THE FLASH AND THE SOUND
The Romans attributed a mysterious influence to each manifestation of
electricity. They divided lightning into individual and family
lightning, lightning of advice, monitory, explanatory, expostulatory,
confirmatory, auxiliary, disagreeable, perfidious, pestiferous,
menacing, murderous, etc., etc.
They adapted it to every taste and circumstance, but modern science
has come to put order into this Capharnaum.
When a cloud is superabundantly charged with electricity, this
electricity, which is compressed in the cloudy envelope, tries to
escape in order to join the electricity accumulated either in another
cloud or on the ground. An electric deflagration ensues, and a long
ignited dart precipitates itself into space, showing us on a large
scale what our experience of physics has taught us in a small way in
our laboratories. This luminous and often dazzling trail constitutes
lightning.
Lightning is not always the same, and in order to classify the
different forms it takes more easily, it can be divided into three
groups--diffused lightning, linear lightning, and fireballs. This last
is the most curious of the three. The variety and eccentricity of
fireballs are celebrated in the history of lightning, and I propose to
devote the following chapter to their vagaries.
Diffused lightning is the commonest of all. You can count hundreds of
flashes on a stormy night. Occasionally they succeed one another with
such rapidity that the sky is momentarily entirely illumined with a
fantastic brightness. At these times great sombre clouds can be seen
surging from the darkness of the night, to shine suddenly with an
ephemeral brightness of a diffused red, blue, or violet tinge. Their
irregular shapes, with their jagged edges of light, are visible
against the dark background of the heavens, and the thunder growls
monotonously. Whether the exchange of electricity is produced on a
vast stretch between two rows of clouds, or whether it is manifested
by a long thin spark launched like an arrow and veiled by the curtain
of clouds, all that can be seen is a strange light, vague, diaphanous,
instantaneous, which sometimes spreads itself like a sheet of fire all
over the horizon.
It is diffused lightning which gives us the finest storm effects on
those heavy summer evenings when the air is breathless and saturated
with electricity. Suddenly the clouds are illumined, nebulous veils
of light on which can be seen, in sombre fantastic, fugitive vision,
the outlines of the trees, houses, and other landmarks. Then, all at
once, heaven and earth fall back into a darkness deeper than before,
owing to the contrast.
Linear lightning is more terrible. It is regarded by astronomers as
the most perfect form of destructive lightning. It is a strong
flash--a thin trail of light--very clear, and extraordinarily rapid,
which shoots from an electric cloud to the earth, or from one cloud to
another.
Like a supple and undulating serpent of fire, it twists itself
luminously into space, spreading itself menacingly in the heavens with
its long spirals of light.
Sometimes--in a hurry, no doubt, to reach its prey--it effects its
passage in a straight line, but as a rule it follows a sinuous track,
and forms itself into a zigzag at an obtuse angle. The different forms
which this lightning takes are no doubt attributable to various
causes. One of the chief of these seems to be the unequal distribution
of humidity in the air, which renders it a more or less good
conductor. In fact, fulminic matter is strongly attracted towards damp
regions, and goes quickly from one point to another, guided in its
chosen way by the hygrometrical conditions of the atmosphere; and it
is these constant changes of direction which determine the meanderings
of its course. Thus the lightning would trace a sort of plan of the
hygrometrical state of the air for a certain portion of the
atmosphere. For it, the short road is hardly ever the straight line.
On the other hand, the variability of the overloading of electricity
has something to say to the form it takes.
Sometimes lightning forms itself into two or three branches, and
becomes forked lightning. Or it even divides itself into a number of
points from a principal branch, out of which a great many sparks burst
forth.
These incandescent sheaves move through space with extraordinary
agility. It has not been possible to measure their speed with absolute
accuracy, but their rapidity is such that their transit appears to be
instantaneous. The latest researches seem to have proved that their
speed is superior to that of light, which is 300,000 kilometres a
second.
