Mediterranean, and we read as follows in the log of the ship: “North
end of Minorca; heavy squalls; hail, rain, thunder, and lightning.
12.15, ship struck by lightning, which knocked three men from the
masthead, one killed. 12.30, ship again struck; main top-mast shivered
in pieces; many men struck senseless on the decks. Ship again struck,
and set on fire in the masts and rigging; mainmast shivered in pieces;
fore top-mast shivered; men benumbed on the decks, and knocked out
of the top; one man killed on the spot. 1.30, cut away the mainmast;
employed clearing wreck. 4, moderate; set the foresail.”[22]
Again, in 1810, the _Repulse_, a ship of seventy-four guns, was struck,
off the coast of Spain. “The wind had been variable in the morning--and
at 12.35 there was a heavy squall, with rain, thunder, and lightning.
The ship was struck by two vivid flashes of lightning, which shivered
the maintop-gallant mast, and severely damaged the mainmast. Seven men
were killed on the spot; three others only survived a few days; and ten
others were maimed for life. After the second discharge the rain fell
in torrents. The ship was more completely crippled than if she had been
in action, and the squadron, then engaged on a critical service, lost
for a time one of its fastest and best ships.”[23]
=Destruction of Powder Magazines.=--Not less appalling is the
devastation caused by lightning when it falls on a powder magazine.
Here is a striking example: On the eighteenth of August, 1769, the
tower of St. Nazaire, at Brescia, was struck by lightning. Underneath
the tower about 200,000 pounds of gunpowder, belonging to the Republic
of Venice, were stored in vaults. The powder exploded, leveling to
the ground a great part of the beautiful city of Brescia, and burying
thousands of its inhabitants in the ruins. It is said that the tower
itself was blown up bodily to a great height in the air, and came down
in a shower of stones. This is, perhaps, the most fearful disaster of
the kind on record. But we are not without examples in our own times.
In the year 1856 the lightning fell on the Church of St. John, in the
Island of Rhodes. A large quantity of gunpowder had been deposited in
the vaults of the church. This was ignited by the flash; the building
was reduced to a mass of ruins, a large portion of the town was
destroyed, and a considerable number of the inhabitants were killed.
Again, in the following year, the magazine of Joudpore, in the Bombay
Presidency, was struck by lightning. Many thousand pounds of gunpowder
were blown up, five hundred houses were destroyed, and nearly a
thousand people are said to have been killed.[24]
=Experimental Illustrations.=--And now, before proceeding further,
I will make one or two experiments, with a view of showing that the
electricity of our machines is capable of producing effects similar
to those produced by lightning, though immeasurably inferior in point
of magnitude. Here is a common tumbler, about three-quarters full of
water. Into it I introduce two bent rods of brass, which are carefully
insulated below the surface of the water by a covering of india-rubber.
The points, however, are exposed, and come to within an inch of one
another, near the bottom of the tumbler. Outside the tumbler, the
brass rods are mounted on a stand, by means of which I can send the
full charge of this Leyden jar battery through the water, from point
to point. Since water is a bad conductor of electricity, as compared
with metals, the charge encounters great resistance in passing through
it, and in overcoming this resistance produces considerable mechanical
commotion, which is usually sufficient to shiver the glass to pieces.
To charge the battery will take about twenty turns of this large Holtz
machine. Observe how the pith ball of the electroscope rises as the
machine is worked, showing that the charge is going in. And now it
remains stationary; which is a sign that the battery is fully charged,
and can receive no more. You will notice that the outside coating of
the battery has been already connected with one of the brass rods
dipping into the tumbler of water. By means of this discharger I will
now bring the inside coating into connection with the other rod. And
see, before contact is actually made, the spark has leaped across, and
our tumbler is violently burst asunder from top to bottom.
[Illustration: GLASS VESSEL BROKEN BY DISCHARGE OF LEYDEN JAR BATTERY.]
