discharge. To make clear to you the influence of this circumstance, I
must ask your attention for a moment to the diagram on next page. Let
the broken line represent the path of a flash of lightning, and let O
represent the position of an observer. The sound will reach him first
from the point A, which is nearest to him, and then it will continue to
arrive in successive instants from the successive points along the line
A N and along the line A M, thus producing the effect of a continuous
peal. Meanwhile the sound waves have been traveling from the point B,
and in due time will reach the observer at O. Coming as they do in a
different direction from the former, they will strike the ear as the
beginning of a new peal which, in its turn, will be prolonged by the
sound waves arriving, in successive instants, from the successive
points along the line B M and B H. A little later, the sound will
arrive from the more distant point C, and a third peal will begin. And
so there will be several distinct peals proceeding, so to speak, from
several distinct points in the path of the lightning flash.
[Illustration: ORIGIN OF SUCCESSIVE PEALS OF THUNDER.]
A third cause to which the succession of peals may be referred is to
be found in the minor electrical discharges that must often take place
within the thundercloud itself. A thundercloud is not a continuous mass
like the metal cylinder of this electric machine--it has many outlying
fragments, more or less imperfectly connected with the principal body.
Moreover, the material of which the cloud is composed is only a very
imperfect conductor as compared with our brass cylinder. For these
two reasons it must often happen, about the time a flash of lightning
passes, that different parts of the cloud will be in such different
electrical conditions as to give rise to electrical discharges within
the cloud itself. Each of these discharges produces its own peal of
thunder; and thus we may have a number of minor peals, sometimes
preceding and sometimes following the great crash which is due to the
principal discharge.
Lastly, the influence of echo has often a considerable share in
multiplying the number of peals of thunder. The waves of sound, going
forth in all directions, are reflected from the surfaces of mountains,
forests, clouds, and buildings, and coming back from different
quarters, and with varying intensity, reach the ear like the roar of
distant artillery. The striking effect of these reverberations in a
mountain district has been described by a great poet in words which, I
daresay, are familiar to most of you:
“Far along,
From peak to peak, the rattling crags among,
Leaps the live thunder! Not from one lone cloud,
But every mountain now has found a tongue,
And Jura answers from her misty shroud
Back to the joyous Alps, that call to her aloud!”
=Variations of Intensity in Thunder.=--From what has been said, it
is easy to understand how the general roar of thunder is subject to
great changes of intensity, during the time it lasts, according to
the number of peals that may be arriving at the ear of an observer in
each particular moment. But every one must have observed that even an
individual peal of thunder often undergoes similar changes, swelling
out at one moment with great power, and the next moment rapidly
dying away. To account for this phenomenon, I would observe, first,
that there is no reason to suppose that the disturbance caused by
lightning is of exactly the same magnitude at every point of its path.
On the contrary, it would seem very probable that the amount of this
disturbance is, in some way, dependent on the resistance which the
discharge encounters. Hence the intensity of the sound waves sent forth
by a flash of lightning is probably very different at different parts
of its course; and each individual peal will swell out on the ear or
die away, according to the greater or less intensity of the sound waves
that reach the ear in each successive moment of time.
But there is another influence at work which must produce variations
in the loudness of a peal of thunder, even though the sound waves, set
in motion by the lightning, were everywhere of equal intensity. This
influence depends on the position of the observer in relation to the
path of the lightning flash. At one part of its course the lightning
may follow a path which remains for a certain length at nearly the
same distance from the observer; then all the sound produced along
this length will reach the observer nearly at the same moment, and
will burst upon the ear with great intensity. At another part, the
lightning may for an equal length go right away from the observer; and
it is evident that the sound produced along this length will reach the
observer in successive instants, and consequently produce an effect
comparatively feeble.
With a view to investigate this interesting question a little more
closely, let me suppose the position of the observer taken as a
centre, and a number of concentric circles drawn, cutting the path of
the lightning flash, and separated from one another by a distance of
110 feet, measured along the direction of the radius. It is evident
that all the sound produced between any two consecutive circles will
reach the ear within a period which must be measured by the time that
sound takes to travel 110 feet, that is, within the tenth of a second.
