LIGHTNING, THUNDER
AND
LIGHTNING CONDUCTORS.
_WITH AN APPENDIX ON THE RECENT CONTROVERSY
ON LIGHTNING CONDUCTORS._
BY
GERALD MOLLOY, D. D., D. Sc.
_ILLUSTRATED._
[Illustration]
NEW YORK:
THE HUMBOLDT PUBLISHING CO.,
28 LAFAYETTE PLACE.
LIGHTNING, THUNDER, AND LIGHTNING CONDUCTORS.
CONTENTS.
LECTURE I. Pages 5-26
LIGHTNING AND THUNDER.
Identity of Lightning and Electricity--Franklin’s Experiment--Fatal
Experiment of Richman--Immediate Cause of Lightning--Illustration from
Electric Spark--What a Flash of Lightning Is--Duration of a Flash of
Lightning--Experiments of Professor Rood--Wheatstone’s
Experiments--Experiment with Rotating Disc--Brightness of a Flash of
Lightning--Various Forms of Lightning--Forked Lightning, Sheet
Lightning, Globe Lightning--St. Elmo’s Fire--Experimental
Illustration--Origin of Lightning--Length of a Flash of
Lightning--Physical Cause of Thunder--Rolling of Thunder--Succession
of Peals--Variation of Intensity--Distance of a Flash of Lightning
LECTURE II. Pages 26-53
LIGHTNING CONDUCTORS.
Destructive Effects of Lightning--Destruction of
Buildings--Destruction of Ships at Sea--Destruction of Powder
Magazines--Experimental Illustrations--Destruction of Life by
Lightning--The Return Shock--Franklin’s Lightning Rods--Introduction
of Lightning Rods into England--The Battle of Balls and
Points--Functions of a Lightning Conductor--Conditions of a Lightning
Conductor--Mischief Done by Bad Conductors--Evil Effects of a Bad
Earth Contact--Danger from Rival Conductors--Insulation of Lightning
Conductors--Personal Safety in a Thunder Storm--Practical
Rules--Security Afforded by Lightning Rods
APPENDIX. Pages 55-62
RECENT CONTROVERSY ON LIGHTNING CONDUCTORS.
Theory of Lightning Conductors Challenged--Lectures of Professor
Lodge--Short Account of his Views and Arguments--Effect of
Self-Induction on a Lightning Rod--Experiment on the Discharge of a
Leyden Jar--Outer Shell only of a Lightning Rod Acts as a
Conductor--Discussion at the Meeting of the British Association,
September, 1888--Statement by Mr. Preece--Lord Rayleigh and Sir
William Thomson--Professor Rowland and Professor Forbes--M. de
Fonvielle, Sir James Douglass, and Mr. Symons--Reply of Professor
Lodge--Concluding Remarks of Professor Fitzgerald, President
of the Section--Summary Showing the Present State of the Question
LIST OF ILLUSTRATIONS.
PAGE
THE ELECTRIC SPARK: A TYPE OF A FLASH OF LIGHTNING, 8
CARDBOARD DISC WITH BLACK AND WHITE SECTORS; AS SEEN WHEN AT REST, 12
SAME DISC; AS SEEN WHEN IN RAPID ROTATION, 12
THE BRUSH DISCHARGE, ILLUSTRATING ST. ELMO’S FIRE, 17
ORIGIN OF SUCCESSIVE PEALS OF THUNDER, 22
VARIATIONS OF INTENSITY IN A PEAL OF THUNDER, 24
DISCHARGE OF LEYDEN JAR BATTERY THROUGH THIN WIRES, 27
GLASS VESSEL BROKEN BY DISCHARGE OF LEYDEN JAR BATTERY, 32
GUN COTTON SET ON FIRE BY ELECTRIC SPARK, 33
VOLTA’S PISTOL; EXPLOSION CAUSED BY ELECTRIC SPARK, 34
THE RETURN SHOCK ILLUSTRATED, 35
PROTECTION FROM LIGHTNING BY A CLOSED CONDUCTOR, 48
INDUCTION EFFECT OF LEYDEN JAR DISCHARGE, 56
LECTURE I.
LIGHTNING AND THUNDER.
The electricity produced by an ordinary electric machine exhibits,
under certain conditions, phenomena which bear a striking resemblance
to the phenomena attendant on lightning. In both cases there is a flash
of light; in both there is a report, which, in the case of lightning,
we call thunder; and, in both cases, intense heat is developed, which
is capable of setting fire to combustible bodies. Further, the spark
from an electric machine travels through space with extraordinary
rapidity, and so does a flash of lightning; the spark follows a zig-zag
course, and so does a flash of lightning; the spark moves silently and
harmlessly through metal rods and stout wires, while it forces its
way, with destructive effect, through bad conductors, and it is so,
too, with a flash of lightning. Lastly, the electricity of a machine
is capable of giving a severe shock to the human body; and we know
that lightning gives a shock so severe as usually to cause immediate
death. For these reasons it was long conjectured by scientific men
that lightning is, in its nature, identical with electricity; and that
it differs from the electricity of our machines only in this, that it
exists in a more powerful and destructive form.
