely the puff of an 18-pounder, with a 3-lb. charge, was
seen at Dover Castle, which was about a mile further off than the South
Foreland. Thirty-six seconds afterward the loud report of the gun was
heard, its complete superiority over the trumpets being thus, to all
appearance, demonstrated.
We clinched this observation by steaming out to a distance of 8-1/2
miles, where the report of a second gun was well heard by all of us.
At a distance of 10 miles the report of a third gun was heard by some,
and at 9·7 miles the report of a fourth gun was heard by all.
The result seemed perfectly decisive. Applying the law of inverse
squares, the sound of the gun at a distance of 6 miles from the
Foreland must have had more than two and a half times the intensity
of the sound of the trumpets. It would not have been rash under the
circumstances to have reported without qualification the superiority
of the gun as a fog-signal. No single experiment is, to my knowledge,
on record to prove that a sound once predominant would not be always
predominant, or that the atmosphere on different days would show
preferences to different sounds. On many subsequent occasions, however,
the sound of the horns proved distinctly superior to that of the
gun. This _selective_ power of the atmosphere revealed itself more
strikingly in our autumn experiments than in our summer ones; and it
was sometimes illustrated within a few hours of the same day: of two
sounds, for example, one might have the greatest range at 10 A.M., and
the other the greatest range at 2 P.M.
In the experiments on May 19th and 20th the superiority of the trumpets
over the whistles was decided; and indeed, with few exceptions, this
superiority was maintained throughout the inquiry. But there were
exceptions. On June 2d, for example, the whistles rose in several
instances to full equality with, and on rare occasions subsequently
even surpassed, the horns. The sounds were varied from day to day,
and various shiftings of the horns and reeds were resorted to, with a
view of bringing out their maximum power. On the date last mentioned
a single horn was sounded, two were sounded, and three were sounded
together; but the utmost range of the loudest sound, even with the
paddles stopped, did not exceed 6 miles. With the view of concentrating
their power, the axes of the horns had been pointed in the same
direction, and, unless stated to the contrary, this in all subsequent
experiments was the case.
On June 3d the three guns already referred to were permanently mounted
at the South Foreland. They were ably served by gunners from Dover
Castle.
On the same day dense clouds quite covered the firmament, some of them
particularly black and threatening, but a marked advance was observed
in the transmissive power of the air. At a distance of 6 miles the
horn-sounds were not quite quenched by the paddle-noises; at 8 miles
the whistles were heard, and the horns better heard; while at 9 miles,
with the paddles stopped, the horn-sounds alone were fairly audible.
During the day’s observations a remarkable and instructive phenomenon
was observed. Over us rapidly passed a torrential shower of rain,
which, according to Derham, is a potent damper of sound. We could,
however, notice no subsidence of intensity as the shower passed. It
is even probable that, had our minds been free from bias, we should
have noticed an augmentation of the sound, such as occurred with the
greatest distinctness on various subsequent occasions during violent
rain.
The influence of “beats” was tried on June 3d, by throwing the horns
slightly out of unison; but though the beats rendered the sound
characteristic, they did not seem to augment the range. At a distance
from the station curious fluctuations of intensity were noticed. Not
only did the different blasts vary in strength, but sudden swellings
and fallings off, even of the same blast, were observed. This was not
due to any variation on the part of the instruments, but purely to the
changes of the medium traversed by the sound. What these changes were
shall be indicated subsequently.
The range of our best horns on June 10th was 8-3/4 miles. The guns at
this distance were very feeble. That the loudness of the sound depends
on the shape of the gun was proved by the fact that thus far the
howitzer, with a 3-lb. charge, proved more effective than the other
guns.
On June 25th a gradual improvement in the transmissive power of the
air was observed from morning to evening; but at the last the maximum
range was only moderate. The fluctuations in the strength of the sound
were remarkable, sometimes sinking to inaudibility and then rising to
loudness. A similar effect, due to a similar cause, is often noticed
with church-bells. The acoustic transparency of the air was still
further augmented on the 26th: at a distance of 9-1/4 miles from the
station the whistles and horns were plainly heard against a wind with a
force of 4; while on the 25th, with a favoring wind, the maximum range
was only 6-1/2 miles. Plainly, therefore, something else than the wind
must be influential in determining the range of the sound.
On Tuesday, July 1st, observations were made on the decay of the sound
at various angular distances from the axis of the horn. As might be
expected, the sound in the axis was loudest, the decay being gradual on
both sides. In the case of the gun, however, the direction of pointing
has very little influence.
The day was acoustically clear; at a distance of 10 miles the horn
yielded a plain sound, while the American whistle seemed to surpass
the horn. Dense haze at this time quite hid the Foreland. At 10-1/2
miles occasional blasts of the horn came to us, but after a time all
sound ceased to be audible; it seemed as if the air, after having been
exceedingly transparent, had become gradually more opaque to the sound.
At 4.45 P.M. we took the master of the Varne light-ship on board the
“Irene.” He and his company had heard the sound at intervals during the
day, although he was dead to windward and distant 12-3/4 miles from the
source of sound.
Here a word of reflection on our observations may be fitly introduced.
It is, as already shown, an opinion entertained in high quarters that
the waves of sound are reflected at the limiting surfaces of the minute
particles which constitute haze and fog, the alleged waste of sound in
fog being thus explained. If, however, this be an efficient practical
cause of the stoppage of sound, and if clear calm air be, as alleged,
the best vehicle, it would be impossible to understand how to-day, in
a thick haze, the sound reached a distance of 12-3/4 miles, while on
May 20th, in a calm and hazeless atmosphere, the maximum range was only
from 5 to 6 miles. Such facts foreshadow a revolution in our notions
regarding the action of haze and fogs upon sound.
An interval of 12 hours sufficed to change in a surprising degree the
acoustic transparency of the air. On the 1st of July the sound had a
range of nearly 13 miles; on the 2d the range did not exceed 4 miles.
§ 3. _Contradictory Results_
Thus far the investigation proceeded with hardly a gleam of a principle
to connect the inconstant results. The distance reached by the sound
on the 19th of May was 3-1/2 miles; on the 20th it was 5-1/2 miles; on
the 2d of June 6 miles; on the 3d more than 9 miles; on the 10th it
was also 9 miles; on the 25th it fell to 6-1/2 miles; on the 26th it
rose again to more than 9-1/4 miles; on the 1st of July, as we have
just seen, it reached 12-3/4, whereas on the 2d the range shrunk to 4
miles. None of the meteorological agents observed could be singled out
as the cause of these fluctuations. The wind exerts an acknowledged
power over sound, but it could not account for these phenomena. On the
25th of June, for example, when the range was only 6-1/2 miles, the
wind was favorable; on the 26th, when the range exceeded 9-1/4 miles,
it was opposed to the sound. Nor could the varying optical clearness
of the atmosphere be invoked as an explanation; for on July 1st, when
the range was 12-3/4 miles, a thick haze hid the white cliffs of the
Foreland, while on many other days, when the acoustic range was not
half so great, the atmosphere was optically clear. Up to July 3d all
remained enigmatical; but on this date observations were made which
seemed to me to displace surmise and perplexity by the clearer light of
physical demonstration.
§ 4. _Solution of Contradictions_
On July 3d we first steamed to a point 2·9 miles S.W. by W. of the
signal-station. No sounds, not even the guns, were heard at this
distance. At 2 miles they were equally inaudible. But this being a
position at which the sounds, though strong in the axis of the horn,
invariably subsided, we steamed to the exact bearing from which our
observations had been made on July 1st. At 2.15 P.M., and at a distance
of 3-3/4 miles from the station, with calm, clear air and a smooth sea,
the horns and whistle (American) were sounded, but they were inaudible.
Surprised at this result, I signalled for the guns. They were all
fired, but, though the smoke seemed at hand, no sound whatever reached
us. On July 1st, in this bearing, the observed range of both horns and
guns was 10-1/2 miles, while on the bearing of the Varne light-vessel
it was nearly 13 miles. We steamed in to 3 miles, paused, and listened
with all attention; but neither horn nor whistle was heard. The guns
were again signalled for; five of them were fired in succession, but
not one of them was heard. We steamed on in the same bearing to 2
miles, and had the guns fired pointblank at us. The howitzer and the
mortar, with 3-lb. charges, yielded a feeble thud, while the 18-pounder
was wholly unheard. Applying the law of inverse squares, it follows
that, with the air and sea, according to accepted notions, in a far
worse condition, the sound at 2 miles’ distance on July 1st must have
had more than forty times the intensity which it possessed at the same
distance at 3 P.M. on the 3d.
“On smooth water,” says Sir John Herschel, “sound is propagated with
remarkable clearness and strength.” Here was the condition; still,
with the Foreland so close to us, the sea so smooth, and the air
so transparent, it was difficult to realize that the guns had been
fired or the trumpets blown at all. What could be the reason? Had
the sound been converted by internal friction into heat? or had
it been wasted in partial reflections at the limiting surfaces of
non-homogeneous masses of air? I ventured, two or three years ago, to
say something regarding the function of the Imagination in Science,
and, notwithstanding the care then taken, to define and illustrate
its real province, some persons, among whom were one or two able men,
deemed me loose and illogical. They misunderstood me. The faculty to
which I referred was that power of visualizing processes in space,
and the relations of space itself, which must be possessed by all
great physicists and geometers. Looking, for example, at two pieces
of polished steel, we have not a sense, or the rudiment of a sense,
to distinguish the inner condition of the one from that of the other.
And yet they may differ materially, for one may be a magnet, the other
not. What enabled Ampère to surround the atoms of such a magnet with
channels in which electric currents ceaselessly run, and to deduce from
these pictured currents all the phenomena of ordinary magnetism? What
enabled Faraday to visualize his lines of force, and make his mental
picture a guide to discoveries which have rendered his name immortal?
Assuredly it was the disciplined imagination. Figure the observers on
the deck of the “Irene,” with the invisible air stretching between
them and the South Foreland, knowing that it contained something which
stifled the sound, but not knowing what that something is. Their senses
are not of the least use to them; nor could all the philosophical
instruments in the world render them any assistance. They could not, in
fact, take a single step toward the solution without the formation of a
mental image—in other words, without the exercise of the imagination.
Sulphur, in homogeneous crystals, is exceedingly transparent to radiant
heat, whereas the ordinary brimstone of commerce is highly impervious
to it—the reason being that the brimstone does not possess the
molecular continuity of the crystal, but is a mere aggregate of minute
grains not in perfect optical contact with each other. Where this is
the case, a portion of the heat is always reflected on entering and on
quitting a grain; hence, when the grains are minute and numerous, this
reflection is so often repeated that the heat is entirely wasted before
it can plunge to any depth into the substance. The same remark applies
to snow, foam, clouds, and common salt, indeed, to all transparent
substances in powder; they are all impervious to light, not through the
immediate absorption or extinction of the light, but through repeated
internal reflection.