Lightning is not always of a dazzling whiteness, it is often yellow,
red, blue, violet, or green. Its colour depends on the quantity of the
electricity thrown on the atmosphere by the discharge; on the density
of the air at the time of the passage of the ignited matter; on its
hygrometrical state, and on the substances which it contains during
suspension. It has been remarked in the study of physics that the
electric spark is white in the open air, but that it gets a violet
tinge in the vacuum of a pneumatic machine.
This proves that violet lightning comes from the far-off regions of
the atmosphere. It traverses a bed of rarified air, and shows the
great height of the storm-clouds from which it emanates.
The fulminating spark is so fugitive that it is difficult to form an
idea of its length. One could easily take it to be a yard or so long,
so illusory and deceptive are our impressions. As a matter of fact, it
is proved that flashes of lightning cover a distance of several
kilometres.
There are various methods to which one can have recourse in these
scientific researches. The first, which gives the length of horizontal
lightning, is based on a minute comparison between the trajectory
described by the meteor and the known distance of the terrestrial
points between which it travels. In order to gauge the extent of
vertical lightning, you must estimate approximately the height of the
clouds from which it comes, based on the irregularities of the earth
of which the height is known.
But there is a still simpler method for approximate measurement within
the reach of every one. It consists in multiplying 337 (the number of
yards traversed by sound in a second) by the number of seconds during
which the thunder lasts.
These methods all give the same result, and prove that lightning is
often 1, 5, and 10 kilometres in length. The greatest length proved up
to the present has been 18 kilometres. When one thinks of the
instantaneousness of these flashes, one marvels at their incomparable
agility, and we can only be lost in admiration of the magic force of
the heavenly sling, which is capable of hurling these whole rivers of
fire to roll in their sinuous course right through space, and in a
space of time almost inappreciable to our senses.
Yet, in spite of the extreme rapidity of the lightning, it has been
possible to determine that these meteors do not last the thousandth
part of a second. To prove this, we take a circle of cardboard,
divided from the middle into black and white sections. This circle can
be turned like a wheel almost as quickly as one can wish. We know that
luminous impressions remain on the retina the tenth part of a second;
thus, if we imitate the childish game of turning a lighted coal--if
the turn is made in the tenth of a second, each successive position of
the coal remaining impressed on the retina for the same length of
time--we have a continuous circle. In turning our cardboard wheel with
the black and white spokes, if each spoke passes before our eyes in
less than the tenth of a second, we can no longer distinguish between
the sections, but can only see a grey circle. But we can make it
rotate a hundred turns or more in a second; this being done, if we
continue to observe the circle, we can no longer see the lines, they
succeed each other more quickly in our eyes than the impression they
produce. But if the circle turns before us in the darkness, and it is
suddenly lighted up and as suddenly darkened, the impression produced
on our eyes by each of the sections would last less than the tenth of
a second, and the circle would appear to us as if it were stationary.
In applying a calculated rotation to this contrivance, Charles
Wheatstone has proved that some lightning does not last the thousandth
part of a second. This measure is probably a minimum; in the majority
of cases the duration of lightning is longer than this.
Often during the hot, transparent summer nights, we see a considerable
number of flashes, which furrow the firmament with their gentle,
bluish light. These fugitive gleams remind us in the sky of the
will-o'-the-wisps, which come forth silently from marshy ground. The
atmosphere is pure; there are no apparent traces of a storm, and yet
the sky is glistening with thousands of small flames. The flashes
succeed one another almost without interruption. These electric
sparks are known as heat-lightning, but this is quite inaccurate, and
has no meaning in the language of modern science.
In a great number of cases an astronomer would be able to discover
certain characteristic signs indicating that a storm is taking place
under the horizon at a very great distance from the point of
observation. It is only at the moment when the sky is lighted up that
one can see the ridge of clouds lying low on the horizon. At other
times there is no sign of a storm, as far as the eye can see. The
atmosphere is quite clear, and yet the sky is swept with a number of
electric flashes. But afterwards you hear that a violent storm has
devastated the region over which the gleams have appeared, and that it
is to this that they are attributable. They are only reflected lights.