=Identity of Lightning and Electricity.=--But it was reserved for
the celebrated Benjamin Franklin to demonstrate the truth of this
conjecture by direct experiment. He first conceived the idea of drawing
electricity from a thundercloud in the same way as it is drawn from
the conductor of an electric machine. For this purpose he proposed
to place a kind of sentry-box on the summit of a lofty tower, and to
erect, on the sentry-box, a metal rod, projecting twenty or thirty
feet upward into the air, pointed at the end, and having no electrical
communication with the earth. He predicted that when a thundercloud
would pass over the tower, the metal rod would become charged with
electricity, and that an observer, stationed in the sentry-box, might
draw from it, at pleasure, a succession of electric sparks.
With the magnanimity of a really great man, Franklin published this
project to the world; being more solicitous to extend the domain of
science by new discoveries, than to secure for himself the glory
of having made them. The project was set forth in a letter to Mr.
Collinson, of London, which bears date July 29, 1750, and which, in the
course of a year or two, was translated into the principal languages
of Europe. Two years later the experiment suggested by Franklin was
made by Monsieur Dalibard, a wealthy man of science, at his villa near
Marly-la-Ville, a few miles from Paris. In the middle of an elevated
plain Monsieur Dalibard erected an iron rod, forty feet in length, one
inch in diameter, and ending above in a sharp steel point. The iron rod
rested on an insulating support, and was kept in position by means of
silk cords.
In the absence of Monsieur Dalibard, who was called by business to
Paris, this apparatus was watched by an old dragoon, named Coiffier;
and on the afternoon of the tenth of May, 1752, he drew sparks from the
lower end of the rod at the time that a thundercloud was passing over
the neighborhood. Conscious of the importance that would be attached to
this phenomenon, the old dragoon summoned, in all haste, the prior of
Marly to come and witness it. The prior came without delay, and he was
followed by some of the principal inhabitants of the village. In the
presence of the little group, thus gathered together, the experiment
was repeated--electric sparks were again drawn, in rapid succession,
from the iron rod; the prediction of Franklin was fulfilled to the
letter; and the identity of lightning and electricity was, for the
first time, demonstrated to the world.
=Franklin’s Experiment.=--Meanwhile Franklin had been waiting, with
impatience, for the completion of the tower of Christchurch, in
Philadelphia, on which he intended to make the experiment himself.
He even collected money, it is said, to hasten on the building.
But, notwithstanding his exertions, the progress of the tower was
slow; and his active mind, which could ill brook delay, hit upon
another expedient, remarkable alike for its simplicity and for its
complete success. He constructed a boy’s kite, using, however, a silk
pockethandkerchief, instead of paper, that it might not be damaged by
rain. To the top of the kite he attached a pointed iron wire about a
foot long, and he provided a roll of hempen twine, which he knew to be
a conductor of electricity, for flying it. This was the apparatus with
which he proposed to explore the nature of a thundercloud.
The thundercloud came late in the afternoon of the fourth of July,
1752, and Franklin sallied out with his kite, accompanied by his son,
and taking with him a common door-key and a Leyden jar. The kite
was soon high in air, and the philosopher awaited the result of his
experiment, standing, with his son, under the lee of a cowshed, partly
to protect himself from the rain that was coming, and partly, it is
said, to shield himself from the ridicule of passers-by, who, having
no sympathy with his philosophical speculations, might be inclined to
regard him as a lunatic. To guard against the danger of receiving a
flash of lightning through his body, he held the kite by means of a
silk ribbon, which was tied to the door-key, the door-key being itself
attached to the lower end of the hempen string.
A flash of lightning soon came from the cloud, and a second, and a
third; but no sign of electricity could be observed in the kite, or
the hempen cord, or the key. Franklin was almost beginning to despair
of success, when suddenly he noticed that the little fibres of the
cord began to bristle up, just as they would if it were placed near an
electric machine in action. He presented the door-key to the knob of
the Leyden jar, and a spark passed between them. Presently a shower
began to fall; the cord, wetted by the rain, became a better conductor
than it had been before, and sparks came more freely. With these sparks
he now charged the Leyden jar, and found, to his intense delight, that
he could exhibit all the phenomena of electricity by means of the
lightning he had drawn from the clouds.
In the following year a similar experiment, with even more striking
results, was carried out, in France, by de Romas. Though it is said he
had no knowledge of what Franklin had done in America, he, too, used
a kite; and, with a view of making the string a better conductor, he
interlaced with it a thin copper wire. Then, flying his kite in the
ordinary way, when it had risen to a height of about 550 feet, he drew
sparks from it which, we are told, were upwards of nine feet long, and
emitted a sound like the report of a pistol.
=Fatal Experiment of Richman.=--There can be no doubt that experiments
of this kind, made with the electricity of a thundercloud, were
extremely dangerous; and this was soon proved by a fatal accident.