This will probably appear to you a very small affair, when compared
with the tearing asunder of solid masonry, and the hurling about of
stones by the ton weight. No doubt it is; and that is just one of
the lessons we have to learn from the experiment we have made. For,
not only does it show us that effects of this kind may be caused by
electricity artificially produced, but it brings home forcibly to the
mind how incomparably more powerful is the lightning of the clouds than
the electricity of our machines.
The property which electricity has of setting fire to combustible
substances may be easily illustrated. This india-rubber tube is
connected with the gas pipe under the floor, and to the end of the
tube is fitted a brass stop-cock which I hold in my hand. I open
the cock, and allow the jet of gas to flow toward the conductor of
Carré’s machine, while my assistant turns the handle; a spark passes,
and the gas is lit. Again, my assistant stands on this insulating
stool, placing his hand on the large conductor of the machine, while
I turn the handle. His body becomes electrified, and when he presents
his knuckle to this vessel of spirits of wine, which is electrically
connected with the earth, a spark leaps across, and the spirits of wine
are at once in a blaze. Once more; I tie a little gun-cotton around
one knob of the discharging rod, and then use it to discharge a small
Leyden jar; at the moment of the discharge the gun-cotton is set on
fire.
It would be easy to explode gunpowder with the electric spark, but the
smoke of the explosion would make the lecture-hall very unpleasant for
the remainder of the lecture. I propose, therefore, to substitute for
gunpowder an explosive mixture of oxygen and hydrogen, with which I
have filled this little metal flask, commonly known as Volta’s pistol.
By a very simple contrivance, the electric spark is discharged through
the mixture, when I hold the flask toward the conductor of the machine.
A cork is fitted tightly into the neck of the flask, and at the moment
the spark passes you hear a loud explosion, and you see the cork driven
violently up to the ceiling.
[Illustration: GUN-COTTON SET ON FIRE BY ELECTRIC SPARK.]
=Destruction of Life.=--The last effect of lightning to which I shall
refer, and which, perhaps, more than any other, strikes us with terror,
is the sudden and utter extinction of life, when the lightning flash
descends on man or on beast. So swift is this effect, in most cases,
that death is, in all probability, absolutely painless, and the victim
is dead before he can feel that he is struck. I cannot give you, with
any degree of exactness, the number of people killed every year by
lightning, because the record of such deaths has been hitherto very
imperfectly kept, in almost all countries, and is, beyond doubt, very
incomplete. But perhaps you will be surprised to learn that the number
of deaths by lightning actually recorded is, on an average, in England
about 22 every year, in France 80, in Prussia 110, in Austria 212, in
European Russia 440.[25]
So far as can be gathered from the existing sources of information,
it would seem that the number of persons killed by lightning is, on
the whole, about one in three of those who are struck. The rest are
sometimes only stunned, sometimes more or less burned, sometimes
made deaf for a time, sometimes partially paralyzed. On particular
occasions, however, especially when the lightning falls on a large
assembly of people, the number of persons struck down and slightly
injured, in proportion to the number killed, is very much increased.
An interesting case of this kind is reported by Mr. Tomlinson. “On
the twenty-ninth of August, 1847, at the parish church of Welton,
Lincolnshire, while the congregation were engaged in singing the hymn
before the sermon, and the Rev. Mr. Williamson had just ascended the
pulpit, the lightning was seen to enter the church from the belfry,
and instantly an explosion occurred in the centre of the edifice. All
that could move made for the door, and Mr. Williamson descended from
the pulpit, endeavoring to allay the fears of the people. But attention
was now called to the fact that several of the congregation were lying
in different parts of the church, apparently dead, some of whom had
their clothing on fire. Five women were found injured, and having their
faces blackened and burned, and a boy had his clothes almost entirely
consumed. A respected old parishioner, Mr. Brownlow, aged sixty-eight,
was discovered lying at the bottom of his pew, immediately beneath one
of the chandeliers, quite dead. There were no marks on the body, but
the buttons of his waistcoat were melted, the right leg of his trousers
torn down, and his coat literally burnt off. His wife in the same pew
received no injury.”[26]
[Illustration: VOLTA’S PISTOL; EXPLOSION CAUSED BY ELECTRIC SPARK.]
