distant cumuli sailed near the horizon, but the zenith and a vast
angular space all round it were absolutely free from cloud. From the
deck of the 'Galatea' a rocket was discharged, which reached a great
elevation, and exploded with a loud report. Following this solid
nucleus of sound was a continuous train of echoes, which retreated to
a continually greater distance, dying gradually off into silence after
seven seconds' duration. These echoes were of the same character as
those so frequently noticed at the South Foreland in 1872-73, and
called by me 'aerial echoes.'
On the 23rd of March the experiments were resumed, the most noteworthy
results of that day's observations being that the sounds were heard at
Tillingham, 10 miles to the N.E.; at West Mersea, 15.75 miles to the
N.E. by E; at Brightlingsea, 17.5 miles to the N.E.; and at Clacton
Wash, 20.5 miles to the N.E. by 1/2 E. The wind was blowing at the
time from the S.E. Some of these sounds were produced by rockets,
some by a 24-lb. howitzer, and some by an 8-inch Maroon.
In December, 1876, Mr. Gardiner, the managing director of the
Cotton-powder Company, had proposed a trial of this material against
the gun-cotton. The density of the cotton he urged was only 1.03,
while that of the powder was 1.70. A greater quantity of explosive
material being thus compressed into the same volume, Mr. Gardiner
thought that a greater sonorous effect must be produced by the powder.
At the instance of Mr. Mackie, who had previously gone very
thoroughly into the subject, a Committee of the Elder Brethren visited
the cotton-powder manufactory, on the banks of the Swale, near
Faversham, on the 16th of June, 1877. The weights of cotton-powder
employed were 2 oz, 8 oz, 1 lb, and 2 lbs, in the form of rockets and
of signals fired a few feet above the ground. The experiments
throughout were arranged and conducted by Mr. Mackie. Our desire on
this occasion was to get 'as near to windward as possible, but the
Swale and other obstacles limited our distance to 1.5 mile. We stood
here E.S.E. from the firing-point while the wind blew fresh from the
N.E.
The cotton-powder yielded a very effective report. The rockets in
general had a slight advantage over the same quantities of material
fired near the ground. The loudness of the sound was by no means
proportional to the quantity of the material exploded, 8 oz. yielding
very nearly as loud a report as 1 lb. The 'aerial echoes,' which
invariably followed the explosion of the rockets, were loud and
long-continued.
On the 17th of October, 1877, another series of experiments with
howitzers and rockets was carried out at Shoeburyness. The charge of
the howitzer was 3 lbs. of L. G. powder. The charges of the rockets
were 12 oz, 8 oz, 4 oz, and 2 oz. of gun-cotton respectively. The gun
and the four rockets constituted a series, and eight series were fired
during the afternoon of the 17th. The observations were made from the
'Vestal' and the 'Galatea,' positions being successively assumed which
permitted the sound to reach the observers with the Wind, against the
wind, and across the wind. The distance of the 'Galatea' varied from
3 to 7 miles, that of the 'Vestal,' which was more restricted in her
movements, being 2 to 3 miles. Briefly summed up, the result is that
the howitzer, firing a 3-lb. charge, which it will be remembered was
our best gun at 'the South Foreland, was beaten by the 12-oz. rocket,
by the 8-oz. rocket, and by the 4-oz. rocket. The 2-oz. rocket alone
fell behind the howitzer.
It is worth while recording the distances at which some of the sounds
were heard on the day now referred to:
1. Leigh 6.5 miles W.N.W. 24 out of 40 sounds heard.
2. Girdler 12 miles S.E. by E. 5 out of 40 sounds heard.
Light-vessel
3. Reculvers 171 miles S.E. by S. 18 out of 40 sounds heard.
4. St. Nicholas 20 miles S.E. 3 out of 40 sounds heard.
5. Epple Bay 22 miles S.E. by E. 19 out of 40 sounds heard.
6. Westgate 23 miles S.E. by E. 9 out of 40 sounds heard.
7. Kingsgate 25 miles S.E. by E. 8 out of 40 sounds heard.
The day was cloudy, with occasional showers of drizzling rain; the
wind about N.W. by N. all day; at times squally, rising to a force of
6 or 7 and sometimes dropping to a force of 2 or 3. The station at
Leigh excepted, all these places were to leeward of Shoeburyness. At
four other stations to leeward, varying in distance from 15.5 to 24.5
miles, nothing was heard, while at eleven stations to windward,
varying from 8 to 26 miles, the sounds were also inaudible. It was
found, indeed, that the sounds proceeding directly against the wind
did not penetrate much beyond 3 miles.