Professor Richman, of St. Petersburgh, had erected on the roof of his
house a pointed iron rod, the lower end of which passed into a glass
vessel, intended, as we are informed, to measure the strength of the
charge which he expected to receive from the clouds. On the sixth of
August, 1753, observing the approach of a thunderstorm, he hastened
to his apparatus; and as he stood near it, with his head bent down,
to watch the effect, a flash of lightning passed through his body and
killed him on the spot. This catastrophe served to fix public attention
on the danger of such experiments, and gave occasion to the saying of
Voltaire: “There are some great lords whom we should always approach
with extreme precaution, and lightning is one of them.”[1] From this
time the practice of making experiments directly with the lightning of
the clouds seems to have been, by common consent, abandoned.
=Immediate Cause of Lightning.=--And now, having set before you some
of the most memorable experiments by which the identity of lightning
and electricity has been demonstrated, I will try to give you a clear
conception regarding the immediate cause of lightning, so far as the
subject is understood at the present day by scientific men. You know
that there are two kinds of electricity, which are called _positive_
and _negative_; and that each of them repels electricity of the same
kind as itself, while it attracts electricity of the opposite kind.
Now, every thundercloud is charged with electricity of one kind or
the other, positive or negative; and, as it hovers over the earth, it
develops, by what is called _induction_, or influence, electricity
of the opposite kind in that part of the earth which is immediately
under it. Thus we have two bodies--the cloud and the earth--charged
with opposite kinds of electricity, and separated by a stratum of the
atmosphere. The two opposite electricities powerfully attract each
other; but for a time they are prevented from rushing together by the
intervening stratum of air, which is a non-conductor of electricity,
and acts as a barrier between them. As the electricity, however,
continues to accumulate, the attraction becomes stronger and stronger,
until at length it is able to overcome the resistance of this barrier;
a violent disruptive discharge then takes place between the cloud
and the earth, and the flash of lightning is the consequence of the
discharge.
[Illustration: THE ELECTRIC SPARK; A TYPE OF A FLASH OF LIGHTNING.]
The whole phenomenon may be illustrated, on a small scale, by means
of this electric machine of Carré’s which you see before you. When
my assistant turns the handle of the machine negative electricity is
developed in that large brass cylinder, which in our experiment will
represent the thundercloud. At a distance of five or six inches from
the cylinder I hold a brass ball, which is in electrical communication
with the earth through my body. The electrified brass cylinder acts
by induction, or influence on the brass ball, and develops in it, as
well as in my body, a charge of positive electricity. Now, the positive
electricity of the ball and the negative electricity of the cylinder
are mutually attracting each other, but the intervening stratum of air
offers a resistance which prevents a discharge from taking place. My
assistant, however, continues to work the machine; the two opposite
electricities rapidly accumulate on the cylinder and the ball; at
length their mutual attraction is strong enough to overcome the
resistance interposed between them; a disruptive discharge follows,
and at the same moment a spark is seen to pass, accompanied by a sharp
snapping report.
This spark is a miniature flash of lightning; and the snapping report
is a diminutive peal of thunder. Furthermore, at the moment the spark
passes you may observe a slight convulsive movement in my hand and
wrist. This convulsive movement represents, on a small scale, the
violent shock, generally fatal to life, which is produced by a flash of
lightning when it passes through the body.
I can continue to take sparks from the conductor as long as the machine
is worked; and it is interesting to observe that these sparks follow
an irregular zig-zag course, just as lightning does. The reason is the
same in both cases: a discharge between two electrified bodies takes
place along the line of least resistance; and, owing to the varying
condition of the atmosphere, as well as of the minute particles of
matter floating in it, the line of least resistance is almost always a
zig-zag line.
=What a Flash of Lightning is.=--Lightning, then, may be conceived as
an electrical discharge, sudden and violent in its character, which
takes place, through the atmosphere, between two bodies highly charged
with opposite kinds of electricity. Sometimes this electrical discharge
passes, as I have said, between a cloud and the earth; sometimes it
passes between one cloud and another; sometimes, on a smaller scale,
it takes place, between the great mass of a cloud and its outlying
fragments.
But, if you ask me in what the discharge itself consists, I am utterly
unable to tell you. It is usual to speak and write on this subject as
if electricity were a material substance, a very subtle fluid, and as
if, at the moment the discharge takes place, this fluid passes like a
rapid stream, from the body that is positively electrified to the body
that is negatively electrified. But we must always remember that this
is only a conventional mode of expression, intended chiefly to assist
our conceptions, and to help us to talk about the phenomena. It does
not even profess to represent the objective truth. All that we know
for certain is this: that immediately before the discharge the two
bodies are highly electrified with opposite kinds of electricity; and,
that immediately after the discharge, they are found to have returned
to their ordinary condition, or, at least, to have become less highly
electrified than they were before.
The flash of light that accompanies an electric discharge is often
supposed to be the electricity itself, passing from one body to
the other. But it is not; it is simply an effect produced by the
discharge. Heat is generated by the expenditure of electrical energy,
in overcoming the resistance offered by the atmosphere; and this heat
is so intense, that it produces a brilliant incandescence along the
path of the discharge. When a spark appears, for example, between the
conductor of the machine and this brass ball, it can be shown, by very
satisfactory evidence, that minute particles of these solid bodies are
first converted into vapor, and then made to glow with intense heat.