Not less striking is the story told by Dr. Plummer, surgeon of the
Illinois Volunteers, in the _Medical and Surgical Reporter_ of June 19,
1865: “Our regiment was yesterday the scene of one of the most terrible
calamities which it has been my lot to witness. About two o’clock a
violent thunderstorm visited us. While the old guard was being turned
out to receive the new, a blinding flash of lightning was seen,
accompanied instantly by a terrific peal of thunder. The whole of the
old guard, together with part of the new, were thrown violently to the
earth. The shock was so severe and sudden that, in most cases, the rear
rank men were thrown across the front rank men. One man was instantly
killed, and thirty-two men were more or less severely burned by the
electric fluid. In some instances the men’s boots and shoes were rent
from their feet and torn to pieces, and, strange as it may appear, the
men were injured but little in the feet. In all cases the burns appear
as if they had been caused by scalding-hot water, in many instances the
skin being shriveled and torn off. The men all seem to be doing well,
and a part of them will be able to resume their duties in a few days.”
=The Return Shock.=--It sometimes happens that people are struck down
and even killed at the moment a discharge of lightning takes place
between a cloud and the earth, though they are very far from the
point where the flash is actually seen to pass; while others, who are
situated between them and the lightning, suffer very little, or perhaps
not at all. This curious phenomenon was first carefully investigated by
Lord Mahon in the year 1779, and was called by him the “return shock.”
His theory, which is now commonly accepted, may be easily understood
with the aid of the sketch before you.
[Illustration: THE RETURN SHOCK ILLUSTRATED.]
Let us suppose ABC to represent the outline of a thundercloud which
dips down toward the earth at A and at C. The electricity of the cloud
develops by inductive action a charge of the opposite kind in the earth
beneath it. But the inductive action is most powerful at E and F, where
the cloud comes nearest to the earth. Hence, bodies situated near these
points may be very highly electrified as compared with bodies at a
point between them, such as D. Now, when a flash of lightning passes at
E, the under part of the cloud is at once relieved of its electricity,
its inductive action ceases, and, therefore, a person situated at F
suddenly ceases to be electrified. This sudden change from a highly
electrified to a neutral state involves a shock to his system which
may be severe enough to stun or even to kill him. Meanwhile, people
at _D_, having been also electrified to some extent by the influence
of the thundercloud, must in like manner undergo a change in their
electrical condition when the flash of lightning passes, but this
change will be less violent because they were less highly electrified.
Many experiments have been devised to illustrate this theory of Lord
Mahon. But the best illustration I know is furnished by this electric
machine of Carré’s. If you stand near one end of the large conductor
when the machine is in action and sparks are taken from the other end,
you will feel a distinct electric shock every time a spark passes.
The large conductor here takes the place of the cloud, the spark that
passes at one end represents the flash of lightning, and the observer
at the other end gets the return shock, though he is at a considerable
distance from the point where the flash is seen.
An experiment of this kind, of course, cannot be made sensible to a
large audience like the present. But I can give you a good idea of the
effect by means of this tuft of colored papers. While the machine is in
action I hold the tuft of papers near that end of the conductor which
is farthest from the point where the discharge takes place. You see the
paper ribbons are electrified by induction, and, in virtue of mutual
repulsion, stand out from one another “like quills upon the fretful
porcupine.” But, when a spark passes, the inductive action ceases, the
paper ribbons cease to be electrified, and the whole tuft suddenly
collapses into its normal state.
While fully accepting Lord Mahon’s theory of the return shock as
perfectly good so far as it goes, I would venture to point out another
influence which must often contribute largely to produce the effect in
question, and which is not dependent on the form of the cloud. It may
easily happen, from the nature of the surface in the district affected
by a thundercloud, that the point of most intense electrification--say
E in the figure--is in good electrical communication with a distant
point, such as F, while it is very imperfectly connected with a much
nearer point, D. In such a case it is evident that bodies at F will
share largely in the highly-electrified condition of E, and also share
largely in the sudden change of that condition the moment the flash of
lightning passes; whereas bodies at D will be less highly electrified
before the discharge, and less violently disturbed when the discharge
takes place.