On the following day, viz. the 18th October, we proceeded to Dungeness
with the view of making a series of strict comparative experiments
with gun-cotton and cotton-powder. Rockets containing 8 oz, 4 oz, and
2 oz. of gun-cotton had been prepared at the Royal Arsenal; while
others, containing similar quantities of cotton-powder, had been
supplied by the Cotton-powder Company at Faversham. With these were
compared the ordinary 18-pounder gun, which happened to be mounted at
Dungeness, firing the usual charge of 3 lbs. of powder, and a syren.
From these experiments it appeared that the guncotton and
cotton-powder were practically equal as producers of sound.
The effectiveness of small charges was illustrated in a very striking
manner, only a single unit separating the numerical value of the 8-oz.
rocket from that of the 2-oz. rocket. The former was recorded as 6.9
and the latter as 5.9, the value of the 4-oz. rocket being
intermediate between them. These results were recorded by a number of
very practised observers on board the 'Galatea.' They were completely
borne out by the observations of the Coastguard, who marked the value
of the 8-oz rocket 6-1, and that of the 2-oz. rocket 5.2. The
18-pounder gun fell far behind all the rockets, a result, possibly, to
be in part ascribed to the imperfection of the powder. The
performance of the syren was, on the whole, less satisfactory than
that of the rocket. The instrument was worked, not by steam of 70
lbs. pressure, as at the South Foreland, but by compressed air,
beginning with 40 lbs. and ending with 30 lbs. pressure. The trumpet
was pointed to windward, and in the axis of the instrument the sound
was about as effective as that of the 8-oz. rocket. But in a
direction at right angles to the axis, and still more in the rear of
this direction, the syren fell very sensibly behind even the 2-oz.
rocket.
These are the principal comparative trials made between the gun-cotton
rocket and other fog-signals; but they are not the only ones. On the
2nd of August, 1877, for example, experiments were made at Lundy
Island with the following results. At 2 miles distant from the
firing-point, with land intervening, the 18-pounder, firing a 3-lb.
charge, was quite unheard. Both the 4-oz. rocket and the 8-oz.
rocket, however, reached an elevation which commanded the acoustic
shadow, and yielded loud reports. When both were in view the rockets
were still superior to the gun. On the 6th of August, at St. Ann's,
the 4-oz. and 8-oz. rockets proved superior to the syren. On the
Shambles Light-vessel, when a pressure of 13 lbs. was employed to
sound the syren, the rockets proved greatly superior to that
instrument. Proceeding along the sea margin at Flamboro' Head, Mr.
Edwards states that at a distance of 1.25 mile, with the 18-pounder
previously used as a fog-signal hidden behind the cliffs, its report
was quite unheard, while the 4-oz. rocket, rising to an elevation
which brought it clearly into view, yielded a powerful sound in the
face of an opposing wind.
On the evening of February 9th, 1877, a remarkable series of
experiments were made by Mr. Prentice at Stowmarket with the
gun-cotton rocket. From the report with which he has kindly furnished
me I extract the following particulars. The first column in the
annexed statement contains the name of the place of observation, the
second its distance from the firing-point, and the third the result
observed:
Stoke Hill, Ipswich 10 miles Rockets clearly seen and sounds
distinctly heard 53 seconds after the flash.
Melton 15 miles Signals distinctly heard. Thought at
first that sounds were reverberated
from the sea.
Framlingham 18 miles Signals very distinctly heard, both in
the open air and in a closed room.
Wind in favour of sound.
Stratford. 19 miles St. Andrews Reports loud; startled
pheasants in a cover close by.
Tuddenham. 10 miles St. Martin Reports very loud; rolled
away like thunder.