The gases, too, of which the air is composed, and the solid particles
floating in the air, are likewise raised to incandescence. So, too,
with lightning; the flash of light is due to the intense heat generated
by the electrical discharge, and owes its character to the composition
and the density of the atmosphere through which the discharge passes.
=Duration of a Flash of Lightning.=--How long does a flash of lightning
last? You are aware, I dare say, that when an impression of light is
made on the eye, the impression remains for a sensible interval of
time, not less than the tenth of a second, after the source of light
has been extinguished or removed. Hence we continue, in fact, to see
the light, for at least the tenth of a second, after the light has
ceased. Now, if you reflect how brief is the moment for which a flash
of lightning is visible, and if you deduct the tenth of a second from
that brief moment, you will see, at once, that the period of its actual
duration must be very short indeed.
The exact duration of a flash of lightning is a question on which no
settled opinion has yet been accepted generally by scientific men.
Indeed, the most widely different statements have been made on the
subject, quite recently, by the highest authorities, each speaking
apparently with unhesitating confidence. Thus, for example, Professor
Mascart describes an experiment, which he says was made by Wheatstone,
and which showed that a flash of lightning lasts for less than
_one_-thousandth of a second;[2] Professor Everett describes the same
experiment, without saying by whom it was made, and gives, as the
result, that “the duration of the illumination produced by lightning
is certainly less than the _ten_-thousandth of a second;”[3] Professor
Tyndall, in his own picturesque way, tells us that “a flash of
lightning cleaves a cloud, appearing and disappearing in less than the
_hundred_-thousandth of a second;”[4] and according to Professor Tait,
of Edinburgh, “Wheatstone has shown that lightning certainly lasts less
than the _millionth_ of a second.”[5]
=Experiments of Professor Rood.=--I cannot say which of these
statements is best supported by actual observation; for none of the
writers I have quoted gives any reference to the original memoir from
which his statement is derived. As far as my own reading goes, I have
only come across one original record of experiments, made directly on
the flash of lightning itself, with a view to determine the period of
its duration. These experiments were carried out by Professor Ogden
Rood, of Columbia College, New York, between the years 1870 and 1873,
and are recorded in the _American Journal of Science and Arts_.[6]
For the description of his apparatus, and for the details of his
observations, I must refer you to the memoir itself; but I may tell you
briefly that the results at which he arrived, if they be accepted, must
lead to a considerable modification of the views previously entertained
on the subject. In the first place, he satisfied himself that what
appears to the eye a single flash of lightning is usually, if not
always, multiple in its character; consisting, in fact, of a succession
of distinct flashes, which follow one another with such rapidity as
to make a continuous impression on the retina. Next, he proceeded to
measure approximately the duration of these several component flashes;
and he found that it varied over a wide range, amounting sometimes to
fully the twentieth of a second, and being sometimes less than the
sixteen-hundredth of a second.
=Wheatstone’s Experiments.=--These results are extremely interesting;
but we can hardly regard them as finally established, until they have
been confirmed by other observers. I may remark, however, that they
fit in very well with the experiments made by Professor Wheatstone,
many years ago, on the duration of the electric spark, which, as I told
you, is a miniature flash of lightning. In these classical experiments,
which leave nothing to be desired in point of accuracy, Professor
Wheatstone showed that a spark taken directly from a Leyden jar, or a
spark taken from the conductor of a powerful electric machine, that is,
just such a spark as you have seen here to-day, lasts for less than the
millionth of a second.
But he also showed that the duration of the spark is greatly increased,
when a resisting wire is introduced into the path of the discharge.
Thus, for example, when the discharge from a Leyden jar was made to
pass through half a mile of copper wire, with breaks at intervals, the
sparks that appeared at these breaks were found to last for ¹⁄₂₄₀₀₀
of a second.[7] Hence we should naturally expect that the period of
illumination would be still further increased, in the case of a flash
of lightning, where the resistance interposed is enormously greater
than in either of the experiments made by Wheatstone.[8]
=Experiment of the Rotating Disk.=--It would be tedious, on an occasion
like the present, to enter into an account of Wheatstone’s beautiful
and ingenious method of investigation, by which the above facts have
been established; but I will show you a much more simple experiment
which brings home very forcibly to the mind how exceedingly short
must be the duration of the electric spark. Here is a circular disk
of cardboard, the outer part of which, as you see, is divided into
sectors, black and white alternately, while the space about the centre
is entirely white. The disk is mounted on a stand, by means of which
I can make it rotate with great velocity. When it is put in rotation,
the effect on the eye is very striking--the central space remains white
as before, but in the outer rim the distinction of black and white
absolutely disappears and gives place to a uniform gray. This color is
due to the blending together of black and white in equal proportions;
the blending being effected, not on the cardboard disk, but on the
retina of the eye.
[Illustration: CARDBOARD DISK AS SEEN WHEN AT REST.]
[Illustration: SAME DISK AS SEEN WHEN IN RAPID ROTATION.]