Humboldt, in his observations at the Falls of the Orinoco, is known to
have applied these principles to sound. He found the noise of the falls
far louder by night than by day, though in that region the night is far
noisier than the day. The plain between him and the falls consisted
of spaces of grass and rock intermingled. In the heat of the day he
found the temperature of the rock to be considerably higher than that
of the grass. Over every heated rock, he concluded, rose a column of
air rarefied by the heat; its place being supplied by the descent of
heavier air. He ascribed the deadening of the sound to the reflections
which it endured at the limiting surfaces of the rarer and denser
air. This philosophical explanation made it generally known that a
non-homogeneous atmosphere is unfavorable to the transmission of sound.
But what on July 3d, not with the variously-heated plain of Antures,
but with a calm sea as a basis for the atmosphere, could so destroy its
homogeneity as to enable it to quench in so short a distance so vast a
body of sound? My course of thought at the time was thus determined:
As I stood upon the deck of the “Irene” pondering the question, I
became conscious of the exceeding power of the sun beating against
my back and heating the objects near me. Beams of equal power were
falling on the sea, and must have produced copious evaporation. That
the vapor generated should so rise and mingle with the air as to form
an absolutely homogeneous medium, was in the highest degree improbable.
It would be sure, I thought, to rise in invisible streams, breaking
through the superincumbent air now at one point, now at another, thus
rendering the air _flocculent_ with wreaths and striæ, charged in
different degrees with the buoyant vapor. At the limiting surfaces of
these spaces, though invisible, we should have the conditions necessary
to the production of partial echoes and the consequent waste of sound.
Ascending and descending air-currents, of different temperatures, as
far as they existed, would also contribute to the effect.
Curiously enough, the conditions necessary for the testing of this
explanation immediately set in. At 3.15 P.M. a solitary cloud threw
itself athwart the sun, and shaded the entire space between us and the
South Foreland. The heating of the water and the production of vapor-
and air-currents were checked by the interposition of this screen;
hence the probability of suddenly-improved transmission. To test
this inference, the steamer was immediately turned and urged back to
our last position of inaudibility. The sounds, as I expected, were
distinctly though faintly heard. This was at 3 miles’ distance. At
3-3/4 miles, the guns were fired, both pointblank and elevated. The
faintest pop was all that we heard; but we did hear a pop, whereas
we had previously heard nothing, either here or three-quarters of a
mile nearer. We steamed out to 4-1/4 miles, where the sounds were
for a moment faintly heard; but they fell away as we waited; and
though the greatest quietness reigned on board, and though the sea
was without a ripple, we could hear nothing. We could plainly see the
steam-puffs which announced the beginning and the end of a series of
trumpet-blasts, but the blasts themselves were quite inaudible.
It was now 4 P.M., and my intention at first was to halt at this
distance, which was beyond the sound-range, but not far beyond it,
and see whether the lowering of the sun would not restore the power
of the atmosphere to transmit the sound. But after waiting a little
the anchoring of a boat was suggested, so as to liberate the steamer
for other work; and though loth to lose the anticipated revival of the
sounds myself, I agreed to this arrangement. Two men were placed in the
boat and requested to give all attention, so as to hear the sound if
possible. With perfect stillness around them they heard nothing. They
were then instructed to hoist a signal if they should hear the sounds,
and to keep it hoisted as long as the sounds continued.
At 4.45 we quitted them and steamed toward the South Sand Head
light-ship. Precisely 15 minutes after we had separated from them the
flag was hoisted; the sound had at length succeeded in piercing the
body of air between the boat and the shore.
We continued our journey to the light-ship, went on board, heard the
report of the lightsmen, and returned to our anchored boat. We then
learned that when the flag was hoisted the horn-sounds were heard, that
they were succeeded after a little time by the whistle-sounds, and that
both increased in intensity as the evening advanced. On our arrival, of
course, we heard the sounds ourselves.
We pushed the test further by steaming further out. At 5-3/4 miles we
halted and heard the sounds: at 6 miles we heard them distinctly, but
so feebly that we thought we had reached the limit of the sound-range;
but while we waited the sounds rose in power. We steamed to the Varne
buoy, which is 7-3/4 miles from the signal-station, and heard the
sounds there better than at 6 miles’ distance. We continued our course
outward to 10 miles, halted there for a brief interval, but heard
nothing.
Steaming, however, on to the Varne light-ship, which is situated at
the other end of the Varne shoal, we hailed the master, and were
informed by him that up to 5 P.M. nothing had been heard, but that at
that hour the sounds began to be audible. He described one of them as
“very gross, resembling the bellowing of a bull,” which very accurately
characterizes the sound of the large American steam-whistle. At the
Varne light-ship, therefore, the sounds had been heard toward the close
of the day; though it is 12-3/4 miles from the signal-station. I think
it probable that, at a point 2 miles from the Foreland, the sound at 5
P.M. possessed fifty times the intensity which it possessed at 2 P.M.
To such undreamed-of fluctuations is the atmosphere liable. On our
return to Dover Bay, at 10 P.M., we heard the sounds, not only distinct
but loud, where nothing could be heard in the morning.
§ 5. _Other Remarkable Instances of Acoustic Opacity_
In his excellent lecture entitled “Wirkungen aus der Ferne,” Dove has
collected some striking cases of the interception of sound. The Duke of
Argyll has also favored me with some highly-interesting illustrations;
but nothing of this description that I have read equals in point of
interest the following account of the battle of Gaines’s Farm, for
which I am indebted to the Rector of the University of Virginia:
“LYNCHBURG, VIRGINIA, _March 19, 1874_.
“SIR—I have just read with great interest your lecture of
January 16th, on the acoustic transparency and opacity of the
atmosphere. The remarkable observations you mention induce me
to state to you a fact which I have occasionally mentioned,
but always, where I am not well known, with the apprehension
that my veracity would be questioned. It made a strong
impression on me at the time, but was an insoluble mystery
until your discourse gave me a possible solution.
“On the afternoon of June 28, 1862, I rode, in company
with General G. W. Randolph, then Secretary of War of the
Confederate States, to Price’s house, about nine miles from
Richmond; the evening before General Lee had begun his attack
on McClellan’s army, by crossing the Chickahominy about four
miles above Price’s, and driving in McClellan’s right wing.
The battle of Gaines’s Farm was fought the afternoon to
which I refer. The valley of the Chickahominy is about one
mile and a half wide from hilltop to hilltop. Price’s is on
one hilltop, that nearest to Richmond; Gaines’s farm, just
opposite, is on the other, reaching back in a plateau to Cold
Harbor.
“Looking across the valley I saw a good deal of the battle,
Lee’s right resting in the valley, the Federal left wing the
same. My line of vision was nearly in the line of the lines of
battle. I saw the advance of the Confederates, their repulse
two or three times, and in the gray of the evening the final
retreat of the Federal forces.
“I distinctly saw the musket-fire of both lines, the smoke,
individual discharges, the flash of the guns. I saw batteries
of artillery on both sides come into action and fire rapidly.
Several field-batteries on each side were plainly in sight.
Many more were hid by the timber which bounded the range of
vision.
“Yet looking for nearly two hours, from about 5 to 7 P.M. on
a midsummer afternoon, at a battle in which at least 50,000
men were actually engaged, and doubtless at least 100 pieces
of field-artillery, through an atmosphere optically as limpid
as possible, _not a single sound of the battle_ was audible to
General Randolph and myself. I remarked it to him at the time
as astonishing.
“Between me and the battle was the deep broad valley of the
Chickahominy, partly a swamp shaded from the declining sun by
the hills and forest in the west (my side). Part of the valley
on each side of the swamp was cleared; some in cultivation,
some not. Here were conditions capable of providing several
belts of air, varying in the amount of watery vapor (and
probably in temperature), arranged like laminæ at right angles
to the acoustic waves as they came from the battlefield to me.
“Respectfully,
“Your obedient servant,
“R. G. H. KEAN.
“PROF. JOHN TYNDALL.”
I learn from a subsequent letter that during the battle the air was
still.—J. T.
§ 6. _Echoes from Invisible Acoustic Clouds_
But both the argument and the phenomena have a complementary side,
which we have now to consider. A stratum of air less than 3 miles thick
on a calm day has been proved competent to stifle both the cannonade
and the horn-sounds employed at the South Foreland; while, according
to the foregoing explanation, this result was due to the reflection of
the sound from invisible _acoustic clouds_ which filled the atmosphere
on a day of perfect _optical_ transparency. But, granting this, it is
incredible that so great a body of sound could utterly disappear in so
short a distance without rendering some account of itself. Supposing,
then, instead of placing ourselves behind the acoustic cloud, we were
to place ourselves in front of it, might we not, in accordance with the
law of conservation, expect to receive by reflection the sound which
had failed to reach us by transmission? The case would then be strictly
analogous to the reflection of light from an ordinary cloud to an
observer between it and the sun.
My first care in the early part of the day in question was to assure
myself that our inability to hear the sound did not arise from any
derangement of the instruments on shore. Accompanied by the private
secretary of the Deputy Master of the Trinity House, at 1 P.M. I was
rowed to the shore, and landed at the base of the South Foreland
Cliff. The body of air which had already shown such extraordinary
power to intercept the sound, and which manifested this power still
more impressively later in the day, was now in front of us. On it
the sonorous waves impinged, and from it they were sent back with
astonishing intensity. The instruments, hidden from view, were on the
summit of a cliff 235 feet above us, the sea was smooth and clear of
ships, the atmosphere was without a cloud, and there was no object
in sight which could possibly produce the observed effect. From the
perfectly transparent air the echoes came, at first with a strength
apparently little less than that of the direct sound, and then dying
away. A remark made by my talented companion in his notebook at the
time shows how the phenomenon affected him: “Beyond saying that the
echoes seemed to come from the expanse of ocean, it did not appear
possible to indicate any more definite point of reflection.” Indeed
no such point was to be seen; the echoes reached us, as if by magic,
from the invisible acoustic clouds with which the optically transparent
atmosphere was filled. The existence of such clouds in all weathers,
whether optically cloudy or serene, is one of the most important points
established by this inquiry.
Here, in my opinion, we have the key to many of the mysteries and
discrepancies of evidence which beset this question. The foregoing
observations show that there is no need to doubt either the veracity
or the ability of the conflicting witnesses, for the variations of
the atmosphere are more than sufficient to account for theirs. The
mistake, indeed, hitherto has been, not in reporting incorrectly, but
in neglecting the monotonous operation of repeating the observations
during a sufficient time. I shall have occasion to remark subsequently
on the mischief likely to arise from giving instructions to mariners
founded on observations of this incomplete character.