Christ Church Park. 11 miles Report arrived a little more than a
minute after flash.
Nettlestead Hall 6 miles Distinct in every part of observer's
house. Very loud in the open air.
Bildestone 6 miles Explosion very loud, wind against sound.
Nacton 14 miles Reports quite distinct--mistaken by
inhabitants for claps of thunder.
Aldboro' 25 miles Rockets seen through a very hazy
atmosphere; a rumbling detonation heard.
Capel Mills 11 miles Reports heard within and without the
observer's house. Wind opposed to sound.
Lawford 15.5 miles Reports distinct: attributed to
distant thunder.
In the great majority of these cases, the direction of the sound
enclosed a large angle with the direction of the wind. In some cases,
indeed, the two directions were at right angles to each other. It is
needless to dwell for a moment on the advantage of possessing a signal
commanding ranges such as these.
The explosion of substances in the air, after having been carried to a
considerable elevation by rockets, is a familiar performance. In
1873, moreover, the Board of Trade proposed a light-and-sound rocket
as a signal of distress, which proposal was subsequently realized, but
in a form too elaborate and expensive for practical use. The idea of
a gun-cotton rocket fit for signalling in fogs is, I believe, wholly
due to Sir Richard Collinson, the Deputy Master of the Trinity House.
Thanks to the skilful aid given by the authorities of Woolwich, by Mr.
Prentice, and Mr. Brock, that idea is now an accomplished fact; a
signal of great power, handiness, and economy, being thus placed at
the service of our mariners. Not only may the rocket be applied in
association with lighthouses and lightships, but in the Navy also it
may be turned to important account. Soon after the loss of the
'Vanguard' I ventured to urge upon an eminent naval officer the
desirability of having an organized code of fog-signals for the fleet.
He shook his head doubtingly, and referred to the difficulty of
finding room for signal guns. The gun-cotton rocket completely
surmounts this difficulty, It is manipulated with ease and rapidity,
while its discharges may be so grouped and combined as to give a most
important extension to the voice of the admiral in command. It is
needless to add that at any point upon our coasts, or upon any other
coast, where its establishment might be desirable, a fog-signal
station might be extemporised without difficulty.
*****
I have referred more than once to the train of echoes which
accompanied the explosion of gun-cotton in free air, speaking of them
as similar in all respects to those which were described for the first
time in my Report on Fog-signals, addressed to the Corporation of
Trinity House in 1874. [Footnote: See also 'Philosophical
Transactions' for 1874, p. 183.] To these echoes I attached a
fundamental significance. There was no visible reflecting surface
from which they could come. On some days, with hardly a cloud in the
air and hardly a ripple on the sea, they reached a magical intensity.
As far as the sense of hearing could judge, they came from the body of
the air in front of the great trumpet which produced them. The
trumpet blasts were five seconds in duration, but long before the
blast had ceased the echoes struck in, adding their strength to the
primitive note of the trumpet. After the blast had ended the echoes
continued, retreating further and further from the point of
observation, and finally dying away at great distances. The echoes
were perfectly continuous as long as the sea was clear of ships,
'tapering' by imperceptible gradations into absolute silence. But
when a ship happened to throw itself athwart the course of the sound,
the echo from the broadside of the vessel was returned as a shock
which rudely interrupted the continuity of the dying atmospheric
music.
These echoes have been ascribed to reflection from the crests of the
sea-waves. But this hypothesis is negatived by the fact, that the
echoes were produced in great intensity and duration when no waves
existed--when the sea, in fact, was of glassy smoothness. It has been
also shown that the direction of the echoes depended not on that of
waves, real or assumed, but on the direction of the axis of the
trumpet. Causing that axis to traverse an arc of 210°, and the
trumpet to sound at various points of the arc, the echoes were always,
at all events in calm weather, returned from that portion of the
atmosphere towards which the trumpet was directed. They could not,
under the circumstances, come from the glassy sea; while both their
variation of direction and their perfectly continuous fall into
silence, are irreconcilable with the notion that they came from fixed
objects on the land. They came from that portion of the atmosphere
into which the trumpet poured its maximum sound, and fell in intensity
as the direct sound penetrated to greater atmospheric distances.