This principle may be illustrated by a very simple experiment. Here
is a brass chain about twenty feet long. One end of it I hand to any
one among the audience who will kindly take hold of it; the other end
I hold in my hand. I now stand near the conductor of the machine; and
will ask some one to stand about ten feet away from me, near the middle
of the chain, but without touching it. Now observe what happens when
the machine is worked and I take a spark from the conductor: My friend
at the far end of the chain, twenty feet away, gets a shock nearly
as severe as the one I get myself, because he is in good electrical
communication with the point where the discharge takes place. But
my more fortunate friend, who is ten feet nearer to the flash, is
hardly sensible of any effect, because he is connected with me only
through the floor of the hall, which is, comparatively speaking, a bad
conductor of electricity.
=Summary.=--Let me now briefly sum up the chief destructive effects
of lightning. First, with regard to good conductors: though it passes
harmlessly through them if they be large enough to afford it an easy
passage, it melts and converts them into vapor if they be of such
small dimensions as to offer considerable resistance. Secondly,
lightning acts with great mechanical force on bad conductors; it is
capable of tearing asunder large masses of masonry, and of projecting
the fragments to a considerable distance. Thirdly, it sets fire to
combustible materials. And lastly, it causes the instantaneous death of
men and animals.
=Franklin’s Lightning Rods.=--The object of lightning conductors is to
protect life and property from these destructive effects. Their use was
first suggested by Franklin, in 1749, even before his famous experiment
with the kite; and immediately after that experiment, in 1752, he set
up, on his own house, in Philadelphia, the first lightning conductor
ever made. He even devised an ingenious contrivance, by means of which
he received notice when a thundercloud was approaching. The contrivance
consisted of a peal of bells, which he hung on his lightning conductor,
and which were set ringing whenever the lightning conductor became
charged with electricity.
Franklin’s lightning rods were soon adopted in America; and he himself
contributed very much to their popularity by the simple and lucid
instructions he issued every year, for the benefit of his countrymen,
in the annual publication known as “Poor Richard’s Almanac.” It is
very interesting at this distance of time to read the homely practical
rules laid down by this great philosopher and statesman; and, though
some modifications have been suggested by the experience of a hundred
and thirty years, especially as regards the dimensions of the lightning
conductor, it is surprising to find how accurately the general
principles of its construction, and of its action, are here set forth.
“It has pleased God,” he says, “in His goodness to mankind, at length
to discover to them the means of securing their habitations and other
buildings from mischief by thunder and lightning. The method is this:
Provide a small iron rod, which may be made of the rod-iron used by
nailors, but of such a length that one end being three or four feet in
the moist ground, the other may be six or eight feet above the highest
part of the building. To the upper end of the rod fasten about a foot
of brass wire, the size of a common knitting needle, sharpened to a
fine point; the rod may be secured on the house by a few small staples.
If the house or barn be long, there may be a rod and point at each end,
and a middling wire along the ridge from one to the other. A house thus
furnished will not be damaged by lightning, it being attracted by the
points and passing through the metal into the ground, without hurting
anything. Vessels also having a sharp-pointed rod fixed on the top of
their masts, with a wire from the foot of the rod reaching down round
one of the shrouds to the water, will not be hurt by lightning.”
=Introduction of Lightning Rods into England.=--The progress of
lightning conductors was more slow in England and on the Continent of
Europe, owing to a fear, not unnatural, that they might, in some cases,
draw down the lightning where it would not otherwise have fallen.
People preferred to take their chance of escaping as they had escaped
before, rather than invite, as it were, the lightning to descend on
their houses, in the hope that an iron rod would convey it harmless
to the earth. But the immense amount of damage done every year by
lightning, soon led practical men to entertain a proposal which offered
complete immunity from all danger on such easy terms; and when it was
found that buildings protected by lightning conductors were, over and
over again, struck by lightning without suffering any harm, a general
conviction of their utility was gradually established in the public
mind.