A sailor tells us that once when he was out at sea, more than 100
kilometres from Lima, he saw a number of bright flashes, without any
thunder, to the east and north-east of the horizon. The weather was
perfect, and the sky absolutely serene. Now we know that storms, and
the electric phenomena which they produce, are unknown upon that
coast; but this immunity does not extend for more than 100 kilometres
to the interior of this country, so that this lightning which was
observed at sea, 100 kilometres from the shore, must have taken place
more than 200 kilometres away.
One of our correspondents, M. Soleyre of Constantine, sent us word, in
1899, of an interesting case of lightning without thunder.
"In August," he says, "I noticed it in the valley of the Arve above
Salambes; when I came back to Algiers I saw it again on September 16,
and on October 19.
"It was not sheet lightning, but ordinary lightning concentrated in
very thin lines. This lasted long, and was very near. Another thing,
there was no hail. This is not very rare in Algiers."
On September 1, 1901, I happened to be in Geneva at about 6 p.m. The
weather was heavy but very fine. I noticed a good deal of lightning on
the south-west of the horizon. It went on almost without interruption
above the Savoy Alps. Each flash illuminated at the same time the
ridge of the mountains and the fringed edge of the great sombre clouds
lying low on the horizon. This lightning was silent; the noise of the
thunder did not reach Geneva. The next day I learnt that a terrible
storm had devastated the neighbourhood of Chambéry and Aix-les-Bains.
Moreover, apart from storms, there have been other records of this
lighting up of the sky being observed at great distances.
Thus, in 1803, a service of luminous communications was established on
Mount Brocken in the Hartz Mountains in order to determine the
differences of longitude. The combustion of 180 to 200 grammes of
powder, burnt in the open air, for each of the signals, produced a
light which was observed by astronomers stationed on Mount Kenlenberg,
although they were 240 kilometres from Brocken, which is itself
invisible from Kenlenberg.
On certain fête-days, July 14, for example, when the principal
monuments in Paris are illuminated, at a distance of 20 and 30
kilometres we can see a sort of luminous vapour which floats above the
town and reflects the lights of the boulevards, although the lights
themselves are invisible from the point of observation.
Here is another example which any Parisian can verify: the captive
balloon of the Aërodrome at Porte-Maillot, which soars some hundreds
of yards above Paris during the spring and summer, as seen from the
dark paths of the Bois de Boulogne, appears against the azure of the
sky like a magnificent globe bathed in light, resembling an enormous
moon. Well, this gentle, pale light is only the reflection of the
lights of Paris which are invisible from the Bois de Boulogne.
The earth and all the planets which are dark in themselves, shine in
space lighted up by the sun.
The silent lightning which flashes in the sky is only the reflection
of a distant storm. Whether on account of the spherical shape of the
earth or on account of the irregularities of the land, the clouds are
invisible, but the effluvium which escapes from them can be seen at a
great distance.
These poetic and ephemeral flames which glide through the sky, appeal
to the imagination of the dreamer, and yet they are quite as terrible
as the flashes which are accompanied by thunder. If the noise which
accompanies these is not perceptible, it is because the sound of the
thunder does not carry far, and has been lost in space before reaching
us.
It is the same with the silent lightning which gleams in a stormy sky.
This phenomenon is particularly frequent in the Antilles. Either the
storm breaks too far from the observer, or the discharge has taken
place between two beds of clouds, the lower of which intercepts the
waves of sound without preventing the escape of the electric spark,
and the thunder is not heard.
As a rule we imagine that lightning always descends, that it comes to
us from the higher celestial regions to be lost in the common
reservoir. But this is quite inaccurate. Lightning sometimes ascends.
Sometimes it descends and reascends. That is to say, after it reaches
the ground, either there is no attraction there, or a stronger force
draws it back to the aerial regions, and it flies back to the clouds
whence it came.
As a rule we only fear the direct lightning. This is a great mistake.
There are many cases of lightning striking from a distance.