Hence, in order to determine the quantity of sound that reaches the
ear in successive periods of one-tenth of a second, we have only to
observe how much is produced between each two consecutive circles.
But on the supposition that the sound waves, set in motion by the
flash of lightning, are of equal intensity at every point of its path,
it is clear that the quantity of sound developed between each two
consecutive circles will be simply proportional to the length of the
path enclosed between them.
With these principles established, let us now follow the course of a
peal of thunder, in the diagram before us. This broken line, drawn
almost at random, represents the path of a flash of lightning; the
observer is supposed to be placed at O, which is the centre of the
concentric circles; these circles are separated from one another by a
distance of 110 feet, measured in the direction of the radius; and we
want to consider how any one peal of thunder may vary in loudness in
the successive periods of one-tenth of a second.
[Illustration: VARIATIONS OF INTENSITY IN A PEAL OF THUNDER.]
Let us take, for example, the peal which begins when the sound waves
reach the ear from the point A. In the first unit of time the sound
that reaches the ear is the sound produced along the lines A B and A
C; in the second unit, the sound produced along the lines B D and C E;
in the third unit, the sound produced along D F and E G. So far the
peal has been fairly uniform in its intensity; though there has been a
slight falling off in the second and third units of time, as compared
with the first. But in the fourth unit there is a considerable falling
away of the sound; for the line F K is only about one-third as long
as D F and E G taken together; therefore the quantity of sound that
reaches the ear in the fourth unit of time is only one-third of that
which reaches it in each of the three preceding units; and consequently
the sound is only one-third as loud. In the fifth unit, however, the
peal must rise to a sudden crash; for the portion of the lightning path
inclosed between the fifth and sixth circles is about six times as
great as that between the fourth and fifth; therefore the intensity of
the sound will be suddenly increased about six-fold. After this sudden
crash, the sound as suddenly dies away in the sixth unit of time; it
continues feeble as the path of the lightning goes nearly straight
away from the observer; it swells again slightly in the ninth unit of
time; and then continues without much variation to the end. This is
only a single illustration, but it seems quite sufficient to show that
the changes of intensity in a peal of thunder must be largely due to
the position of the spectator in relation to the several parts of the
lightning flash.
=Distance of a Flash of Lightning.=--I need hardly remind you that, by
observing the interval that elapses between the flash of lightning and
the peal of thunder that follows it, we may estimate approximately the
distance of the nearest point of the discharge. Light travels with such
amazing velocity that we may assume, without any sensible error, that
we see the flash of lightning at the very moment in which the discharge
takes place. But sound, as we have seen, takes a sensible time to
travel even short distances; and therefore a measurable interval almost
always elapses between the moment in which the flash is seen and the
moment in which the peal of thunder first reaches the ear. And the
distance through which sound travels in this interval will be the
distance of the nearest point through which the discharge has passed.
Now, the velocity of sound in air varies slightly with the temperature;
but, at the ordinary temperature of our climate, we shall not be far
astray if we allow 1,100 feet for every second, or about one mile for
every five seconds.
You will observe also that, by repeating this observation, we can
determine whether the thundercloud is coming toward us, or going away
from us. So long as the interval between each successive flash and the
corresponding peal of thunder, continues to get shorter and shorter,
the thundercloud is approaching; when the interval begins to increase,
the thundercloud is receding from us, and the danger is passed.
The crash of thunder is terrific when the lightning is close at hand;
but it is a curious fact, that the sound does not seem to travel as
far as the report of an ordinary cannon. We have no authentic record
of thunder having been heard at a greater distance than from twelve to
fifteen miles, whereas the report of a single cannon has been heard
at five times that distance; and the roar of artillery, in battle,
at a greater distance still. On the occasion of the Queen’s visit to
Cherbourg, in August, 1858, the salute fired in honor of her arrival
was heard at Bonchurch, in the Isle of Wight, a distance of sixty
miles. It was also heard at Lyme Regis, in Dorsetshire, which is
eighty-five miles from Cherbourg, as the crow flies; and we are told
that, not only was it audible in its general effect, but the report of
individual guns was distinctly recognized. The artillery of Waterloo is
said to have been heard at the town of Creil, in France, 115 miles from
the field of battle; and the cannonading at the siege of Valenciennes,
in 1793, was heard, from day to day, at Deal, on the coast of England,
a distance of 120 miles.[15]
So far, I have endeavored to set forth some general ideas on the nature
and origin of lightning, and of the thunder that accompanies it. In
my next Lecture I propose to give a short account of the destructive
effects of lightning, and to consider how these effects may best be
averted by means of lightning conductors.