It required, however, long pondering and repeated observation before
this conclusion took firm root in my mind; for it was opposed to
the results of great observers, and to the statements of celebrated
writers. In science as elsewhere, a mind of any depth which accepts
a doctrine undoubtingly, discards it unwillingly. The question of
aërial echoes has a historic interest. While cloud-echoes have been
accepted as demonstrated by observation, it has been hitherto held as
established that audible echoes never occur in optically clear air. We
owe this opinion to the admirable report of Arago on the experiments
made to determine the velocity of sound at Montlhéry and Villejuif in
1822.[65] Arago’s account of the phenomenon observed by him and his
colleagues is as follows: “Before ending this note we will only add
that the shots fired at Montlhéry were accompanied by a rumbling like
that of thunder, which lasted from 20 to 25 seconds. Nothing of this
kind occurred at Villejuif. Once we heard two distinct reports, a
second apart, of the Montlhéry cannon. In two other cases the report
of the same gun was followed by a prolonged rumbling. These phenomena
never occurred without clouds. Under a clear sky the sounds were
single and instantaneous. May we not, therefore, conclude that the
multiple reports of the Montlhéry gun heard at Villejuif were echoes
from the clouds, and may we not accept this fact as favorable to the
explanation given by certain physicists of the rolling of thunder?”
This explanation of the Montlhéry echoes is an inference from
observations made at Villejuif. The inference requires qualification.
Some hundreds of cannon-shots have been fired at the South Foreland,
many of them when the heavens were completely free from clouds, and
never in a single case has a _roulement_ similar to that noticed at
Montlhéry been absent. It follows, moreover, so hot upon the direct
sound as to present hardly a sensible breach of continuity between the
sound and the echo. This could not be the case if the clouds were its
origin. A reflecting cloud, at the distance of a mile; would leave
a silent interval of nearly ten seconds between sound and echo; and
had such an interval been observed at Montlhéry, it could hardly have
escaped record by the philosophers stationed there; but they have not
recorded it.
I think both the fact and the inference need reconsideration. For
our observations prove to demonstration that air of perfect visual
transparency is competent to produce echoes of great intensity and
long duration. The subject is worthy of additional illustration. On
the 8th of October, as already stated, the siren was established at
the South Foreland. I visited the station on that day, and listened
to its echoes. They were far more powerful than those of the horn.
Like the others, they were perfectly continuous, and faded, as if into
distance, gradually away. The direct sound seemed rendered complex and
multitudinous by its echoes, which resembled a band of trumpeters,
first responding close at hand, and then retreating rapidly toward the
coast of France. The siren echoes on that day had 11 seconds’, those
of the horn 8 seconds’, duration.
In the case of the siren, moreover, the reinforcement of the direct
sound by its echo was distinct. About a second after the commencement
of the siren-blast the echo struck in as a new sound. This first echo,
therefore, must have been flung back by a body of air not more than 600
or 700 feet in thickness. The few detached clouds visible at the time
were many miles away, and could clearly have had nothing to do with the
effect.
On the 10th of October I was again at the Foreland listening to the
echoes, with results similar to those just described. On the 15th I had
an opportunity of remarking something new concerning them at Dungeness,
where a horn similar to, but not so powerful as, those at the South
Foreland, has been mounted. It rotates automatically through an arc of
210°, halting at four different points on the arc and emitting a blast
of 6 seconds’ duration, these blasts being separated from each other by
intervals of silence of 20 seconds.
The new point observed was this: as the horn rotated the echoes were
always returned along the line in which the axis of the horn pointed.
Standing either behind or in front of the lighthouse tower, or closing
the eyes so as to exclude all knowledge of the position of the horn,
the direction of its axis when sounded could always be inferred from
the direction in which the aërial echoes reached the shore. Not only,
therefore, is knowledge of _direction_ given by a sound, but it may
also be given by the aërial echoes of the sound.
On the 17th of October, at about 5 P.M., the air being perfectly free
from clouds, we rowed toward the Foreland, landed, and passed over the
seaweed to the base of the cliff. As I reached the base the position of
the “Galatea” was such that an echo of astonishing intensity was sent
back from her side; it came as if from an independent source of sound
established on board the steamer. This echo ceased suddenly, leaving
the aërial echoes to die gradually into silence.
At the base of the cliff a series of concurrent observations made the
duration of the aërial siren-echoes from 13 to 14 seconds.
Lying on the shingle under a projecting roof of chalk, the somewhat
enfeebled diffracted sound reached me, and I was able to hear
with great distinctness, about a second after the starting of the
siren-blast, the echoes striking in and reinforcing the direct sound.
The first rush of echoed sound was very powerful, and it came, as
usual, from a stratum of air 600 or 700 feet in thickness. On again
testing the duration of the echoes, it was found to be from 14 to 15
seconds. The perfect clearness of the afternoon caused me to choose
it for the examination of the echoes. It is worth remarking that this
was our day of longest echoes, and it was also our day of greatest
acoustic transparency, this association suggesting that the duration
of the echo is a measure of the atmospheric _depths_ from which it
comes. On no day, it is to be remembered, was the atmosphere free from
invisible acoustic clouds; and on this day, and when their presence
did not prevent the direct sound from reaching to a distance of 15 or
16 nautical miles, they were able to send us echoes of 15 seconds’
duration.
On various occasions, when fully three miles from the shore, the
Foreland bearing north, we have had the distinct echoes of the siren
sent back to us from the cloudless _southern_ air.
To sum up this question of aërial echoes. The siren sounded three
blasts a minute, each of 5 seconds’ duration. From the number of days
and the number of hours per day during which the instrument was in
action we can infer the number of blasts. They reached nearly twenty
thousand. The blasts of the horns exceeded this number, while hundreds
of shots were fired from the guns. Whatever might be the state of the
weather, cloudy or serene, stormy or calm, the aërial echoes, though
varying in strength and duration from day to day, were never absent;
and on many days, “under a perfectly clear sky,” they reached, in the
case of the siren, an astonishing intensity. It is doubtless to these
air-echoes, and not to cloud-echoes, that the rolling of thunder is to
be ascribed.
§ 7. _Experimental Demonstration of Reflection from Gases_
Thus far we have dealt in inference merely, for the interception
of sound through aërial reflection has never been experimentally
demonstrated; and, indeed, according to Arago’s observation, which
has hitherto held undisputed possession of the scientific field, it
does not sensibly exist. But the strength of science consists in
verification, and I was anxious to submit the question of aërial
reflection to an experimental test. The knowledge gained in the last
lecture enables us to apply such a test; but, as in most similar cases,
it was not the simplest combinations that were first adopted. Two gases
of different densities were to be chosen, and I chose carbonic acid and
coal-gas. With the aid of my skillful assistant, Mr. John Cottrell,
a tunnel was formed, across which five-and-twenty layers of carbonic
acid were permitted to fall, and five-and-twenty alternate layers of
coal-gas to rise. Sound was sent through this tunnel, making fifty
passages from medium to medium in its course. These, I thought, would
waste in aërial echoes a sensible portion of sound.
To indicate this waste an objective test was found in one of the
sensitive flames described in the last chapter. Acquainted with it, we
are prepared to understand a drawing and description of the apparatus
first employed in the demonstration of aërial reflection. The following
clear account of the apparatus was given by a writer in “Nature,”
February 5, 1874:
“A tunnel _t t′_ (Fig. 146), 2 inches square, 4 feet 8 inches long,
open at both ends, and having a glass front, runs through the
box _a b c d_. The spaces above and below are divided into cells
opening into the tunnel by transverse orifices exactly corresponding
vertically. Each alternate cell of the upper series—the 1st, 3d,
5th, etc.—communicates by a bent tube (_e e e_) with a common upper
reservoir (_g_), its counterpart cell in the lower series having a
free outlet into the air. In like manner the 2d, 4th, 6th, etc., of
the lower series of cells are connected by bent tubes (_n n n_) with
the lower reservoir (_i_), each having its direct passage into the
air through the cell immediately above it. The gas-distributors (_g_
and _i_) are filled from both ends at the same time, the upper with
carbonic-acid gas, the lower with coal-gas, by branches from their
respective supply-pipes (_f_ and _h_). A well-padded box (P) open to
the end of the tunnel forms a little cavern, whence the sound-waves are
sent forth by an electric bell (dotted in the figure). A few feet
from the other end of the tunnel, and in a direct line with it, is a
sensitive flame (_k_), provided with a funnel as sound-collector, and
guarded from chance currents by a shade.
[Illustration: FIG. 146.]
“The bell was set ringing. The flame, with quick response to each
blow of the hammer, emitted a sort of musical roar, shortening and
lengthening as the successive sound-pulses reached it. The gases were
then admitted. Twenty-five flat jets of coal-gas ascended from the
tubes below, and twenty-five cascades of carbonic acid fell from the
tubes above. That which was a homogeneous medium had now fifty limiting
surfaces, from each of which a portion of the sound was thrown back.
In a few moments these successive reflections became so effective that
no sound having sufficient power to affect the flame could pierce the
clear, optically-transparent, but acoustically-opaque, atmosphere in
the tunnel. So long as the gases continued to flow the flame remained
perfectly tranquil. When the supply was cut off, the gases rapidly
diffused into the air. The atmosphere of the tunnel became again
homogeneous, and therefore acoustically transparent, and the flame
responded to each sound-pulse as before.”
Not only do gases of different densities act thus upon sound, but
atmospheric air in layers of different temperatures does the same.
Across a tunnel resembling _t t′_, Fig. 146, sixty-six platinum wires
were stretched, all of them being in metallic connection. The bell, in
its padded box, was placed at one end of the tunnel, and the sensitive
flame _k_, near its flaring-point, at the other. When the bell rang
the flame flared. A current from a strong voltaic battery being sent
through the platinum wires, they became heated: layers of warm air rose
from them through the tunnel, and immediately the agitation of the
flame was stilled. On stopping the current, the agitation recommenced.
In this experiment the platinum wires had not reached a red heat.
Employing half the number and the same battery, they were raised to
a red heat, the action in this case upon the sound-waves being also
energetic. Employing one-third of the number of wires, and the same
strength of battery, the wires were raised to a white heat. Here also
the flame was immediately rendered tranquil by the stoppage of the
sound.
§ 8. _Reflection from Vapors_
But not only do gases of different densities, and air of different
temperatures, act thus upon sound, but air saturated, in different
degrees, with the vapors of volatile liquids can be shown by experiment
to produce the same effect. Into the path pursued by the carbonic acid
in our first experiment a flask, which I have frequently employed to
charge air with vapor, was introduced. Through a volatile liquid,
partially filling the flask, air was forced into the tunnel _t t_′,
which was thus divided into spaces of air saturated with the vapor, and
other spaces in their ordinary condition. The action of such a medium
upon the sound-waves issuing from the bell is very energetic, instantly
reducing the violently-agitated flame to stillness and steadiness.
The removal of the heterogeneous medium instantly restores the noisy
flaring of the flame.