I mentioned just now that an impression made on the retina lasts for
the tenth of a second after the cause of it has been removed. Now, when
this disk is in rotation, the sectors follow one another so rapidly
that the particular part of space occupied at any moment by a white
sector will be occupied by a black sector within a time much less than
the tenth of a second. It follows that the impression made by each
white sector remains on the retina until the following black sector
comes into the same position; and, in like manner, the impression made
by each black sector remains until the following white sector takes up
the position of the black. Therefore, the impression made by the whole
outer rim is the impression of black and white combined--that is, the
impression of gray.
So far, I dare say, the phenomenon is already familiar to you all.
But I propose now to show you the revolving disk illuminated by
the electric spark; and you will observe that, at the moment of
illumination, the black and white sectors come out as clearly and
distinctly as if the disk were standing still.
For the success of this experiment it is desirable, not only to have
a brilliant spark in order to secure a good illumination of the disk,
but also to have a succession of such sparks, that you may see the
phenomenon frequently repeated, and thus be able to observe it at your
leisure. To attain these two objects, I have made the arrangement which
is here before you.
In front of the disk is a large and very powerful Leyden jar. The
rod connected with the inner coating rises well above the mouth of
the jar, and ends in a brass ball nearly opposite the centre of the
disk. Connected with the outer coating of the jar is another rod
which likewise ends in a brass ball, and which is so adjusted that
the distance between the two balls is about an inch. The two rods are
connected respectively with the two conductors of a Holtz machine, so
that, when the machine is worked, the jar is first quickly charged,
and then it discharges itself, with a brilliant spark, between the
two brass balls. Thus, by continuing to work the machine, we can get,
as long as we choose, a succession of sparks following one another at
short and regular intervals right in front of the disk.
Everything being now ready, and the room partially darkened, the
disk is put in rapid rotation; and you can see, by the twilight that
remains, the outer rim a uniform gray, and the central space white.
But when my assistant begins to turn the Holtz machine, and brilliant
sparks leap out at intervals, the revolving disk, illuminated for a
moment at each discharge, seems to be standing still, and shows the
black and white sectors distinctly visible.
The reason of this is clear: So brief is the moment for which the spark
endures, that the disk, though in rapid motion, makes no sensible
advance during that small fraction of time; therefore, in the image on
the retina, the impression made by the white sectors remains distinct
from the impression made by the black, and the eye sees the disk as it
really is.
I may notice, in passing, a very interesting consideration, suggested
by this experiment. A cannon ball is now commonly discharged with a
velocity of about 1,600 feet a second. Moving with this velocity it
is, as you know, under ordinary circumstances, altogether invisible to
the eye. But suppose it were illuminated, in the darkness of night,
by this electric spark, which lasts, we will say, for the millionth
of a second. During the moment of illumination, the cannon ball moves
through the millionth part of 1,600 feet, which is a little less than
the fiftieth of an inch. Practically, we may say that the cannon ball
does not sensibly change its place while the spark lasts. Further, the
impression it makes on the eye, from the position it occupies at the
moment of illumination, remains on the retina for at least the tenth
of a second. Therefore, if we are looking toward that particular part
of space where the cannon ball happens to be at the moment the spark
passes, we must see the cannon ball hanging motionless in the air,
though we know it is traveling at the rate of 1,600 feet a second, or
about 1,000 miles an hour.
=Brightness of a Flash of Lightning.=--I should like to say one word
about the brightness of a flash of lightning. Somewhat more than
thirty years ago, Professor Swan, of Edinburgh, showed that the eye
requires a sensible time--about the tenth of a second--to perceive
the full brightness of a luminous object. Further, he proved, by a
series of interesting experiments, that when a flash of light lasts
for less than the tenth of a second, its apparent brilliancy to the
eye is proportional to the time of its duration.[9] Now consider the
consequence of these facts in reference to the brightness of our
electric spark. If the spark lasted for the tenth of a second, we
should perceive its full brightness; if it lasted for the tenth part of
that time, we should see only the tenth part of its brightness; if it
lasted for the hundredth part, we should see only the hundredth part
of its brightness; and so on. But we know, in point of fact, that it
lasts for less than the millionth of a second, that is, less than the
hundred-thousandth part of the tenth of a second. Therefore we see only
the hundred-thousandth part of its real brightness.
Here is a startling conclusion, and one, I may say, fully justified
by scientific evidence. That electric spark, brilliant as it appears
to us, is really a hundred thousand times as bright as it seems to
be. We cannot speak with the same precision of a flash of lightning;
because its duration has not yet been so exactly determined. But if we
suppose that a flash of lightning, in a particular case, lasts for the
thousandth of a second, it would follow, from the above experiments,
that the flash is a hundred times as bright, in fact, as it appears to
the eye.
=Various Forms of Lightning.=--The lightning of which I have spoken
hitherto is commonly called _forked_ lightning; a name which seems
to have been derived from the zig-zag line of light it presents to
the eye. But there are other forms under which the electricity of the
clouds often makes itself manifest; and to these I would now invite
your attention for a few moments. The most common of them all, at least
in this country, is that which is familiarly known by the name of
_sheet_ lightning. This is, probably, nothing else than the lighting up
of the atmosphere, or of the clouds, by forked lightning, which is not
itself directly visible.