The first public building protected by a lightning rod in England was
St. Paul’s Cathedral, in London. On the eighteenth of June, 1764, the
beautiful steeple of Saint Bride’s Church, in the city, was struck by
lightning and reduced to ruin. This incident awakened the attention of
the dean and chapter of St. Paul’s to the danger of a similar calamity,
which seemed, as it were, impending over their own church. After long
deliberation, they referred the matter to the Royal Society, asking for
advice and instruction. A committee of scientific men was appointed by
the Royal Society to consider the question. Benjamin Franklin himself,
who happened to be in London at the time, as the representative of the
American States in their dispute with England, was nominated a member
of the committee. And the result of its deliberation was that, in the
year 1769, a number of lightning conductors were erected on St. Paul’s
Cathedral.
It was on this occasion that arose the celebrated controversy about
the respective merits of points and balls. Franklin had recommended a
pointed conductor; but some members of the committee were of opinion
that the conductor should end in a ball and not in a point. The
decision of the committee was in favor of Franklin’s opinion, and
pointed conductors were accordingly adopted for St. Paul’s Cathedral.
But the controversy did not end here. The time was one of great
political excitement, and party spirit infused itself even into the
peaceful discussions of science. The weight of scientific opinion was
on the side of Franklin; but it was hinted, on the other side, that the
pointed conductors were tainted with republicanism, and pregnant with
danger to the empire. As a rule, the whigs were strongly in favor of
points; while the Tories were enthusiastic in their support of balls.
For a time the Tories seemed to prevail. The king was on their side.
Experiments on a grand scale were conducted in his presence, at the
Pantheon, a large building in Oxford street; he was assured that these
experiments proved the great superiority of balls over points; and to
give practical effect to his convictions, his majesty directed that a
large cannon ball should be fixed on the end of the lightning conductor
attached to the royal palace at Kew. But the committee of the Royal
Society remained unconvinced. In course of time the heat of party
spirit abated; experience as well as reason was found to be in favor of
Franklin’s views; and the battle of the balls and points has long since
passed into the domain of history.[27]
=Functions of a Lightning Conductor.=--A lightning conductor fulfills
two functions. First, it favors a silent and gradual discharge of
electricity between the cloud and the earth, and thus tends to prevent
that accumulation which must of necessity take place before a flash
of lightning will pass. Secondly, if a flash of lightning come, the
lightning conductor offers it a safe channel through which it may pass
harmless to the earth.
These two functions of a lightning conductor may be easily illustrated
by experiment. When our machine is in action, if I present my closed
hand to the large brass conductor, a spark passes between them, and I
feel, at the same moment, a slight electric shock. Here the conductor
of the machine, as usual, holds the place of the electrified cloud;
my closed hand represents, as it were, a lofty building that stands
out prominently on the surface of the earth; the spark is the flash of
lightning, and the electric shock just suggests the destructive power
of the sudden disruptive discharge.
Now let me protect this building by a lightning conductor. For this
purpose, I take in my hand a brass rod, which I connect with the
earth by a brass chain. In the first instance, I will have a metal
ball on the end of my lightning conductor. You see the effect; sparks
pass rapidly, but I feel no shock. I can increase the strength of
the discharge by hanging this condensing jar on the conductor of the
machine. Sparks pass now, much more brilliant and powerful than before,
but still I get no shock. It is evident, therefore, that my lightning
rod does not prevent the flash from passing, but it conveys it harmless
to the ground.
I next take a rod which is sharply pointed, and connecting it as before
with the earth by a brass chain, I present the sharp point to the
conductor of the machine. Observe how different is the result; there is
no disruptive discharge; no spark passes; no shock is felt. Electricity
still continues to be generated in the machine, and electricity is
generated, by induction, in the brass rod, and in my body. But these
two opposite electricities discharge themselves silently, by means of
this pointed rod, and no sensible effect of any kind is exhibited.