A few illustrations of the action of non-homogeneous atmospheres,
produced by the saturation of layers of air with the vapors of volatile
liquids, may follow here:
_Bisulphide of Carbon._—Flame very sensitive, and noisily responsive to
the sound. The action of the non-homogeneous atmosphere was prompt and
strong, stilling the agitated flame.
_Chloroform._—Flame still very sensitive; action similar to the last.
_Iodide of Methyl._—Action prompt and energetic.
_Amylene._—Very fine action; a short and violently-agitated flame was
immediately rendered tall and quiescent.
_Sulphuric Ether._—Action prompt and energetic.
The vapor of water at ordinary temperatures is so small in quantity and
so attenuated that it requires special precautions to bring out its
action. But with such precautions it was found competent to reduce to
quiescence the sensitive flame.
As the skill and knowledge of the experimenter augment he is often
able to simplify his experimental combinations. Thus, in the present
instance, by the suitable arrangement of the source of sound and the
sensitive flame, it was found that not only twenty-five layers, but
three or four layers of coal-gas and carbonic acid sufficed to still
the agitated flame. Nay, with improved manipulation, the action of a
single layer of either gas was rendered perfectly sensible. So also as
regards heated layers of air, not only were sixty-six or twenty-two
heated platinum wires found sufficient, but the heated air from two or
three candle-flames, or even from a single flame, or a heated poker,
was found perfectly competent to stop the flame’s agitation. The same
remark applies to vapors. Three or four heated layers of air, saturated
with the vapor of a volatile liquid, stilled the flame; and, by
improved manipulation, the action of a single saturated layer could be
rendered sensible. In all these cases, moreover, a small, high-pitched
reed might be substituted for the bell.
My assistant has devised the simple apparatus sketched in Fig. 147, for
showing reflection by gases, vapors, and heated air. At the end A of
the square pipe A B is a small vibrating reed of high pitch, the sound
of which violently agitates the sensitive flame _f_. To the horizontal
tube _g g′_ are attached four small burners, and above them four
chimneys, through which the heated gases from the flames can ascend
into A B. When the coverings of the chimneys are removed and the gas is
ignited, the air within A B is rendered rapidly non-homogeneous, and
immediately stills the agitated flame.
[Illustration: FIG. 147.]
The pipe A B may be turned upside down, an orifice seen between A and
B fitting on to the stand which supports the tube. The conduit _t_
leads into a shallow rectangular box, which communicates by a series of
transverse apertures with A B. When air, saturated with the vapor of a
volatile liquid, is forced through these apertures, the atmosphere in
A B is immediately rendered heterogeneous, the agitated flame being as
rapidly stilled.
[Illustration: FIG. 148.]
In the experiments at the South Foreland, not only was it proved that
the acoustic clouds stopped the sound; but, in the proper position, the
sounds which had been refused transmission were received by reflection.
I wished very much to render this echoed sound evident experimentally;
and stated to my assistant that we ought to be able to accomplish this.
Mr. Cottrell met my desire by the following beautiful experiment, which
has been thus described before the Royal Society:
A vibrating reed B (Fig. 148) was placed so as to send sound-waves
through a tin tube, 38 inches long, and 1-3/4 inch diameter, in the
direction B A, the action of the sound being rendered manifest by its
causing a sensitive flame placed at F′ to become violently agitated.
“The invisible heated layer immediately above the luminous portion
of an ignited coal-gas flame issuing from an ordinary bat’s-wing
burner was allowed to stream upward across the end A of the tin tube.
A portion of the sound issuing from the tube was reflected at the
limiting surfaces of the heated layer; the part transmitted being now
only competent to slightly agitate the sensitive flame at F′.
“The heated layer was then placed at such an angle that the reflected
portion of the sound was sent through a second tin tube, A F (of the
same dimensions as B A). Its action was rendered visible by causing a
second sensitive flame placed at the end of the tube at F to become
violently affected. This _echo_ continued active as long as the heated
layer intervened; but upon its withdrawal the sensitive flame placed
at F′, receiving the whole of the direct pulse, became again violently
agitated, and at the same moment the sensitive flame at F, ceasing to
be affected by the echo, resumed its former tranquillity.
“Exactly the same action takes place when the luminous portion of a
gas-flame is made the reflecting layer; but in the experiments above
described the invisible layer above the flame only was used. By proper
adjustment of the pressure of the gas the flame at F′ can be rendered
so moderately sensitive to the direct sound-wave that the portion
transmitted through the reflecting layer shall be incompetent to affect
the flame. Then by the introduction and withdrawal of the bat’s-wing
flame the two sensitive flames can be rendered alternately quiescent
and strongly agitated.
“An illustration is here afforded of the perfect analogy between
light and sound; for if a beam of light be projected from B to F′,
and a plate of glass be introduced at A in the exact position of the
reflecting layer of gas, the beam will be divided, one portion being
reflected in the direction A F, and the other portion transmitted
through the glass toward F′, exactly as the sound-wave is divided into
a reflected and transmitted portion by the layer of heated gas or
flame.”
Thus far, therefore, we have placed our subject in the firm grasp of
experiment; nor shall we find this test failing us further on.
_PART II_
INVESTIGATION OF THE CAUSES WHICH HAVE HITHERTO BEEN SUPPOSED EFFECTIVE
IN PREVENTING THE TRANSMISSION OF SOUND THROUGH THE ATMOSPHERE
Action of Hail and Rain—Action of Snow—Action of Fog;
Observations in London—Experiments on Artificial
Fogs—Observations on Fogs at the South Foreland—Action of
Wind—Atmospheric Selection—Influence of Sound-Shadow
§ 1. _Action of Hail and Rain_
In the first part of this chapter it was demonstrated that the optic
transparency and acoustic transparency of our atmosphere were by no
means necessarily coincident; that on days of marvellous optical
clearness the atmosphere may be filled with impervious acoustic clouds,
while days optically turbid may be acoustically clear. We have now
to consider, in detail, the influence of various agents which have
hitherto been considered potent in reference to the transmission of
sound through the atmosphere.
Derham, and after him all other writers, considered that falling rain
tended powerfully to obstruct sound. An observation on June 3d has
been already referred to as tending to throw doubt on this conclusion.
Two other crucial instances will suffice to show its untenability. On
the morning of October 8th, at 7.45 A.M., a thunderstorm accompanied by
heavy rain broke over Dover. But the clouds subsequently cleared away,
and the sun shone strongly on the sea. For a time the optical clearness
of the atmosphere was extraordinary, but it was acoustically opaque.
At 2.30 P.M. a densely-black scowl again overspread the heavens to the
W.S.W. The distance being 6 miles, and all hushed on board, the horn
was heard very feebly, the siren more distinctly, while the howitzer
was better than either, though not much superior to the siren.
A squall approached us from the west. In the Alps or elsewhere I have
rarely seen the heavens blacker. Vast cumuli floated to the N.E. and
S.E.; vast streamers of rain descended in the W.N.W.; huge scrolls of
cloud hung in the N.; but spaces of blue were to be seen to the N.N.E.
At 7 miles’ distance the siren and horn were both feeble, while the gun
sent us a very faint report. A dense shower now enveloped the Foreland.
The rain at length reached us, falling heavily all the way between
us and the Foreland; but the sound, instead of being deadened, rose
perceptibly in power. Hail was now added to the rain, and the shower
reached a tropical violence, the hailstones floating thickly on the
flooded deck. In the midst of this furious squall both the horns and
the siren were distinctly heard; and as the shower lightened, thus
lessening the local pattering, the sounds so rose in power that we
heard them at a distance of 7-1/2 miles distinctly louder than they
had been heard through the rainless atmosphere at 5 miles.
At 4 P.M. the rain had ceased and the sun shone clearly through the
calm air. At 9 miles’ distance the horn was heard feebly, the siren
clearly, while the howitzer sent us a loud report. All the sounds were
better heard at this distance than they had previously been at 5-1/2
miles; from which, by the law of inverse squares, it follows that
the intensity of the sound at 5-1/2 miles’ distance must have been
augmented at least threefold by the descent of the rain.
On the 23d of October our steamer had forsaken us for shelter, and I
sought to turn the weather to account by making other observations on
both sides of the fog-signal station. Mr. Douglass, the chief engineer
of the Trinity House, was good enough to undertake the observations
N.E. of the Foreland; while Mr. Ayers, the assistant engineer, walked
in the other direction. At 12.50 P.M. the wind blew a gale, and broke
into a thunderstorm with violent rain. Inside and outside the Cornhill
Coast-guard Station, a mile from the instruments in the direction of
Dover, Mr. Ayers heard the sound of the siren through the storm; and
after the rain had ceased, all sounds were heard distinctly louder
than before. Mr. Douglass had sent a fly before him to Kingsdown, and
the driver had been waiting for fifteen minutes before he arrived.
During this time no sound had been heard, though 40 blasts had
been blown in the interval; nor had the coast-guard man on duty, a
practiced observer, heard any of them throughout the day. During the
thunderstorm, and while the rain was actually falling with a violence
which Mr. Douglass describes as perfectly torrential, the sounds
became audible and were heard by all.
To rain, in short, I have never been able to trace the slightest
deadening influence upon sound. The reputed barrier offered by “thick
weather” to the passage of sound was one of the causes which tended to
produce hesitation in establishing sound-signals on our coasts. It is
to be hoped that the removal of this error may redound to the advantage
of coming generations of seafaring men.
§ 2. _Action of Snow_
Falling snow, according to Derham, is the most serious obstacle of all
to the transmission of sound. We did not extend our observations at the
South Foreland into snowy weather; but a previous observation of my own
bears directly upon this point. On Christmas night, 1859, I arrived at
Chamouni, through snow so deep as to obliterate the road-fences, and to
render the labor of reaching the village arduous in the extreme. On the
26th and 27th it fell heavily. On the 27th, during a lull in the storm,
I reached the Montavert, sometimes breast deep in snow. On the 28th,
with great difficulty, two lines of stakes were set out across the
glacier, with the view of determining its winter motion. On the 29th
the entry in my journal, written in the morning, is: “Snow, heavy snow;
it must have descended through the entire night, the quantity freshly
fallen is so great.”
Under these circumstances I planted my theodolite beside the Mer de
Glace, having waded to my position through snow, which, being dry,
reached nearly to my breast. Assistants were sent across the glacier
with instructions to measure the displacement of a transverse line of
stakes planted previously in the snow. A storm drifted up the valley,
darkening the air as it approached. It reached us, the snow falling
more heavily than I had ever seen it elsewhere. It soon formed a heap
on the theodolite, and thickly covered my own clothes. Here, then,
was a combination of snow in the air, and of soft fresh snow on the
ground, such as Derham could hardly have enjoyed; still through such
an atmosphere I was able to make my instructions audible quite across
the glacier, the distance being half a mile, while the experiment was
rendered reciprocal by one of my assistants making his voice audible to
me.