Generally speaking, after a flash of sheet lightning, we hear the
rolling of distant thunder. But it sometimes happens, especially in
summer time, that the atmosphere is again and again lit up by a sudden
glow of light, and yet no thunder is heard. This phenomenon is commonly
called _summer_ lightning, or _heat_ lightning. It is probably due,
in many cases, to electrical discharges in the higher regions of the
atmosphere, where the air is greatly rarified; and, in these cases, it
would seem to resemble the discharges obtained by means of an induction
coil in glass tubes containing rarified gases. But there is little
doubt that in many cases, too, summer lightning, like ordinary sheet
lightning, is due to forked lightning, which is so remote that we can
neither see the flash itself directly, nor hear the rolling of the
thunder.
Perhaps the most distinct and satisfactory evidence on this subject,
derived from actual observation, is contained in the following letter
of Professor Tyndall, written in May, 1883: “Looking to the south
and south-east from the Bel Alp, the play of silent lightning among
the clouds and mountains is sometimes very wonderful. It may be seen
palpitating for hours, with a barely appreciable interval between
the thrills. Most of those who see it regard it as lightning without
thunder--Blitz ohne Donner, Wetterleuchten, I have heard it named by
German visitors. The Monte Generoso, overlooking the Lake of Lugano, is
about fifty miles from the Bel Alp, as the crow flies. The two points
are connected by telegraph; and frequently when the Wetterleuchten,
as seen from the Bel Alp, was in full play, I have telegraphed to the
proprietor of the Monte Generoso Hotel and learned, in every instance,
that our silent lightning co-existed in time with a thunderstorm more
or less terrific in upper Italy.”[10]
Another form of lightning, described by many writers, is called _globe_
lightning. It is said to appear as a ball of fire, about the size of
a child’s head, or even larger, which moves for a time slowly about,
and then, after the lapse of several seconds, explodes with a terrific
noise, sending forth flashes of fire in all directions, which burn
whatever they strike. Many accounts are on record of such phenomena;
but they are derived, for the most part, from the evidence of persons
who were not specially competent to observe, and to describe with
precision, the facts that fell under their observation. Hence these
accounts, while they are accepted by some, are rejected by others;
and it seems to me, in the present state of the question, that the
existence of globe lightning can hardly be regarded as a demonstrated
fact. At all events, if phenomena of this kind have really occurred, I
can only say that nothing we know about electricity, at present, will
enable us to account for them.[11]
=St. Elmo’s Fire.=--A much more authentic and, at the same time, very
interesting form, under which the electricity of the clouds sometimes
manifests its presence, is known by the name of St. Elmo’s fire. This
phenomenon at one time presents the appearance of a star, shining at
the points of the lances or bayonets of a company of soldiers; at
another, it takes the form of a tuft of bluish light, which seems to
stream away from the masts and spars of a ship at sea, or from the
pointed spire of a church. It was well known to the ancients. Cæsar,
in his Commentaries, tells us that, after a stormy night, the iron
points of the javelins of the fifth legion seemed to be on fire; and
Pliny says that he saw lights, like stars, shining on the lances of
the soldiers, keeping watch by night upon the ramparts. When two
such lights appeared at once, on the masts of a ship, they were
called Castor and Pollux, and were regarded by sailors as a sign of a
prosperous voyage. When only one appeared, it was called Helen, and was
taken as an unfavorable omen.
In modern times St. Elmo’s fire has been witnessed by a host of
observers, and all its various phases have been repeatedly described.
In the memoirs of Forbin we read that, when he was sailing once, in
1696, among the Balearic Islands, a sudden storm came on during the
night, accompanied by lightning and thunder. “We saw on the vessel,” he
says, “more than thirty St. Elmo’s fires. Among the rest there was one
on the vane of the mainmast more than a foot and a half high. I sent a
man up to fetch it down. When he was aloft he cried out that it made
a noise like wetted gunpowder set on fire. I told him to take off the
vane and come down; but, scarcely had he removed it from its place,
when the fire left it and reappeared at the end of the mast, so that
it was impossible to take it away. It remained for a long time, and
gradually went out.”
On the 14th of January, 1824, Monsieur Maxadorf happened to look at a
load of straw in the middle of a field just under a dense black cloud.