For example, at the end of May, 1866, an English coastguard was making
his rounds on the coast of one of the Shetland Isles, when a flash of
lightning passed near him, striking a great rock. The unfortunate man
was completely blinded, and plunged into darkness thus suddenly, he
would inevitably have fallen down an abyss, if his companions,
attracted by his cries, had not come to the rescue and taken him home.
Here is another case:--
On September 24, 1826, a terrible storm burst over Versailles,
accompanied by a great deal of thunder and lightning. At the moment
when the lightning struck Galli's farm, an old man who was in a street
in Versailles, at a distance of two kilometres from the farm, suddenly
felt a violent shock, accompanied by a feeling of oppression and
giddiness and a semi-paralysis of the tongue and the whole of his left
side. Next morning this had passed away, but in the evening at the
same time as the shock had occurred, he felt similar sensations of
fainting, and it was the same to the end of the week. It would be well
to remark here that at the moment of the accident, M. B---- happened
to be near the wall of a house, not far from the metallic tube which
conducted the rain-water into the level of the pavement.
The following phenomenon, to which we have already alluded, is no less
curious:--
On July 22, 1868, at about 7 o'clock in the evening, at Gien-sur-Cure
(Nièvre), the thunder had been growling violently for some time, when
all of a sudden the lightning struck a thatched house, which it set on
fire. At the same time a woman who was in a house ten yards away, felt
a shock, and saw the tiled floor rise beneath her. Her two sabots were
broken on her feet, and a bottle of Holy Water with which she was
blessing the house was broken in her hand, only the neck remained in
her fingers. She herself suffered nothing but the shock. Nineteen of
the tiles were flung in all directions.
Here is another very remarkable case of _ascending lightning_,
published in the _Comptes Rendus_ of the Academie des Sciences:--
At Porto-Alegre, on June 9, 1870, at 2 a.m., during a violent storm,
on the property of M. Laranja e Oliveira, at Brazil, a servant was
entering the house; he was about ten yards away, when a flash of
lightning illuminated it; at the same moment he felt a great tingling
in the flesh of his feet, then in his legs, then all over his body,
and finally in his head, on which the hair stood on end to such an
extent that _he was obliged to hold his hat on_ in order to prevent
its falling off. At the same time, a white flame burst from the ground
about two yards in front of him, accompanied by a shower of sparks.
Terrified by such a phenomenon, which he attributed to souls from
another world, he thought he was petrified to the spot; finally, he
ran away. Anything metallic which he had about him at the time of this
occurrence became magnetized. A key which was in his pocket remained
magnetic for two days.
Thus, as well as the ordinary fulguration, in which the lightning
(which we imagine descends from the clouds) acts directly on the body,
and the lightning which strikes indirectly, there are other electric
shocks which can be experienced by men and animals. Notable among
these is the _striking from the earth_, commonly known as _choc de
retour_, and which is in reality only an instance of the ascending
current, or of lightning striking from a distance. We must also
describe the striking _by a man who has been struck_.
The Abbé Richard, in his _Histoire de l'Air_, tells the following
story:--
In the neighbourhood of the village of Rumigny, in Picardy, on August
20, 1769, at six o'clock in the morning, there was a sudden irruption
of fulminating matter from the bosom of the earth in such quantities
as to produce the most violent results. The sky was cloudy, and looked
like a storm. A young farmer and his wife were following, at some
distance, a vehicle drawn by four horses. Suddenly the driver of this,
_without seeing the lightning or hearing the thunder_, was thrown to
the earth. His four horses were stretched dead on the ground near the
carriage. There was a _smoking hole_ in the ground, from which the
effluvium came forth and killed the young man and his wife at ten
paces off and separated from each other by twenty paces. The current
also knocked down, at a hundred paces, the father of the young man in
the same fashion as it had done the driver, but without injuring one
or the other.
The bodies showed no signs of a wound, only a considerable swelling
and a great deformity of the features. The woman, who was young and
pretty, became hideous; the whole of her body as well as that of her
husband was absolutely yellow. The four horses had their intestines
drawn from their bodies. They were all thrown on the same side. The
man's hat was pierced and his hair burnt, but he had no bruise on his
head.