§ 3. _Passage of Sound through Textile Fabrics, and through Artificial
Showers_
The flakes here were so thick that it was only at intervals that I
was able to pick up the retreating forms of the men. Still the air
through which the flakes fell was continuous. Did the flakes merely
yield passively to the sonorous waves, swinging like the particles
of air themselves to and fro as the sound-waves passed them? Or did
the waves bend by diffraction round the flakes, and emerge from them
without sensible loss? Experiment will aid us here by showing the
astonishing facility with which sound makes its way among obstacles,
and passes through tissues, so long as the continuity of the air in
their interstices is preserved.
A piece of millboard or of glass, a plank of wood, or the hand, placed
across the open end _t′_ of the tunnel _a b c d_, Fig. 146 (page 334),
intercepts the sound of the bell, placed in the padded box P, and
stills the sensitive flame _k_.
An ordinary cambric pocket-handkerchief, on the other hand, placed
across the tunnel-end produced hardly an appreciable effect upon the
sound. Through two layers of the handkerchief the flame was strongly
agitated; through four layers it was still agitated; while through six
layers, though nearly stilled, it was not entirely so.
Dipping the same handkerchief into water, and stretching a single
wetted layer across the tunnel-end, it stilled the flame as effectually
as the millboard or the wood. Hence the conclusion that the sound-waves
in the first instance passed through the interstices of the cambric.
Through a single layer of thin silk the sound passed without sensible
interruption; through six layers the flame was strongly agitated; while
through twelve layers the agitation was quite perceptible.
A single layer of this silk, when wetted, stilled the flame.
A layer of soft lint produced but little effect upon the sound; a layer
of thick flannel was almost equally ineffectual. Through four layers of
flannel the flame was perceptibly agitated. Through a single layer of
green baize the sound passed almost as freely as through air; through
four layers of the baize the action was still sensible. Through a layer
of close hard felt, half an inch thick, the sound-waves passed with
sufficient energy to sensibly agitate the flame. Through 200 layers of
cotton-net the sound passed freely. I did not witness these effects
without astonishment.
A single layer of thin oiled silk stopped the sound and stilled the
flame. A leaf of common note-paper, or a five-pound note, also stopped
the sound.
The sensitive flame is not absolutely necessary to these experiments.
Let a ticking watch be hung six inches from the ear, a cambric
handkerchief dropped between it and the ear scarcely sensibly affects
the ticking; a sheet of oil-skin or an intensely heated gas-column cuts
it almost wholly off.
But though oiled silk, foreign post, or a banknote, can stop the
sound, a film sufficiently thin to yield freely to the aërial pulses
transmits it. A thick soap-film produces an obvious effect upon the
sensitive flame; a very thin one does not. The augmentation of the
transmitted sound may be observed simultaneously with the generation
and brightening of the colors which indicate the increasing thinness of
the film. A very thin collodion-film acts in the same way.
Acquainted with the foregoing facts regarding the passage of sound
through cambric, silk, lint, flannel, baize, felt, and cotton-net,
you are prepared for the statement that the sound-waves pass without
sensible impediment through heavy artificial showers of rain, hail,
and snow. Water-drops, seeds, sand, bran, and flocculi of various
kinds, have been employed to form such showers; through all of these,
as through the actual rain and hail already described, and through the
snow on the Mer de Glace, the sound passes without sensible obstruction.
§ 4. _Action of Fog. Observations in London_
But the mariner’s greatest enemy, fog, is still to be dealt with;
and here for a long time the proper conditions of experiment were
absent. Up to the end of November we had had frequent days of haze,
sufficiently thick to obscure the white cliffs of the Foreland, but
no real fog. Still those cases furnished demonstrative evidence that
the notions entertained regarding the reflection of sound by suspended
particles were wrong; for on many days of the thickest haze the sound
covered twice the range attained on other days of perfect optical
transparency. Such instances dissolved the association hitherto assumed
to exist between acoustic transparency and optic transparency, but they
left the action of dense fogs undetermined.
On December 9th a memorable fog settled down on London. I addressed a
telegram to the Trinity House suggesting some gun observations. With
characteristic promptness came the reply that they would be made in the
afternoon at Blackwall. I went to Greenwich in the hope of hearing the
guns across the river; but the delay of the train by the fog rendered
my arrival too late. Over the river the fog was very dense, and through
it came various sounds with great distinctness. The signal-bell of an
unseen barge rang clearly out at intervals, and I could plainly hear
the hammering at Cubitt’s Town, half a mile away, on the opposite side
of the river. No deadening of the sound by the fog was apparent.
Through this fog and various local noises, Captain Atkins and Mr.
Edwards heard the report of a 12-pounder carronade with a 1-lb. charge
distinctly better than the 18-pounder with a 3-lb. charge, an optically
clear atmosphere, and all noises absent, on July 3d.
Anxious to turn to the best account a phenomenon for which we had
waited so long, I tried to grapple with the problem by experiments on
a small scale. On the 10th, I stationed my assistant with a whistle
and organ-pipe on the walk below the southwest end of the bridge
dividing Hyde Park from Kensington Gardens. From the eastern end of
the Serpentine I heard distinctly both the whistle and the pipe, which
produced 380 waves a second. On changing places with my assistant, I
heard for a time the distinct blast of the whistle only. The deeper
note of the organ-pipe at length reached me, rising sometimes to great
distinctness, and sometimes falling to inaudibility. The whistle showed
the same intermittence as to period, but in an opposite sense; for,
when the whistle was faint, the pipe was strong, and _vice versa_. To
obtain the fundamental note of the pipe, it had to be blown gently, and
on the whole the whistle proved the most efficient in piercing the fog.
An extraordinary amount of sound filled the air during these
experiments. The resonant roar of the Bayswater and Knightsbridge
roads; the clangor of the great bell of Westminster; the
railway-whistles, which were frequently blown, and the fog-signals
exploded at the various metropolitan stations, were all heard with
extraordinary intensity. This could by no means be reconciled with the
statements so categorically made regarding the acoustic impenetrability
of a London fog.
On the 11th of December, the fog being denser than before, I heard
every blast of the whistle, and occasional blasts of the pipe, over
the distance between the bridge and the eastern end of the Serpentine.
On joining my assistant at the bridge, the loud concussion of a gun
was heard by both of us. A police-inspector affirmed that it came
from Woolwich, and that he had heard several shots about 2 P.M. and
previously. The fact, if a fact, was of the highest importance; so I
immediately telegraphed to Woolwich for information. Prof. Abel kindly
furnished me with the following particulars:
“The firing took place at 1.40 P.M. The guns proved were of
comparatively small size—64-pounders, with 10-lb. charges of powder.
“The concussion experienced at my house and office, about
three-quarters of a mile from the butt, was decidedly more severe than
that experienced when the heaviest guns are proved with charges of
110 to 120 lbs. of powder. There was a dense fog here at the time of
firing.”
These were the guns heard by the police-inspector; on subsequent
inquiry it was ascertained that two guns were fired about 3 P.M. These
were the guns heard by myself.
Prof. Abel also communicated to me the following fact: “Our workman’s
bell at the Arsenal Gate, which is of moderate size and anything but
clear in tone, is pretty distinctly heard by Prof. Bloxam _only_ when
the wind is _northeast_. During the whole of last week the bell was
heard with great distinctness, the wind being _southwesterly_ (opposed
to the sound). The distance of the bell from Bloxam’s house is about
three-quarters of a mile as the crow flies.”
Assuredly no question of science ever stood so much in need of revision
as this of the transmission of sound through the atmosphere. Slowly,
but surely, we mastered the question; and the further we advanced, the
more plainly it appeared that our reputed knowledge regarding it was
erroneous from beginning to end.
On the morning of the 12th the fog attained its maximum density. It
was not possible to read at my window, which fronted the open western
sky. At 10.30 I sent an assistant to the bridge, and listened for his
whistle and pipe at the eastern end of the Serpentine. The whistle
rose to a shrillness far surpassing anything previously heard, but it
sank sometimes almost to inaudibility; proving that, though the air
was on the whole highly homogeneous, acoustic clouds still drifted
through the fog. A second pipe, which was quite inaudible yesterday,
was plainly heard this morning. We were able to discourse across the
Serpentine to-day with much greater ease than yesterday.
During our summer observations I had once or twice been able to fix the
position of the Foreland in thick haze by the direction of the sound.
To-day my assistant, hidden by the fog, walked up to the Watermen’s
Boathouse sounding his whistle; and I walked along the opposite side of
the Serpentine, clearly appreciating for a time that the line joining
us was oblique to the axis of the river. Coming to a point which seemed
to be exactly abreast of him, I marked it; and on the following day,
when the fog had cleared away, the marked position was found to be
perfectly exact. When undisturbed by echoes, the ear, with a little
practice, becomes capable of fixing with great precision the direction
of a sound.
On reaching the Serpentine this morning, a peal of bells, which then
began to ring, seemed so close at hand that it required some reflection
to convince me that they were ringing to the north of Hyde Park. The
sounds fluctuated wonderfully in power. Prior to the striking of
eleven by the great bell of Westminster, a nearer bell struck with
loud clangor. The first five strokes of the Westminster bell were
afterward heard, one of them being extremely loud; but the last six
strokes were inaudible. An assistant was stationed to attend to the 12
o’clock bells. The clock which had struck so loudly at 11 was unheard
at 12, while of the Westminster bell eight strokes out of twelve were
inaudible. To such astonishing changes is the atmosphere liable.
At 7 P.M. the Westminster bell, striking seven, was not at all heard
from the Serpentine, while the nearer bell already alluded to was heard
distinctly. The fog had cleared away, and the lamps on the bridge
could be seen from the eastern end of the Serpentine burning brightly;
but, instead of the sound sharing the improvement of the light, what
might be properly called an acoustic fog took the place of its optical
predecessor. Several series of the whistle and organ-pipe were sounded
in succession; one series only of the whistle-sounds was heard, all
the others being quite inaudible. Three series of the organ-pipe were
heard, but very faintly. On reversing the positions and sounding as
before, nothing whatever was heard.
At 8 o’clock the chimes and hour-bell of the Westminster clock were
both very loud. The “acoustic fog” had shifted its position, or
temporarily melted away.
Extraordinary fluctuations were also observed in the case of the
church-bells heard in the morning: in a few seconds they would sink
from a loudly ringing-peal into utter silence, from which they would
rapidly return to loud-tongued audibility. The intermittent drifting of
fog over the sun’s disk (by which his light is at times obscured, at
times revealed) is the optical analogue of these effects. As regards
such changes, the acoustic deportment of the atmosphere is a true
transcript of its optical deportment.