NOTE TO PAGE 20.
ON THE HIGH POTENTIAL OF A FLASH OF LIGHTNING.
The potential of an electrified sphere is equal to the quantity of
electricity with which the sphere is charged, divided by the radius
of the sphere. Now the minute cloud particles, which go to make up
a drop of rain, may be taken to be very small spheres; and if _v_
represent the potential of each one, _q_ the quantity of electricity
with which it is charged, and _r_ the radius of the sphere, we have _v_
= _q_/_r_. Suppose 1,000 of these cloud particles to unite into one;
the quantity of electricity in the drop, thus formed, will be 1,000_q_;
and the radius, which increases in the ratio of the cube root of the
volume, will be 10_r_. Therefore the potential of the new sphere will
be 1000_q_/10_r_, or 100_q/r_; that is to say, it will be 100 times as
great as the potential of each of the cloud particles which compose
it. When a million of cloud particles are blended into a single drop,
the same process will show that the potential has been increased ten
thousandfold; and when a drop is produced by the agglomeration of a
million of millions of cloud particles, the potential of the drop
will be a hundred million times as great as that of the individual
particles.[16]
FOOTNOTES:
[1] “Il y a des grands seigneurs dont il ne faut approcher qu’avec
d’extrêmes précautions. Le tonnerre est de ce nombre.”--Dict. Philos.
art. Foudre.
[2] Electricité Statique, ii., 561.
[3] Deschanel’s Natural Philosophy, Sixth Edition, p. 641.
[4] Fragments of Science, Fifth Edition, p. 311.
[5] Lecture on Thunderstorms, Nature, vol. xxii., p. 341.
[6] Third Series, vol. v., p. 161.
[7] Phil. Trans. Royal Society, 1834, vol. cxxv., pp. 583-591.
[8] In experiments with a Leyden jar, Feddersen has shown that the
duration of the discharge is increased, not only by increasing the
striking distance, but also by increasing the size of the jar. Now, a
flash of lightning may be regarded as the discharge of a Leyden jar
of immense size, with an enormous striking distance; and therefore we
should expect that the duration of the discharge should be greatly
prolonged. See _American Journal of Science and Arts_, Third Series,
vol. i., p. 15.
[9] See original paper by Swan, Trans. Royal Society, Edinburgh, 1849,
vol. xvi., pp. 581-603; also, a second paper, _ib._ 1861, vol. xxii.,
pp. 33-39.
[10] Nature, vol. xxviii., p. 54.
[11] See, however, an attempt to account for this phenomenon in De
Larive’s Treatise on Electricity, London, 1853-8, vol. iii., pp. 199,
200; and another, quite recently, by Mr. Spottiswoode, in a Lecture on
the Electrical Discharge, delivered before the British Association at
York, in September, 1881, and published by Longmans, London, p. 42.
See also, for recent evidence regarding the phenomenon itself, Scott’s
Elementary Meteorology, pp. 175-8.
[12] See Jamin, “Cours de Physique,” i., 480-1; Tomlinson, “The
Thunderstorm,” Third Edition, pp. 95-103; “Thunderstorms,” a Lecture by
Professor Tait, Nature, vol. xxii., p. 356.
[13] Professor Tait, On Thunderstorms, Nature, vol. xxii., pp. 436-7.
[14] See note at the end of this Lecture, p. 26.
[15] See Tomlinson, The Thunderstorm, pp. 87-9.
[16] See Tait on Thunderstorms, Nature, vol. xxii., p. 436.
LECTURE II.
LIGHTNING CONDUCTORS.