This account, in which we must not be surprised to find the ideas and
language of the time (let us observe in passing that the man who was
struck did not hear the thunder, and had not even time to see the
lightning of which he was the victim)--this account, I say, gives us
an instance of ascending lightning. Here is another.
The traveller Brydone gives the following example, which he himself
observed:--
On July 19, 1785, a storm burst near Coldstream between 12 and 1 a.m.
A woman who was cutting hay on the banks of the Tweed _fell
backwards_. She at once called to her companions, and said she had
just received a violent blow on her foot for which she could in no way
account. At the time there was no thunder or lightning in the sky. The
shepherd of a farm at Lennel Hill saw a sheep fall near him, which a
few minutes before appeared to be in perfect health; he found it stone
dead. The storm then appeared very far off. Two carts laden with coal,
and each driven by a young driver seated on a small seat in front,
crossed over the Tweed. They had just climbed a small hill near the
banks of this river when they heard a great detonation round about,
similar to that which would be produced by the discharge of several
guns. At the same instant the driver of the second cart saw the first,
with his companion and the two horses, fall to the ground. _The driver
and horses were stone dead. The ground was pierced with three circular
holes at the very spot where the wheels had touched it when the
accident happened._ Half an hour after this event the holes emitted an
odour which Brydone compared to that of ether. The two circular iron
bands which covered the felloes of the wheels showed evident signs of
fusion in the two spots which rested on the ground at the moment of
the detonation, and in no other place. The skin of the horses had been
burnt, particularly about the legs and under the stomach. The body of
the driver had marks of burning here and there. His clothes, his
shirt, and, above all, his hat, were reduced to shreds, and gave out a
strong smell.
Orioli gives an example of two men who were surprised by a violent
storm near the village of Benvenide. They lay down on the ground to
let the meteor pass. Some moments later one of them got up feeling
very tired, but the other was dead. The bones of the latter were so
soft that it was easy to bend them; his whole body was of the
consistency of paste. The tongue had been torn from the roots, and no
one knew what had become of it.
Now, just as the earth can strike, so can the human body become
fulminating and act like lightning. After having been struck, it can
effectively acquire the power to strike in its turn.
For instance, on June 30, 1854, a man named Barri was killed by
lightning near the Jardin des Plantes, in Paris, and his body lay for
some time exposed to a beating rain. After the storm had passed, two
soldiers from the neighbouring guard-house tried to remove the body,
and each received a violent blow when they touched it. They got off
with a shock, perhaps because the body had been drenched with rain,
which acted as a conductor to the electricity, and thus it had had
time to lose a part of the fluid.
What a mysterious world is that of atmospheric electricity! It is
truly the New World for the scientific mind--a mine, fruitful in
unknown and even unsuspected marvels, which is perpetually disclosing
its riches for our admiration.
One of our most valued collaborators in our researches on the nature
of lightning is photography. Faithfully and unhesitatingly it
registers an indestructible document of the fugitive lightning, which
imprints itself on the sensitized plate, and the astronomer can
afterwards examine the smallest details of the sudden apparition
comfortably and at his leisure. We have already a considerable number
of plates of the outline of the lightning in flight. An examination of
these electric pictures is very interesting.
Who knows whether, later on, when phonography is brought to
perfection, it will not also register the noisy accompaniment to the
electric flash? Then, with the help of the cinematograph, we could
have dramatic representations of sensational storms. While the
photograph unrolls all the phases of the lightning, from its emerging
from the cloud to its fall to earth, before the gaze of the
spectators, the phonograph will repeat the sonorous accents of the
terrorizing voice of thunder.
Thunder, as all the world knows, is the noise which accompanies
lightning. It is produced when a change of electricity--a
neutralization--takes place between two points more or less distant.
The causes which provoke it are still somewhat of a mystery.