At 9 P.M. three strokes only of the Westminster clock were heard; the
others were inaudible. The air had relapsed in part into its condition
at 7 P.M., when all the strokes were unheard. The quiet of the park
this evening, as contrasted with the resonant roar which filled the air
on the two preceding days, was very remarkable. The sound, in fact, was
stifled in the optically clear but acoustically flocculent atmosphere.
On the 13th, the fog being displaced by thin haze, I went again to
the Serpentine. The carriage-sounds were damped to an extraordinary
degree. The roar of the Knightsbridge and Bayswater roads had subsided,
the tread of troops which passed us a little way off was unheard,
while at 11 A.M. both the chimes and the hour-bell of the Westminster
clock were stifled. Subjectively considered, all was favorable to
auditory impressions; but the very cause that damped the local noises
extinguished our experimental sounds. The voice across the Serpentine
to-day, with my assistant plainly visible in front of me, was
distinctly feebler than it had been when each of us was hidden from the
other in the densest fog.
Placing the source of sound at the eastern end of the Serpentine I
walked along its edge from the bridge toward the end. The distance
between these two points is about 1,000 paces. After 500 of them had
been stepped, the sound was not so distinct as it had been at the
bridge on the day of densest fog; hence, by the law of inverse squares,
the optical cleansing of the air through the melting away of the fog
had so darkened it acoustically that a sound generated at the eastern
end of the Serpentine was lowered to one-fourth of its intensity at a
point midway between the end and the bridge.
To these demonstrative observations one or two subsequent ones may
be added. On several of the moist and warm days, at the beginning of
1874, I stood at noon beside the railing of St. James’s Park, near
Buckingham Palace, three-quarters of a mile from the clock-tower, which
was clearly visible. Not a single stroke of “Big Ben” was heard. On
January 19th fog and drizzling rain obscured the tower; still from the
same position I not only heard the strokes of the great bell, but also
the chimes of the quarter-bells.
During the exceedingly dense and “dripping” fog of January 22d, from
the same railings, I heard every stroke of the bell. At the end of the
Serpentine, when the fog was densest, the Westminster bell was heard
striking loudly eleven. Toward evening this fog began to melt away,
and at 6 o’clock I went to the end of the Serpentine to observe the
effect of the optical clearing upon the sound. Not one of the strokes
reached me. At 9 o’clock and at 10 o’clock my assistant was in the same
position, and on both occasions he failed to hear a single stroke of
the bell. It was a case precisely similar to that of December 13th,
when the dissolution of the fog was accompanied by a decided acoustic
thickening of the air.[66]
§ 5. _Observations at the South Foreland_
Satisfactory, and indeed conclusive, as these results seemed, I desired
exceedingly to confirm them by experiments with the instruments
actually employed at the South Foreland. On the 10th of February I had
the gratification of receiving the following note and inclosure from
the Deputy Master of Trinity House:
“MY DEAR TYNDALL—The inclosed will show how accurately
your views have been verified, and I send them on at once
without waiting for the details. I think you will be glad
to have them, and as soon as I get the report it shall be
sent to you. I made up my mind ten days ago that there
would be a chance in the light foggy-disposed weather at
home, and therefore sent the ‘Argus’ off at an hour’s
notice, and requested the Fog Committee to keep one
member on board. On Friday I was so satisfied that the
fog would occur that I sent Edwards down to record the
observations.
“Very truly yours,
“FRED. ARROW.”
The inclosure referred to was notes from Captain Atkins and Mr.
Edwards. Captain Atkins writes thus:
“As arranged, I came down here by the mail express, meeting Mr. Edwards
at Cannon Street. We put up at the Dover Castle, and next morning at
7 I was awoke by sounds of the siren. On jumping up I discovered that
the long-looked-for fog had arrived, and that the ‘Argus’ had left her
moorings.
“However, had I been on board, the instructions I left with Troughton
(the master of the ‘Argus’) could not have been better carried out.
About noon the fog cleared up, and the ‘Argus’ returned to her
moorings, when I learned that they had taken both siren and horn
sounds to a distance of 11 miles from the station, where they dropped
a buoy. This I knew to be correct, as I have this morning recovered
the buoy, and the distances both in and out agree with Troughton’s
statement. I have also been to the Varne light-ship (12-3/4 miles from
the Foreland), and ascertained that during the fog of Saturday forenoon
they ‘distinctly’ heard the sounds.”
Mr. Edwards, who was constantly at my side during our summer and autumn
observations, and who is thoroughly competent to form a comparative
estimate of the strength of the sounds, states that those of the 7th
were “extraordinarily loud,” both Captain Atkins and himself being
awoke by them. He does not remember ever before hearing the sounds so
loud in Dover; it seemed as though the observers were close to the
instruments.
Other days of fog preceded this one, and they were all days of acoustic
transparency, the day of densest fog being acoustically the clearest of
all.
The results here recorded are of the highest importance, for they bring
us face to face with a dense fog and an actual fog-signal, and confirm
in the most conclusive manner the previous observations. The fact of
Captain Atkins and Mr. Edwards being awakened by the siren proves,
beyond all our previous experience, its power during this dense fog.
It is exceedingly interesting to compare the transmission of sound on
February 7th with its transmission on October 14th. The wind on both
days had the same strength and direction. My notes of the observations
show the latter to have been throughout a day of extreme optical
clearness. The range was 10 miles. During the fog of February 7th the
“Argus” heard the sound at 11 miles; and it was also heard at the Varne
light-vessel, which is 12-3/4 miles from the Foreland.
It is also worthy of note that through the same fog the sounds were
well heard at the South Sand Head light-vessel, which is in the
opposite direction from the South Foreland, and was actually behind
the siren. For this important circumstance is to be borne in mind: on
February 7th the siren happened to be pointed, not toward the “Argus,”
but toward Dover. Had the yacht been in the axis of the instrument it
is highly probable that the sound would have been heard all the way
across to the coast of France.
It is hardly necessary for me to say a word to guard myself against the
misconception that I consider sound to be assisted by the fog itself.
The fog-particles have no more influence upon the waves of sound than
the suspended particles stirred up over the banks of Newfoundland
have upon the waves of the Atlantic. A homogeneous air is the usual
associate of fog, and hence the acoustic clearness of foggy weather.
§ 6. _Experiments on Artificial Fogs_
These observations are clinched and finished by being brought within
the range of laboratory experiment. Here we shall learn incidentally a
lesson as to the caution required from an experimenter.
The smoke from smouldering brown paper was allowed to stream upward
through its rectangular apertures, into the tunnel _a b c d_ (Fig.
146); the action upon the sound-waves was strong, rendering the short
and agitated sensitive flame _k_ tall and quiescent.
Air first passed through ammonia, then through hydrochloric acid, and,
thus loaded with thick fumes, was sent into the tunnel; the agitated
flame was rendered immediately quiescent, indicating a very decided
action on the part of the artificial fog.
Air passed through perchloride of tin and sent into the tunnel produced
exceedingly dense fumes. The action upon the sound-waves was very
strong.
The dense smoke of resin, burned before the open end of the tunnel, and
blown into it with a pair of bellows, had also the effect of stopping
the sound-waves, so as to still the agitated flame.
The conclusion seems clear, and its perfect harmony with the prevalent
_à priori_ notions as to the action of fog upon sound makes it almost
irresistible. But caution is here necessary. The smoke of the brown
paper was _hot_; the flask containing the hydrochloric acid was _hot_;
that containing the perchloride of tin was _hot_; while the resin fumes
produced by a red-hot poker were also obviously hot. Were the results,
then, due to the fumes or to the differences of temperature? The
observations might well have proved a trap to an incautious reasoner.
Instead of the smoke and heated air, the heated air alone from four
red-hot pokers was permitted to stream upward into the tunnel; the
action on the sound-waves was very decided, though the tunnel was
optically empty. The flame of a candle was placed at the upper end, and
the hot air just above its tip was blown into the tunnel; the action on
the sensitive flame was decided. A similar effect was produced when the
air, ascending from a red-hot iron, was blown into the tunnel.
In these latter cases the tunnel remained optically clear, while
the same effect as that produced by the resin, smoke, and fumes was
observed. Clearly, then, we are not entitled to ascribe, without
further investigation, to the artificial fog an effect which may have
been due to the air which accompanied it.
Having eliminated the fog and proved the non-homogeneous air effective,
our reasoning will be completed by eliminating the heat, and proving
the fog ineffective.
Instead of the tunnel _a b c d_, Fig. 146, a cupboard with glass sides,
3 feet long, 2 feet wide, and about 5 feet high, was filled with fumes
of various kinds. Here it was thought the fumes might remain long
enough for differences of temperature to disappear. Two apertures were
made in two opposite panes of glass 3 feet asunder. In front of one
aperture was placed the bell in its padded box, and behind the other
aperture, and at some distance from it, the sensitive flame.
Phosphorus placed in a cup floating on water was ignited within the
closed cupboard. The fumes were so dense that considerably less than
the three feet traversed by the sound extinguished totally a bright
candle-flame.
At first there was a slight action upon the sound; but this rapidly
vanished, the flame being no more affected than if the sound had passed
through pure air. The first action was manifestly due to differences of
temperature, and it disappeared when the temperature was equalized.
The cupboard was next filled with the dense fumes of gunpowder. At
first there was a slight action; but this disappeared even more rapidly
than in the case of the phosphorus, the sound passing as if no fumes
were there. It required less than half a minute to abolish the action
in the case of the phosphorus, but a few seconds sufficed in the case
of the gunpowder. These fumes were far more than sufficient to quench
the candle-flame.
The dense smoke of resin, when the temperature had become equable,
exerted no action on the sound.
The fumes of gum-mastic were equally ineffectual.
The fumes of the perchloride of tin, though of extraordinary density,
exerted no sensible effect upon the sound.
Exceedingly dense fumes of chloride of ammonium next filled the
cupboard. A fraction of the length of the 3-foot tube sufficed to
quench the candle-flame. Soon after the cupboard was filled, the sound
passed without the least sensible deterioration. An aperture at the top
of the cupboard was opened; but though a dense smoke-column ascended
through it, many minutes elapsed before the candle-flame could be seen
through the attenuated fog.
Steam from a copper boiler was so copiously admitted into the
cupboard as to fill it with a dense cloud. No real cloud was ever so
dense; still the sound passed through it without the least sensible
diminution. This being the case, cloud-echoes are not a likely
phenomenon.
In all of these cases, when a couple of Bunsen’s burners were ignited
within the cupboard containing the fumes, less than a minute’s action
rendered the air so heterogeneous that the sensitive flame was
completely stilled.
These acoustically inactive fogs were subsequently proved competent to
cut off the electric light.
Experiment and observation go, therefore, hand in hand in demonstrating
that fogs have no sensible action upon sound. The notion of their
impenetrability, which so powerfully retarded the introduction of
phonic coast-signals, being thus abolished, we have solid ground for
the hope that disasters due to fogs and thick weather will in the
future be materially mitigated.