The day on which our latest observations were made was particularly
fine. Before reaching Dungeness, the smoothness of the sea and the
serenity of the air caused me to test the echoing power of the
atmosphere. A single ship lay about half a mile distant between us
and the land. The result of the proposed experiment was clearly
foreseen. It was this. The rocket being sent up, it exploded at a
great height; the echoes retreated in their usual fashion, becoming
less and less intense as the distances of the invisible surfaces of
reflection from the observers increased. About five seconds after the
explosion, a single loud shock was sent back to us from the side of
the vessel lying between us and the land. Obliterated for a moment by
this more intense echo the aerial reverberation continued its retreat,
dying away into silence in two or three seconds afterwards. [Footnote:
The echoes of the gun fired on shore this day were very brief; those
of the 12-oz. gun-cotton rocket were 12" and those of the 8-oz.
cotton-powder rocket 11" in duration.]
I have referred to the firing of an 8-oz. rocket from the deck of the
'Galatea' on March 8, 1877, stating the duration of its echoes to be
seven seconds. Mr. Prentice, who was present at the time, assured me
that in his experiments similar echoes had been frequently heard of
more than twice this duration. The ranges of his sounds alone would
render this result in the highest degree probable.
To attempt to interpret an experiment which I have not had an
opportunity of repeating, is an operation of some risk; and it is not
without a consciousness of this that I refer here to a result
announced by Professor Joseph Henry, which he considers adverse to the
notion of aerial echoes. He took the trouble to point the trumpet of
a syren towards the zenith, and found that when the syren was sounded
no echo was returned. Now the reflecting surfaces which give rise to
these echoes are for the most part due to differences of temperature
between sea and air. If, through any cause, the air above be chilled,
we have descending streams--if the air below be warmed, we have
ascending streams as the initial cause of atmospheric flocculence. A
sound proceeding vertically does not cross the streams, nor impinge
upon the reflecting surfaces, as does a sound proceeding horizontally
across them. Aerial echoes, therefore, will not accompany the
vertical sound as they accompany the horizontal one. The experiment,
as I interpret it, is not opposed to the theory of these echoes which
I have ventured to enunciate. But, as I have indicated, not only to
see but to vary such an experiment is a necessary prelude to grasping
its full significance.
In a paper published in the 'Philosophical Transactions' for 1876,
Professor Osborne Reynolds refers to these echoes in the following
terms Without attempting to explain the reverberations and echoes
which have been observed, I will merely call attention to the fact
that in no case have I heard any attending the reports of the rockets,
[Footnote: These carried 12 oz. of gunpowder, which has been found by
Col. Fraser to require an iron case to produce an effective
explosion.] although they seem to have been invariable with the guns
and pistols. These facts suggest that the echoes are in some way
connected with the direction given to the sound. They are caused by
the voice, trumpets, and the syren, all of which give direction to the
sound; but I am not aware that they have ever been observed in the
case of a sound which has no direction of greatest intensity.' The
reference to the voice, and other references in his paper, cause me to
think that, in speaking of echoes, Professor Osborne Reynolds and
myself are dealing with different phenomena. Be that as it may, the
foregoing observations render it perfectly certain that the condition
as to direction here laid down is not necessary to the production of
the echoes.
There is not a feature connected with the aerial echoes which cannot
be brought out by experiments in the air of the laboratory. I have
recently made the following experiment: A rectangle, x Y (p. 331),
22 inches by 12, was crossed by twenty-three brass tubes (half the
number would suffice and only eleven are shown in the figure), each
having a slit along it from which gas can issue. In this way
twenty-three low flat flames were obtained. A sounding reed a fixed
in a short tube was placed at one end of the rectangle, and a
'sensitive flame,' [Footnote: Fully described in my 'Lectures on
Sound,' 3rd edition, p. 227.] f, at some distance beyond the other
end. When the reed sounded, the flame in front of it was violently
agitated, and roared boisterously. Turning on the gas, and lighting
it as it issued from the slits, the air above the flames became so
heterogeneous that the sensitive flame was instantly stilled, rising
from a height of 6 inches to a height of 18 inches. Here we had the
acoustic opacity of the air in front of the South Foreland strikingly
imitated. [Footnote: Lectures on Sound, 3rd ed, p. 268.] Turning off
the gas, and removing the sensitive flame to f, some distance behind
the reed, it burned there tranquilly, though the reed was sounding.