The luminous rocket which flings itself precipitately from a cloud
saturated with electricity, spreads itself like a trail of flames in
the atmosphere where an infinity of invisible molecules are floating;
these it repels. The passage of this whirlwind of fire in a centre
which is greatly compressed produces a momentary void into which the
surrounding air at once rushes, and it is the same all the way along
the route followed by lightning.
In all probability the equilibrium of the atmosphere, which is
momentarily disturbed by the intrusion of the ignited matter, hastily
re-establishes itself by a rush of the air which the lightning has
ejected, and which is swallowed up with a crash in the opening which
has been made. It is, on a large scale, a similar phenomenon to that
which is produced by opening a case which has been hermetically
sealed. The air rushing in makes a dull noise.
Pouillet objects to this very natural explanation on the ground that
the flight of a cannon-ball ought to produce a similar noise. But
this objection errs in its basis, because, as regards velocity, a
cannon-ball is as a tortoise as compared with the arrow of lightning,
and as regards size, who can compare a few grammes of powder to the
torrents of fire launched into space by the prodigious force of
electricity?
The lightning discharge produces a violent concussion in the cloud,
and very often a shower of rain immediately follows it. The electric
conditions of the different clouds which make a storm being separately
liable the one to the other, the discharge of one must lead to that of
several others more or less distant. In all cases the noise is caused
by the expansion of the air where a more or less partial void has been
made. It is the same with firearms, _crève-vessie_, etc.
One of the chief characteristics of thunder is the rolling, which is
often prolonged, and reverberates on the sides of steep mountains.
This voice, with its lugubrious tone, becomes grave and sometimes
sinister in the revolution of space--this voice, celestial and
infernal, seems to momentarily dominate the world, while the clouds
are enveloped with a thousand diabolical flames. Sometimes it rings in
the air with fierce calls, at others it spreads itself in dull,
languorous complaints.
Nevertheless, the intenseness of thunder undergoes a thousand
fluctuations, and presents astonishing variations. Generally it
strikes and frightens, but the curious thing is that, for the ear, in
reality it is less strong than the crinkling noise of a piece of paper
torn close to it.
Often, too, it may be compared to the discharge of firearms, a pistol
or a cannon.
Thus, when the lightning penetrated Volney's apartments at Naples, the
people present, among whom was Saussure, had the impression of a
pistol-shot in the next room.
There is a case given of M. and Mme. Boddington, who were seated on
the back seat of their coach in order to enjoy the view of the
country, and had given the inside seats to two servants. Suddenly
there was a flash of lightning, which struck M. and Mme. Boddington
and flung the postillion to a great distance. The servants were
untouched, and escaped with a fright. When they got over their terror,
one of them said that a very brilliant flash of lightning had been
immediately followed by a noise similar to that of a heavily charged
musket. He thought some one had shot the horses. His fright had
stunned him so that he hardly knew what had happened.
At other times thunder is accompanied by a whistling noise, but as a
rule it is the rolling which predominates.
We ask ourselves to what it is due that this rolling lasts so long.
There are several causes. The first is due to the length of lightning
and the difference in speed between sound and light. Let us suppose,
for example, a flash of lightning, AE, 11,000 metres long. The
observer stationed at O, underneath extremity E of the lightning
(which is one kilometre high), will see the lightning in its full
length in one indivisible instant. The sound will form itself also at
the same instant all along the line of lightning, but the sound-waves
will only reach the ear of the observer at different times. That which
starts at point E, the nearest, will arrive in 3 seconds, sound
travelling about 337 metres a second. That which is formed at the same
moment at point D, 2000 yards from point O, will take double the time
to arrive. That which comes from point C will not arrive for 12
seconds. The sound formed at B will not arrive until the time
necessary to cover 8 kilometres--that is to say, not for 23
seconds--and the sound formed at A will only reach after 32 seconds.
Thus the rolling will have lasted more than half a minute from start
to finish.
[Illustration: DIAGRAM EXPLAINING THE DURATION OF THE SOUND OF
THUNDER.]