§ 7. _Action of Wind_
In stormy weather we were frequently forsaken by our steamer, which had
to seek shelter in the Downs or Margate Roads, and on such occasions
the opportunity was turned to account to determine the effect of the
wind. On October 11th, accompanied by Mr. Douglass and Mr. Edwards, I
walked along the cliffs from Dover Castle toward the Foreland, the wind
blowing strongly against the sound. About a mile and a half from the
Foreland, we first heard the faint but distinct sound of the siren. The
horn-sound was inaudible. A gun fired during our halt was also unheard.
As we approached the Foreland we saw the smoke of a gun. Mr. Edwards
heard a faint crack, but neither Mr. Douglass nor I heard anything.
The sound of the siren was at the same time of piercing intensity. We
waited for ten minutes, when another gun was fired. The smoke was at
hand, and I thought I heard a faint thud, but could not be certain.
My companions heard nothing. On pacing the distance afterward we were
found to be only 550 yards from the gun. We were shaded at the time
by a slight eminence from both the siren and the gun, but this could
not account for the utter extinction of the gun-sound at so short a
distance, and at a time when the siren sent to us a note of great power.
Mr. Ayres at my request walked windward along the cliff, while Mr.
Douglass proceeded to St. Margaret’s Bay. During their absence I
had three guns fired. Mr. Ayres heard only one of them. Favored by
the wind, Mr. Douglass, at twice the distance, and far more deeply
immersed in the sound-shadow, heard all three reports with the utmost
distinctness.
Joining Mr. Douglass, we continued our walk to a distance of
three-quarters of a mile beyond St. Margaret’s Bay. Here, being dead to
leeward, though the wind blew with unabated violence, the sound of the
siren was borne to us with extraordinary power.[67] In this position
we also heard the gun loudly, and two other loud reports at the proper
interval of ten minutes, as we returned to the Foreland.
It is within the mark to say that the gun on October 11th was heard
five times, and might have been heard fifteen times, as far to leeward
as to windward.
In windy weather the shortness of its sound is a serious drawback to
the use of the gun as a signal. In the case of the horn and siren, time
is given for the attention to be fixed upon the sound; and a single
puff, while cutting out a portion of the blast, does not obliterate it
wholly. Such a puff, however, may be fatal to the momentary gun-sound.
On the leeward side of the Foreland, on the 23d of October, the sounds
were heard at least four times as far as on the windward side, while in
both directions the siren possessed the greatest penetrative power.
On the 24th the wind shifted to E.S.E., and the sounds, which, when the
wind was W.S.W., failed to reach Dover, were now heard in the streets
through thick rain. On the 27th the wind was E.N.E. In our writing-room
in the Lord Warden Hotel, in the bedrooms, and on the staircase, the
sound of the siren reached us with surprising power, piercing through
the whistling and moaning of the wind, which blew through Dover toward
Folkestone. The sounds were heard by Mr. Edwards and myself at 6 miles
from the Foreland on the Folkestone road; and had the instruments not
then ceased sounding, they might have been heard much further. At the
South Sand Head light-vessel, 3-3/4 miles on the opposite side, no
sound had been heard throughout the day. On the 28th, the wind being
N. by E., the sounds were heard in the middle of Folkestone, 8 miles
off, while in the opposite direction they failed to reach 3-3/4 miles.
On the 29th the limits of range were Eastware Bay on the one side, and
Kingsdown on the other; on the 30th the limits were Kingsdown on the
one hand, and Folkestone Pier on the other. With a wind having a force
of 4 or 5 it was a very common observation to hear the sound in one
direction three times as far as in the other.
This well-known effect of the wind is exceedingly difficult to explain.
Indeed, the only explanation worthy of the name is one offered by Prof.
Stokes, and suggested by some remarkable observations of De la Roche.
In Vol. I. of “Annales de Chimie” for 1816, p. 176, Arago introduces De
la Roche’s memoir in these words: “L’auteur arrive à des conclusions,
qui d’abord pourront paraître paradoxales, mais ceux qui savent combien
il mettait de soins et d’exactitude dans toutes ses recherches se
garderont sans doute d’opposer une opinion populaire à des expériences
positives.” The strangeness of De la Roche’s results consisted in his
establishing, by quantitative measurements, not only that sound has
a greater range in the direction of the wind than in the opposite
direction, but that the range at right angles to the wind is the
maximum.
In a short but exceedingly able communication, presented to the British
Association in 1857, the eminent physicist above mentioned points out
a cause which, _if sufficient_, would account for the results referred
to. The lower atmospheric strata are retarded by friction against
the earth, and the upper ones by those immediately below them; the
velocity of transition, therefore, in the case of wind, increases from
the ground upward. It may be proved that this difference of velocity
tilts the sound-wave upward in a direction opposed to, and downward
in a direction coincident with, the wind. In this latter case the
direct wave is reinforced by the wave reflected from the earth. Now
the reinforcement is greatest in the direction in which the direct
and reflected waves inclose the smallest angle; and this is at right
angles to the direction of the wind. Hence the greater range in this
direction. It is not, therefore, according to Prof. Stokes, a stifling
of the sound to windward, but a tilting of the sound-wave over the
heads of the observers, that defeats the propagation in that direction.
This explanation calls for verification, and I wished much to test it
by means of a captive balloon rising high enough to catch the deflected
wave; but on communicating with Mr. Coxwell, who has earned for himself
so high a reputation as an aeronaut, and who has always shown himself
so willing to promote a scientific object, I learned with regret that
the experiment was too dangerous to be carried out.[68]
§ 8. _Atmospheric Selection_
It has been stated that the atmosphere on different days shows
preferences to different sounds. This point is worthy of further
illustration.
After the violent shower which passed over us on October 18th, the
sounds of all the instruments, as already stated, rose in power; but
it was noticed that the horn-sound, which was of lower pitch than
that of the siren, improved most, at times not only equalling, but
surpassing, the sound of its rival. From this it might be inferred that
the atmospheric change produced by the rain favored more especially the
transmission of the longer sonorous waves.
But our programme enabled us to go further than mere inference. It
had been arranged on the day mentioned that up to 3.30 P.M. the siren
should perform 2,400 revolutions a minute, generating 480 waves a
second. As long as this rate continued, the horn, after the shower, had
the advantage. The rate of rotation was then changed to 2,000 a minute,
or 400 waves a second, when the siren-sound immediately surpassed that
of the horn. A clear connection was thus established between aërial
reflection and the length of the sonorous waves.
The 10-inch Canadian whistle being capable of adjustment so as to
produce sounds of different pitch, on the 10th of October I ran through
a series of its sounds. The shrillest appeared to possess great
intensity and penetrative power. The belief is common that a note of
this character (which affects so powerfully, and even painfully, an
observer close at hand) has also the greatest range. Mr. A. Gordon,
in his examination before the Committee on Lighthouses, in 1845,
expressed himself thus: “When you get a shrill sound, high in the
scale, that sound is carried much further than a lower note in the
scale.” I have heard the same opinion expressed by other scientific men.
On the 14th of October the point was submitted to an experimental test.
It had been arranged that up to 11.30 A.M. the Canadian whistle, which
had been heard with such piercing intensity on the 10th, should sound
its shrillest note. At the hour just mentioned we were beside the Varne
buoy, 7-3/4 miles from the Foreland. The siren, as we approached the
buoy, was heard through the paddle-noises; the horns were also heard,
but more feebly than the siren. We paused at the buoy and listened for
the 11.30 gun. Its boom was heard by all. Neither before nor during
the pause was the shrill-sounding Canadian whistle once heard. At the
appointed time it was adjusted to produce its ordinary low-pitched
note, which was immediately heard. Further out the low boom of the
cannon continued audible after all the other sounds had ceased.
But it was only during the early part of the day that this preference
for the longer wave was manifested. At 3 P.M. the case was completely
altered, for then the high-pitched siren was heard when all the other
sounds were inaudible. On many other days we had illustrations of the
varying comparative power of the siren and the gun. On the 9th of
October sometimes the one, sometimes the other, was predominant. On the
morning of the 13th the siren was clearly heard on Shakespeare’s Cliff,
while two guns with their puffs perfectly visible were unheard. On
October 16th, 2 miles from the signal-station, the gun at 11 o’clock
was inferior to the siren, but both were heard. At 12.30, the distance
being 6 miles, the gun was quite unheard, while the siren continued
faintly audible. Later on in the day the experiment was twice repeated.
The puff of the gun was in each case seen, but nothing was heard. In
the last experiment, when the gun was quenched, the siren sent forth a
sound so strong as to maintain itself through the paddle-noises. The
day was clearly hostile to the passage of the longer sonorous waves.
October 17th began with a preference for the shorter waves. At 11.30
A.M. the mastery of the siren over the gun was pronounced; at 12.30
the gun slightly surpassed the siren; at 1, 2, and 2.30 P.M. the gun
also asserted its mastery. This preference for the longer waves was
continued on October 18th. On October 20th the day began in favor
of the gun, then both became equal, and finally the siren gained
the mastery; but the day had become stormy, and a storm is always
unfavorable to the momentary gun-sound. The same remark applies to
the experiments of October 21st. At 11 A.M., distance 6-1/2 miles,
when the siren made itself heard through the noises of wind, sea,
and paddles, the gun was fired; but, though listened for with all
attention, no sound was heard. Half an hour later the result was the
same. On October 24th five observers saw the flash of the gun at a
distance of 5 miles, but heard nothing; all of them at this distance
heard the siren distinctly; a second experiment on the same day yielded
the same result. On the 27th also the siren was triumphant; and on
three distinct occasions on the 29th its mastery over the gun was very
decided.
Such experiments yield new conceptions as to the scattering of sound
in the atmosphere. No sound here employed is a simple sound; in every
case the fundamental note is accompanied by others, and the action
of the atmosphere on these different groups of waves has its optical
analogue in that scattering of the waves of the luminiferous ether
which produces the various shades and colors of the sky.
§ 9. _Concluding Remarks_
A few additional remarks and suggestions will fitly wind up this
chapter. It has been proved that in some states of the weather the
howitzer firing a 3-lb. charge commands a larger range than the
whistles, trumpets, or siren. This was the case, for example, on the
particular day, October 17th, when the ranges of all the sounds reached
their maximum.
On many other days, however, the inferiority of the gun to the siren
was demonstrated in the clearest manner. The gun-puffs were seen with
the utmost distinctness at the Foreland, but no sound was heard, the
note of the siren at the same time reaching us with distinct and
considerable power.
The disadvantages of the gun are these:
_a._ The duration of the sound is so short that, unless the observer is
prepared beforehand, the sound, through lack of attention rather than
through its own powerlessness, is liable to be unheard.