The effects of lightning, on the bodies that it strikes, are analogous
to those which may be produced by the discharge of our electric
machines and Leyden jar batteries. When the discharge of a battery
traverses a metal conductor of sufficient dimensions to allow it an
easy passage, it makes its way along silently and harmlessly. But if
the conductor be so thin as to offer considerable resistance, then the
conductor itself is raised to intense heat, and may be melted, or even
converted into vapor, by the discharge.
On opposite page is shown a board on which a number of very thin wires
have been stretched, over white paper, between brass balls. The wires
are so thin that the full charge of the battery before you, which
consists of nine large Leyden jars, is quite sufficient to convert them
in an instant into vapor. I have already, on former occasions, sent the
charge through two of these wires, and nothing remains of them now
but the traces of their vapor, which mark the path of the electric
discharge from ball to ball. At the present moment the battery stands
ready charged, and I am going to discharge it through a third wire, by
means of this insulated rod which I hold in my hand. The discharge has
passed; you saw a flash, and a little smoke; and now, if you look at
the paper, you will find that the wire is gone, but that it has left
behind the track of its incandescent vapor, marking the path of the
discharge.
[Illustration: DISCHARGE OF LEYDEN JAR BATTERY THROUGH THIN WIRES.]
=Destruction of Buildings by Lightning.=--We learn from this experiment
that the electricity stored up in our battery passes, without visible
effect, through the stout wire of a discharging rod, but that it
instantly converts into vapor the thin wire stretched across the spark
board. And so it is with a flash of lightning. It passes harmlessly, as
every one knows, through a stout metal rod, but when it comes across
bell wires or telegraph wires, it melts them, or converts them into
vapor. On the sixteenth of July, 1759, a flash of lightning struck
a house in Southwark, on the south side of London, and followed the
line of the bell wire. After the lightning had passed, the wire was no
longer to be found; but the path of the lightning was clearly marked
by patches of vapor which were left, here and there, adhering to the
surface of the wall. In the year 1754, the lightning fell on a bell
tower at Newbury, in the United States of America, and having dashed
the roof to pieces, and scattered the fragments about, it reached the
bell. From this point it followed an iron wire, about as thick as a
knitting needle, melting it as it passed along, leaving behind a black
streak of vapor on the surface of the walls.
Again, the electric discharge, passing through a bad conductor,
produces mechanical disturbance, and, if the substance be combustible,
often sets it on fire. So, too, as you know, the lightning flash,
falling on a church spire, dashes it to pieces, knocking the stones
about in all directions, while it sets fire to ships and wooden
buildings; and more than once it has caused great devastation by
exploding powder magazines.
Let me give you one or two examples: In January, 1762, the
lightning fell on a church tower in Cornwall, and a stone--three
hundred-weight--was torn from its place and hurled to a distance of 180
feet, while a smaller stone was projected as far as 1,200 feet from the
building. Again, in 1809, the lightning struck a house not far from
Manchester, and literally moved a massive wall twelve feet high and
three thick to a distance of several feet. You may form some conception
of the enormous force here brought into action, when I tell you that
the total weight of mason-work moved on this occasion was not less than
twenty-three tons.
The church of St. George, at Leicester, was severely damaged by
lightning on the 1st of August, 1846. About 8 o’clock in the evening
the rector of the parish saw a vivid streak of light darting with
incredible velocity against the upper part of the spire. “For the
distance of forty feet on the eastern side, and nearly seventy on the
west, the massive stonework of the spire was instantly rent asunder and
laid in ruins. Large blocks of stone were hurled in all directions,
broken into small fragments, and in some cases, there is reason to
believe, reduced to powder. One fragment of considerable size was
hurled against the window of a house three hundred feet distant,
shattering to pieces the woodwork, and strewing the room within with
fine dust and fragments of glass. It has been computed that a hundred
tons of stone were, on this occasion, blown to a distance of thirty
feet in three seconds. In addition to the shivering of the spire, the
pinnacles at the angles of the tower were all more or less damaged, the
flying buttresses cracked through and violently shaken, many of the
open battlements at the base of the spire knocked away, the roof of the
church completely riddled, the roofs of the side entrances destroyed,
and the stone staircases of the gallery shattered.”[17]
Lightning has been at all times the cause of great damage to property
by its power of setting fire to whatever is combustible. Fuller says,
in his Church History, that “scarcely a great abbey exists in England
which once, at least, has not been burned by lightning from heaven.”