And if, which is very often the case, the astronomer is not exactly
under one of the extremities of the lightning, but at some other point
in its course, he first hears a clap, then an increased noise, then a
diminution. In fact, in this case, the sound which leaves point D just
overhead, which is 1000 metres off, arrives alone in 3 seconds, but
the sounds formed from D to E on one side, and from C to D on the
other, arrive at the same time, having joined each other, taking 9
seconds, which is the necessary time to come from 1000 to 3000 metres.
The sounds beyond C arrive and depart according to distance, as in the
preceding example, and the thunder has lasted 23 seconds instead of 32
seconds.
[Illustration: COMMENCEMENT, AUGMENTATION, AND DIMINUTION OF THE
INTENSITY OF THUNDER.]
I must add that lightning is never straight, but always crooked.
The length of time the thunder rolls has nothing to do with the
distance of the cloud where the phenomenon begins. It is proportionate
to the length of the lightning with which it is associated. The
rolling is often still more prolonged by a succession of small
discharges, which follow each other very rapidly between the stormy
clouds; by the zigzags and ramifications of the lightning caused by
the hygrometrical diversity of the different beds of air; by the
echoes repeated by the mountains, the earth, the water, and the clouds
themselves--to all which must be added also the interferences caused
by the encounter of the different systems of sound-waves.
Its duration is extremely variable, however; it rarely exceeds 30
seconds, though the noise may sometimes seem to last much longer, so
that an observation of this kind may have any value--one must take
into consideration the echo, and isolate a single clap from the series
of discharges which take place in the bosom of the storm. The longest
verified duration of a single discharge is 45 seconds. That is
tremendous if we think of the instantaneousness of the lightning, and
reflect that the flash and the sound are produced in reality at the
same moment, that they are dependent the one on the other, and that in
their various manifestations there is only the difference of motion.
Sound moves like a tortoise behind the swift lightning, whose
vibrations spread in the air with inconceivable rapidity.
Hence these 45 seconds correspond to a flash of lightning more than 15
kilometres in length, but we know that there are even longer ones.
I have already said that we can calculate the distance of the
celestial cannon from which the fulminating discharge comes by
counting the number of seconds which separate the apparition of the
lightning from the first growls of the thunder. Thus the longest
interval that has been proved between the appearance of the lightning
and the noise it produces is 24 kilometres. This, however, is a
maximum.
Numerous observations have proved that thunder is never heard beyond
20 or perhaps 25 kilometres. Lightning pierces the cloudy veil, but
the voice of thunder does not carry so far. In this the great Jupiter
shows himself inferior to the ingenuity of human pigmies, whose
destructive and barbarous art has been able to invent infernal
machines the noise of which can be heard much further.
Cannon can easily be heard at a distance of 40 kilometres. Sometimes,
in sieges and big battles the cannonades can be heard muttering
lugubriously more than 100 kilometres away.
During the siege of Paris, Krupp's cannon--that most expeditious of
all vehicles of civilization in the eyes of the statesmen of this
planet!--could be heard as far as Dieppe, 140 kilometres away, during
the nights when they were bombarding. The cannonade of March 30, 1814,
which crowned the First Empire, as it crowned the Second, was heard
between Lisieux and Caen, a distance of 175 kilometres. Arago even
alleges that the cannon at Waterloo could be heard as far as Creil,
which is 200 kilometres away. Thus man's thunder can be heard at a
greater distance than that of nature. It is true that it is
incomparably more vicious, and that it has a great many more victims.
In its natural state, if we might explain it thus--left to itself--it
comes directly to us from the high regions of the atmosphere, and is
the most terrible of aerial messengers--a subtle messenger, malicious
and violent, it is the terror of the human race. But ruled by the
genius of man, it becomes a powerful agent towards modern
civilization, and we cannot sufficiently admire its many advantages.
If we could tame lightning and guide it safely, its services would
probably become innumerable. Lightning as man's right hand! Why not?
Was it not the auxiliary of the gods in the dark ages? To-day, is it
not regarded by astronomers as one of the most important forces of
nature? Why should it not be the collaborator of man's intelligence
to-morrow?