_b._ Its liability to be quenched by a local sound is so great that it
is sometimes obliterated by a puff of wind taking possession of the
ears at the time of its arrival. This point was alluded to by Arago,
in his report on the celebrated experiments of 1822. By such a puff a
momentary gap is produced in the case of a continuous sound, but not
entire extinction.
_c._ Its liability to be quenched or deflected by an opposing wind, so
as to be practically useless at a very short distance to windward, is
very remarkable. A case has been cited in which the gun failed to be
heard against a violent wind at a distance of 550 yards from the place
of firing, the sound of the siren at the same time reaching us with
great intensity.
Still, notwithstanding these drawbacks, I think the gun is entitled to
rank as a first-class signal. I have had occasion myself to observe its
extreme utility at Holyhead and the Kish light-vessel near Kingstown.
The commanders of the Holyhead boats, moreover, are unanimous in their
commendation of the gun. An important addition in its favor is the fact
that in a fog the flash or glare often comes to the aid of the sound.
On this point, the evidence is quite conclusive.
There may be cases in which the combination of the gun with one of
the other signals may be desirable. Where it is wished to confer an
unmistakable individuality on a fog-signal station, such a combination
might with advantage be resorted to.
If the gun be retained as one form of fog-signal (and I should be sorry
at present to recommend its total abolition), it ought to be of the
most suitable description. Our experiments prove the sound of the gun
to be dependent on its shape; but we do not know that we have employed
the best shape. This suggests the desirability of constructing a gun
with special reference to the production of sound.[69]
An absolutely uniform superiority on all days cannot be conceded
to any one of the instruments subjected to examination; still, our
observations have been so numerous and long-continued as to enable us
to come to the sure conclusion, that, on the whole, the steam-siren is
the most powerful fog-signal which has hitherto been tried in England.
It is specially powerful when local noises, such as those of wind,
rigging, breaking waves, shore-surf, and the rattle of pebbles, have to
be overcome. Its density, quality, pitch, and penetration, render it
dominant over such noises after all other signal-sounds have succumbed.
I have not, therefore, hesitated to recommend the introduction of the
siren as a coast-signal.
It will be desirable in each case to confer upon the instrument a
power of rotation, so as to enable the person in charge of it to point
its trumpet against the wind or in any other required direction.
This arrangement was made at the South Foreland, and it presents no
mechanical difficulty. It is also desirable to mount the siren, so as
to permit of the depression of its trumpet fifteen or twenty degrees
below the horizon.
In selecting the position at which a fog-signal is to be mounted, the
possible influence of a sound-shadow, and the possible extinction of
the sound by the interference of the direct waves with waves reflected
from the shore, must form the subject of the gravest consideration.
Preliminary trials may, in most cases, be necessary before fixing on
the precise point at which the instrument is to be placed.
The siren which has been long known to scientific men is worked with
air; and it would be worth while to try how the fog-siren would behave
supposing compressed air to be substituted for steam. Compressed air
might also be tried with the whistles.
No fog-signal hitherto tried is able to fulfil the condition laid
down in a very able letter already referred to, namely, “_that all
fog-signals should be distinctly audible for at least 4 miles, under
every circumstance_.” Circumstances may exist to prevent the most
powerful sound from being heard at half this distance. What may with
certainty be affirmed is, that in almost all cases the siren may
certainly be relied on at a distance of 2 miles; in the great majority
of cases it may be relied upon at a distance of 3 miles, and in the
majority of cases to a distance greater than 3 miles.
Happily the experiments thus far made are perfectly concurrent in
indicating that at the particular time when fog-signals are needed, the
air holding the fog in suspension is in a highly-homogeneous condition;
hence it is in the highest degree probable that in the case of fog we
may rely upon the signals being effective at far greater distances than
those just mentioned.
I am cautious not to inspire the mariner with a confidence which may
prove delusive. When he hears a fog-signal he ought, as a general rule
(at all events until extended experience justifies the contrary), to
assume the source of sound to be not more than 2 or 3 miles distant,
and to heave his lead or to take other necessary precautions. If he
errs at all in his estimate of distance, it ought to be on the side of
safety.
With the instruments now at our disposal wisely established along our
coasts, I venture to think that the saving of property in ten years
will be an exceedingly large multiple of the outlay necessary for the
establishment of such signals. The saving of life appeals to the higher
motives of humanity.
In a report written for the Trinity House on the subject of
fog-signals, my excellent predecessor, Prof. Faraday, expresses the
opinion that a false promise to the mariner would be worse than no
promise at all. Casting our eyes back upon the observations here
recorded, we find the sound-range on clear, calm days varying from
2-1/2 miles to 16-1/2 miles. It must be evident that an instruction
founded on the latter observation would be fraught with peril in
weather corresponding to the former. Not the maximum but the minimum
sound-range should be impressed upon the mariner. Want of attention to
this point may be followed by disastrous consequences.
This remark is not made without cause. I have before me a “Notice to
Mariners” regarding a fog-whistle recently mounted at Cape Race, which
is reputed to have a range of 20 miles in calm weather, 30 miles with
the wind, and in stormy weather or against the wind 7 to 10 miles.
Now, considering the distance reached by sound in our observations, I
should be willing to concede the possibility, in a more homogeneous
atmosphere than ours, of a sound-range on _some_ calm days of 20 miles,
and on _some_ light windy days of 30 miles, to a powerful whistle; but
I entertain a strong belief that the stating of these distances, or of
the distance 7 to 10 miles against a storm, without any qualification,
is calculated to inspire the mariner with false confidence. I would
venture to affirm that at Cape Race calm days might be found in which
the range of the sound will be less than one-fourth of what this
notice states it to be. Such publications ought to be without a trace
of exaggeration, and furnish only data on which the mariner may with
perfect confidence rely. My object in extending these observations over
so long a period was to make evident to all how fallacious it would
be, and how mischievous it might be, to draw general conclusions from
observations made in weather of great acoustic transparency.
Thus ends, for the present at all events, an inquiry which I trust
will prove of some importance, scientific as well as practical. In
conducting it I have had to congratulate myself on the unfailing aid
and co-operation of the Elder Brethren of the Trinity House. Captain
Drew, Captain Close, Captain Were, Captain Atkins, and the Deputy
Master, have all from time to time taken part in the inquiry. To the
eminent arctic navigator, Admiral Collinson, who showed throughout
unflagging and, I would add, philosophic interest in the investigation,
I am indebted for most important practical aid. He was almost always
at my side, comparing opinions with me, placing the steamer in the
required positions, and making with consummate skill and promptness
the necessary sextant observations. I am also deeply sensible of the
important services rendered by Mr. Douglass, the able and indefatigable
engineer, by Mr. Ayres, the assistant engineer, and by Mr. Price
Edwards, the private secretary of the Deputy Master of the Trinity
House.
The officers and gunners at the South Foreland also merit my best
thanks, as also Mr. Holmes and Mr. Laidlaw, who had charge of the
trumpets, whistles, and siren.
In the subsequent experimental treatment of the subject I have been
most ably aided by my excellent assistant, Mr. John Cottrell.
NOTE
In the Appendix will be found a brief paper on “Acoustic
Reversibility,” in which I offer a solution of a difficulty
encountered by the French philosophers in their experiments
on the velocity of sound in 1822. The solution is based on
the experiments and observations recorded in the foregoing
chapter.—J. T.
SUMMARY OF CHAPTER VII
The paper of Dr. Derham, published in the “Philosophical Transactions”
for 1708, has been hitherto the almost exclusive source of our
knowledge of the causes which affect the transmission of sound through
the atmosphere.
Derham found that fog obstructed sound, that rain and hail obstructed
sound, but that above all things falling snow, or a coating of fresh
snow upon the ground, tended to check the propagation of sound through
the atmosphere.
With a view to the protection of life and property at sea in the
years 1873 and 1874, this subject received an exhaustive examination,
observational and experimental. The investigation was conducted at the
expense of the Government and under the auspices of the Elder Brethren
of the Trinity House.
The most conflicting results were at first obtained. On the 19th of
May, 1873, the sound range was 3-1/3 miles; on the 20th it was 5-1/2
miles; on the 2d of June, 6 miles; on the 3d, more than 9 miles; on the
10th, 9 miles; on the 25th, 6 miles; on the 26th, 9-1/4 miles; on the
1st of July, 12-3/4 miles; on the 2d, 4 miles; while on the 3d, with a
clear calm atmosphere and smooth sea, it was less than 3 miles.
These discrepancies were proved to be due to a state of the air which
bears the same relation to sound that cloudiness does to light. By
streams of air differently heated, or saturated in different degrees
with aqueous vapors, the atmosphere is rendered _flocculent_ to sound.
_Acoustic clouds_, in fact, are incessantly floating or flying through
the air. They have nothing whatever to do with ordinary clouds, fogs,
or haze. The most transparent atmosphere may be filled with them;
converting days of extraordinary optical transparency into days of
equally extraordinary acoustic opacity.
The connection hitherto supposed to exist between a clear atmosphere
and the transmission of sound is therefore dissolved.
The intercepted sound is wasted by repeated reflections in the acoustic
cloud, as light is wasted by repeated reflections in an ordinary cloud.
And as from the ordinary cloud the light reflected reaches the eye, so
from the perfectly invisible acoustic cloud the reflected sound reaches
the ear.
Aërial echoes of extraordinary intensity and of long duration are thus
produced. They occur, contrary to the opinion hitherto entertained, in
the clearest air.
It is to the wafting of such acoustic clouds through the atmosphere
that the fluctuations in the sounds of our public clocks and of
church-bells are due.
The existence of these aërial echoes has been proved both by
observation and experiment. They may arise either from air-currents
differently heated, or from air-currents differently saturated with
vapor.
Rain has no sensible power to obstruct sound.
Hail has no sensible power to obstruct sound.
Snow has no sensible power to obstruct sound.
Fog has no sensible power to obstruct sound.
The air associated with fog is, as a general rule, highly homogeneous
and favorable to the transmission of sound. The notions hitherto
entertained regarding the action of fog are untenable.
Experiments on artificial showers of rain, hail, and snow, and on
artificial fogs of extraordinary density, confirm the results of
observation.
As long as the air forms a continuous medium the amount of sound
scattered by small bodies suspended in it is astonishingly small.
This is illustrated by the ease with which sound traverses layers of
calico, cambric, silk, flannel, baize, and felt. It freely passes
through all these substances in thicknesses sufficient to intercept the
light of the sun.
Through six layers of thin silk, for example, it passes with little
obstruction; it finds its way through a layer of close felt half
an inch thick, and it is not wholly intercepted by 200 layers of
cotton-net.
The atmosphere exercises a selective choice upon the waves of sound
which varies from day to day, and even from hour to hour. It is
sometimes favorable to the transmission of the longer, and at other
times favorable to the transmission of the shorter, sonorous waves.
The recognized action of the wind has been confirmed by this
investigation.