He mentions, as examples, the Abbey of Croyland twice burned, the
Monastery of Canterbury twice, the Abbey of Peterborough twice; also
the Abbey of St. Mary’s, in Yorkshire, the Abbey of Norwich, and
several others. Sir William Snow Harris, writing about twenty years
ago, tells us that “the number of churches and church spires wholly or
partially destroyed by lightning is beyond all belief, and would be too
tedious a detail to enter upon. Within a comparatively few years, in
1822 for instance, we find the magnificent Cathedral of Rouen burned,
and, so lately as 1850, the beautiful Cathedral of Saragossa, in Spain,
struck by lightning during divine service and set on fire. In March of
last year a dispatch from our Minister at Brussels, Lord Howard de
Walden, dated the 24th of February, was forwarded by Lord Russell to
the Royal Society, stating that, on the preceding Sunday, a violent
thunderstorm had spread over Belgium; that twelve churches had been
struck by lightning; and that three of these fine old buildings had
been totally destroyed.”[18]
Even in our own day the destruction caused by fires produced through
the agency of lightning is very great--far greater than is commonly
supposed. No general record of such fires is kept, and consequently
our information on the subject is very incomplete and inexact. I
may tell you, however, one small fact which, so far as it goes,
is precise enough and very significant. In the little province of
Schleswig-Holstein, which occupies an area less than one-fourth of the
area of Ireland, the Provincial Fire Assurance Association has paid in
sixteen years, for damage caused by lightning, somewhat over £100,000,
or at the rate of more than £6,000 a year. The total loss of property
every year in this province, due to fires caused by lightning, is
estimated at not less than £12,500.[19]
=Destruction of Ships at Sea.=--The destructive effects of lightning
on ships at sea, before the general adoption of lightning conductors,
seems almost incredible at the present day. From official records
it appears that the damage done to the Royal Navy of England alone
involved an expenditure of from £6,000 to £10,000 a year. We are
told by Sir William Snow Harris, who devoted himself for many years
to this subject with extraordinary zeal and complete success, that
between the year 1810 and the year 1815--that is, within a period of
five years--“no less than forty sail of the line, twenty frigates, and
twelve sloops and corvettes were placed _hors de combat_ by lightning.
In the merchant navy, within a comparatively small number of years,
no less than thirty-four ships, most of them large vessels with rich
cargoes, have been totally destroyed--been either burned or sunk--to
say nothing of a host of vessels partially destroyed or severely
damaged.”[20]
And these statements, be it observed, take no account of ships that
were simply reported as missing, some of which, we can hardly doubt,
were struck by lightning in the open sea, and went down with all hands
on board. A famous ship of forty-four guns, the _Resistance_, was
struck by lightning in the Straits of Malacca, and the powder magazine
exploding, she went to the bottom. Of her whole crew only three were
saved, who happened to be picked up by a passing boat. It has been well
observed that, were it not for these three chance survivors, nothing
would have been known concerning the fate of the vessel, and she would
have been simply recorded as missing in the Admiralty lists.
Nothing is more fearful to contemplate than the scene on board a ship
when she is struck by lightning in the open sea, with the winds howling
around, the waves rolling mountains high, the rain coming down in
torrents, and the vivid flashes lighting up the gloom at intervals, and
carrying death and destruction in their track. I will read you one or
two brief accounts of such a scene, given in the pithy but expressive
language of the sailor. In January, 1786, the _Thisbe_, of thirty-six
guns, was struck by lightning off the coast of Scilly, and reduced to
the condition of a wreck. Here is an extract from the ship’s log: “Four
A. M., strong gales; handed mainsail and main top-sail; hove to with
storm staysails; blowing very heavy, S. E. 4.15, a flash of lightning,
with tremendous thunder, disabled some of our people. A second flash
set the mainsail, main-top, and mizen staysails on fire. Obliged to cut
away the mainmast; this carried away mizen top-mast and fore top-sail
yard. Found foremast also shivered by the lightning. Fore top-mast went
over the side about 9 A. M. Set the foresail.”[21]