Again lighting the gas as it issued from the brass tubes, the sound
reflected from the heterogeneous air threw the sensitive flame into
violent agitation. Here we had imitated the aerial echoes heard when
standing behind the syren-trumpet at the South Foreland. The
experiment is extremely simple, and in the highest degree impressive.
Fig. 11.
*****
The explosive rapidity of dynamite marks it as a substance specially
suitable for the production of sound. At the suggestion of Professor
Dewar, Mr. McRoberts has carried out a series of experiments on
dynamite, with extremely promising results. Immediately after the
delivery of the foregoing lecture I was informed that Mr. Brock
proposed the employment of dynamite in the Collinson rocket.
********************
XI. ON THE STUDY OF PHYSICS.
[Footnote: From a lecture delivered in the Royal Institution of Great
Britain in the Spring of 1854.]
I HOLD in my hand an uncorrected proof of the syllabus of this course
of lectures, and the title of the present lecture A there stated to be
'On the Importance of the Study of Physics as a Means of Education.'
The corrected proof, however, contains the title: 'On the Importance
of the Study of Physics as a Branch of Education.' Small as this
editorial alteration may seem, the two words suggest two radically
distinct modes of viewing the subject before us. The term Education
is sometimes applied to a single faculty or organ, and if we know
wherein the education of a single faculty consists, this will help us
to clearer notions regarding the education of the sum of all the
faculties, or of the mind. When, for example, we speak of the
education of the voice, what do we mean? There are certain membranes
at the top of the windpipe which throw into vibration the air forced
between them from the lungs, thus producing musical sounds. These
membranes are, to some extent, under the control of the will, and it
is found that they can be so modified by exercise as to produce notes
of a clearer and more melodious character. This exercise we call the
education of the voice. We may choose for our exercise songs new or
old, festive or solemn; the education of the voice being the object
aimed at, the songs may be regarded as the means by which this
education is accomplished. I think this expresses the state of the
case more clearly than if we were to call the songs a branch of
education. Regarding also the education of the human mind as the
improvement and development of the mental faculties, I shall consider
the study of Physics as a means towards the attainment of this end.
From this point of view, I degrade Physics into an implement of
culture, and this is my deliberate design.
The term Physics, as made use of in the present Lecture, refers to
that portion of natural science which lies midway between astronomy
and chemistry. The former, indeed, is Physics applied to 'masses of
enormous weight,' while the latter is Physics applied to atoms and
molecules. The subjects of Physics proper are therefore those which
lie nearest to human perception: light and heat, colour, sound,
motion, the loadstone, electrical attractions and repulsions, thunder
and lightning, rain, snow, dew, and so forth. Our senses stand
between these phenomena and the reasoning mind. We observe the fact,
but are not satisfied with the mere act of observation: the fact must
be accounted for--fitted into its position in the line of cause and
effect. Taking our facts from Nature we transfer them to the domain
of thought: look at them, compare them, observe their mutual relations
and connexions, and bringing them ever clearer before the mental eye,
finally alight upon the cause which unites them. This is the last act
of the mind, in this centripetal direction--in its progress from the
multiplicity of facts to the central cause on which they depend. But,
having guessed the cause, we are not yet contented. We set out from
the centre and travel in the other direction. If the guess be true,
certain consequences must follow from it, and we appeal to the law and
testimony of experiment whether the thing is so. Thus is the circuit
of thought completed,--from without inward, from multiplicity to
unity, and from within outward, from unity to multiplicity. In thus
traversing both ways the line between cause and effect, all our
reasoning powers are called into play. The mental effort involved in
these processes may be compared to those exercises of the body which
invoke the co-operation of every muscle, and thus confer upon the
whole frame the benefits of healthy action.