The straw seemed literally on fire--a streak of light went forth from
every blade; even the driver’s whip shone with a pale-blue flame. As
the black cloud passed away, the light gradually disappeared, after
having lasted about ten minutes. Again, it is related that on the 8th
of May, 1831, in Algiers, as the French artillery officers were walking
out after sunset without their caps, each one saw a tuft of blue light
on his neighbor’s head; and, when they stretched out their hands, a
tuft of light was seen at the end of every finger. Not infrequently a
traveler in the Alps sees the same luminous tuft on the point of his
alpenstock. And quite recently, during a thunderstorm, a whole forest
was observed to become luminous just before each flash of lightning,
and to become dark again at the moment of the discharge.[12]
This phenomenon may be easily explained. It consists in a gradual and
comparatively silent electrical discharge between the earth and the
cloud; and generally, but not always, it has the effect of preventing
such an accumulation of electricity as would be necessary to produce
a flash of lightning. I can illustrate this kind of discharge with
the aid of our machine. If I hold a pointed metal rod toward the
large conductor, you can see, when the machine is worked and the room
darkened, how the point of the rod becomes luminous and shines like a
faint blue star. I substitute for the pointed rod the blunt handles of
a pair of pliers, and a tuft of blue light is at once developed at the
end of each handle, and seems to stream away with a hissing noise. I
now put aside the pliers, and open out my hand under the conductor--and
observe how I can set up, at pleasure, a luminous tuft at the tips
of my fingers. Now and then a spark passes, giving me a smart shock,
and showing how the electricity may sometimes accumulate so fast that
it cannot be sufficiently discharged by the luminous tuft. Lastly, I
present a small bushy branch of a tree to the conductor, and all its
leaves and twigs are aglow with bluish light, which ceases for a moment
when a spark escapes, to be again renewed when electricity is again
developed by the working of the machine.
[Illustration: THE BRUSH DISCHARGE, ILLUSTRATING ST. ELMO’S FIRE.]
Now, if you put a thundercloud in the place of that conductor, you can
easily realize how, through its influence, the lance and bayonet of
the soldier, the alpenstock of the traveler, the pointed spire of a
church, the masts of a ship at sea, the trees of a forest, can all be
made to glow with a silent electrical discharge which may or may not,
according to circumstances, culminate at intervals in a genuine flash
of lightning.
=Origin of Lightning.=--When we seek to account for the origin of
lightning, we are confronted at once with two questions of great
interest and importance--first, What are the sources from which the
electricity of the thundercloud is derived? and, secondly, How does
this electricity come to be developed in a form which so far transcends
in power the electricity of our machines? These questions have long
engaged the attention of scientific men, but I cannot say that they
have yet received a perfectly satisfactory solution. Nevertheless,
some facts of great scientific value have been established, and some
speculations have been put forward, which are well deserving of
consideration.
In the first place, it is quite certain that the atmosphere which
surrounds our globe is almost always in a state of electrification.
Further, the electrical condition of the atmosphere would seem to be
as variable as the wind. It changes with the change of season; it
changes from day to day; it changes from hour to hour. The charge of
electricity is sometimes positive, sometimes negative; sometimes it
is strong, sometimes feeble; and the transition from one condition to
another is sometimes slow and gradual, sometimes sudden and violent.
As a general rule, in fine, clear weather, the electricity of the
atmosphere is positive, and not very strongly developed. In wet weather
the charge may be either positive or negative, and is generally
strong, especially when there are sudden heavy showers. In fog it is
also strong, and almost always positive. In a snowstorm it is very
strong, and most frequently positive. Finally, in a thunderstorm it is
extremely strong, and generally negative; but it is subject to a sudden
change of sign, when a flash of lightning passes or when rain begins to
fall.
So far I have simply stated facts, which have been ascertained
by careful observations, made at different stations by competent
observers, and extending over a period of many years. But as regards
the process by which the electricity of the atmosphere is developed, we
have, up to the present time, no certain knowledge. It has been said
that electricity may be generated in the atmosphere by the friction of
the air itself, and of the minute particles floating in it, against
the surface of the earth, against trees and buildings, against rocks,
cliffs, and mountains. But this opinion, however probable it may be,
has not yet been confirmed by any direct experimental investigation.
The second theory is that the electricity of the atmosphere is due, in
great part at least, to the evaporation of salt water. Many years ago,
Pouillet, a French philosopher, made a series of experiments in the
laboratory, which seemed to show that evaporation is generally attended
with the development of electricity; and, in particular, he satisfied
himself that the vapor which passes off from the surface of salt water
is always positively electrified. Now, the atmosphere is everywhere
charged, more or less, with vapor which comes, almost entirely, from
the salt water of the ocean. Hence Pouillet inferred that the chief
source of atmospheric electricity is the evaporation of sea water.
This explanation would certainly go far to account for the presence
of electricity in the atmosphere, if the fact on which it rests were
established beyond dispute. But there is some reason to doubt whether
the development of electricity, in the experiments of Pouillet, was due
simply to the process of evaporation, and not rather to other causes,
the influence of which he did not sufficiently take into account.
A conjecture has recently been started that electricity may be
generated by the mere impact of minute particles of water vapor against
minute particles of air.[13] If this conjecture could be established as
a fact, it would be amply sufficient to account for all the electricity
of the atmosphere. From the very nature of a gas, the molecules of
which it is composed are forever flying about with incredible velocity;
and therefore the particles of water vapor and the particles of air,
which exist together in the atmosphere, must be incessantly coming
into collision. Hence, however small may be the charge of electricity
developed at each individual impact, the total amount generated over
any considerable area, in a single day, must be very great indeed.
It is evident, however, that this method of explaining the origin of
atmospheric electricity can only be regarded as, at best, a probable
hypothesis, until the assumption on which it rests is supported by the
evidence of observation or experiment.
=Length of a Flash of Lightning.=--It would seem, then, that we are
not yet in a position to indicate with certainty the sources from
which the electricity of the atmosphere is derived. But whatever
these sources may be, there can be little doubt that the electricity
of the atmosphere is intimately associated with the minute particles
of water vapor of which the thundercloud is eventually built up.
This consideration is of great importance when we come to consider
the special properties of lightning, as compared with other forms of
electricity. The most striking characteristic of lightning is the
wonderful power it possesses of forcing its way through the resisting
medium of the air. In this respect it incomparably surpasses all forms
of electricity that have hitherto been produced by artificial means.
The spark of an ordinary electric machine can leap across a space of
three or four inches; the machine we have employed in our experiments
to-day can give, under favorable circumstances, a spark of nine or ten
inches; the longest electric spark ever yet produced artificially is
probably the spark of Mr. Spottiswoode’s gigantic induction coil; and
it does not exceed three feet six inches. But the length of a flash of
lightning is not to be measured in inches, or in feet or in yards; it
varies from one or two miles, for ordinary flashes, to eight or ten
miles in exceptional cases.
This power of discharging itself violently through a resisting
medium, in which the thundercloud so far transcends the conductor of
an electric machine, is due to the property commonly known among
scientific men as electrical _potential_. The greater the distance to
which an electrified body can shoot its flashes through the air, the
higher must be its potential. Hence the potential of a thundercloud
must be exceedingly high, since its flashes can pierce the air to a
distance of several miles. And what I want to point out is, that we
are able to account for this exceedingly high potential, if we may
only assume that the minute particles of water vapor in the atmosphere
have, from any cause, received ever so small a charge of electricity.
The number of such particles that go to make up an ordinary drop of
rain are to be counted by millions of millions; and it is capable of
scientific proof that, as each new particle is added, in the building
up of the drop, a rise of potential is necessarily produced. It is
clear, therefore, that there is practically no limit to the potential
that may be developed by the simple agglomeration of very small cloud
particles, each carrying a very small charge of electricity.[14]
This explanation, which traces the exceedingly high potential of
lightning to the building up of rain drops in the thundercloud,
suggests a reason why it so often happens that immediately after a
flash of lightning “the big rain comes dancing to the earth.” The
potential has been steadily rising as the drops have been getting
larger and larger, until at length the potential has become so high
that the thundercloud is able to discharge itself, and almost at the
same moment the drops have become so large that they can no longer be
held aloft against the attracting force of gravity.
=Physical Cause of Thunder.=--Let us now proceed to consider the
phenomenon of thunder, which is so intimately associated with
lightning, and which, though perfectly harmless in itself, and though
never heard until the real danger is past, often excites more terror
in the mind than the lightning flash itself. The sound of thunder,
like that of the electric spark, is due to a disturbance caused in
the air by the electric discharge. The air is first expanded by the
intense heat that is developed along the line of discharge, and then it
rushes back again to fill up the partial vacuum which its expansion has
produced. This sudden movement gives rise to a series of sound waves,
which reach the ear in the form of thunder. But there are certain
peculiar characteristics of thunder which are deserving of special
consideration.
=Rolling of Thunder.=--They may be classified, I think, under two
heads. First, the sound of thunder is not an instantaneous report
like the sound of the electric spark--it is a prolonged peal lasting,
sometimes, for several seconds. Secondly, each flash of lightning gives
rise, not to one peal only, but to a succession of peals following
one another at irregular intervals. These two phenomena, taken
together, produce that peculiar effect on the ear which is commonly
described as the _rolling_ of thunder; and both of them, I think, may
be sufficiently accounted for in accordance with the well-established
properties of sound.
To understand why the sound of thunder reaches the ear as a prolonged
peal, we have only to remember that sound takes time to travel. Since a
flash of lightning is practically instantaneous, we may assume that the
sound is produced at the same moment all along the line of discharge.
But the sound waves, setting out at the same moment from all points
along the line of discharge, must reach the ear in successive instants
of time, arriving first from that point which is nearest to the
observer, and last from that point which is most distant. Suppose, for
example, that the nearest point of the flash is a mile distant from the
observer, and the farthest point two miles--the sound will take about
five seconds to come from the nearest point, and about ten seconds to
come from the farthest point; and moreover, in each successive instant
from the time the first sound reaches the ear, sound will continue
to arrive from the successive points between. Therefore the thunder,
though instantaneous in its origin, will reach the ear as a prolonged
peal extending over a period of five seconds.
=Succession of Peals.=--The succession of peals produced by a single
flash of lightning is due to several causes, each one of which may
contribute more or less, according to circumstances, toward the general
effect. First, if we accept the results arrived at by Professor Ogden
Rood, of Columbia College, what appears to the eye as a single flash
of lightning, consists, in fact, as a general rule, of a succession
of flashes, each one of which must naturally produce its own peal of
thunder; and although the several flashes, if they follow one another
at intervals of the tenth of a second, will make one continuous
impression on the eye, the several peals of thunder, under the same
conditions, will impress the ear as so many distinct peals.