ATMOSPHERIC ELECTRICITY AND STORM-CLOUDS
With such strange facts before us--facts the strangeness and diversity
of which baffle all hypotheses and forbid all definite conclusions--we
can but keep adding to our observations and accumulating other facts
which may tend to elucidate the mystery. The terrible ravages caused
every year by lightning make it necessary for us to find some means of
preventing the recurrence of certain memorable catastrophes. It is
only in the actual investigation of the phenomenon, in the study of
all its smallest manifestations, that we can hope to discover the
methods of the mysterious power.
From the earliest times mankind has devoted much thought to the
subject. If we glance back towards past centuries we find that thunder
and lightning have ever been regarded as a terrible agent of the will
of the powers above.
The strongest and subtlest brains of antiquity, Anaxagoras, Aristotle,
Seneca, were unable to form any kind of reasonable view regarding the
fantastic phenomena resulting from the force of nature and held so
mysterious to us moderns. Thunder and lightning were generally
believed by them to be due to emanations from the earth or to vapours
contained in the air.
The Etruscans, who flourished fifteen hundred years before Christ, and
who were much given to the study of nature, are said to have observed
the tendency of lightning to make for points, but no theory upon the
subject has come down to us from them.
Electricity for the ancients was an unplumbed ocean, whose slightest
fluctuations affected them in ways they could not understand. In vain
they appealed to their gods to help them to explain the enigma.
Olympus turned a deaf ear to their prayers.
Their imagination exhausted itself in researches into the nature of
such things as amber, in which they recognized the curious attribute
of attraction and repulsion for objects of slight weight. The poets
attributed it to the tears of Phaëton's sisters, lamenting over the
dreams of Eridan. Certain naturalists regarded it as a kind of gum
issuing from trees during the dry days. No one gave any thought to
electricity, by whose subtle fluid the earth and everything upon it is
penetrated and enveloped.
The superstitions connected with lightning would furnish forth
material in themselves for a very curious volume of stories--half
comic, half tragic.
With the Romans the fall of a thunderbolt was always taken as an omen.
In the reign of Domitian, thunder was to be heard once so constantly
during a period of eight months that the tyrant, frightened by the
bombardment from on high, at last cried out in his terror: "Let the
blow come, then, where it will!" The stroke fell upon the capitol, and
upon the temple of the Flavian family, as well as upon the emperor's
palace and the very room in which he slept. The inscription beneath
the triumphal statue was even torn away by the tempest and thrown into
a neighbouring garden.
Otto de Guérike, burgomaster of Magdeburg and inventor of the
air-pump, was the first person to discover the means of producing the
electric spark, about 1650. About the same time, Dr. Wall, while
watching electricity being released from a roll of amber, noticed a
spark and a sudden sharp report, suggestive of a minute flash of
lightning, followed by a minute peal of thunder. The analogy was
striking. This discovery opened out a new horizon to physicists, and
almost immediately the feeble electric light produced by the hand of
man came to be associated with the monstrous sheaves of fire let loose
in space by unknown forces.
L'Abbé Nollet, considered in the France of his time as an oracle in
regard to natural philosophy, expressed himself as follows upon this
subject:--"If some one, after comparing the phenomena, were to
undertake to prove that thunder is in the hands of nature what
electricity is in ours, that those electrical wonders with which we
are now able to make so much play are petty imitations of those great
lightning effects which frighten us; that both result from the same
mechanism; and if he could make it evident that a cloud produced by
the action of the winds, by heat, and by the mingling of exhalations,
bears the same relation to a terrestrial object as an electrified body
bears to an unelectrified body in its close proximity, I admit that
the idea, if well worked out, would captivate me greatly; and, to work
it out, how many plausible arguments there are at the disposal of a
man who is properly versed in electricity!"
The invention of the Leyden jar in 1746, and Franklin's brilliant
investigations, make these conjectures the more probable. Since then
electricity has gone ahead and become one of the most important
branches of modern natural philosophy.
When Franklin demonstrated that the air is in a permanent condition of
electrification even when the sky is clear, people began to study not
thunder alone but the general electrical state of the atmosphere. And
ever since meteorological observatories have made it a practice to
register every day the degree and nature of atmospheric electricity by
the use of very ingenious instruments.
But the records obtained up till now leave us in doubt upon many
points. The subject is still full of new surprises.
Whence come those masses of electricity which move about in the
clouds, sometimes escaping from them in thunderclaps and causing such
tremendous ravages upon this earth of ours? The evaporation of the sea
is one of their principal causes.
The atmosphere is continually impregnated with electric effluvia which
flow silently through the soil through the medium of all bodies,
organized or not, attached to the earth's surface. Plants afford an
especially welcome pathway to this fluid. The green leaves you see
rustling in the wind are often being traversed by electrical currents,
luckily harmless, of precisely the same nature as those of the deadly
lightning. On the other hand, the earth itself emits a certain
quantity of electricity, and it is from the attraction exerted by
these two fluids upon each other that thunder comes into existence. To
put it in another way, thunder is a sudden striking of a balance
between two different masses of electricity.
Minute researches have established the fact that in ordinary
conditions the terrestrial globe is charged with resinous, or negative
electricity, while the atmosphere holds in suspension _vitrée_, or
positive electricity.
In two words, our planet and its aerial envelope are two great
reservoirs of electricity, between which take place continual
exchanges which play a _rôle_ in the life of plants and animals
complementary to that which is played by warmth and moisture.
The aurora borealis, which sometimes illumines, with a brilliancy as
of fairyland, the darkness of night in the Arctic and all the regions
of the North, finds its explanation in the same phenomenon. It also is
a striking of a balance, silent but visible, between two opposing
tensions of the atmosphere and the earth; thus the apparition of the
aurora borealis in Sweden or Norway is accompanied by electric
currents moving through the earth to a distance sufficiently great to
cause the magnetic needle to record the occurrence in the Paris
Observatory.
Indeed, the electricity which pervades the earth, silently and
invisibly, is identical with that which moves in the heights of the
enveloping atmosphere, and, whether it be positive or negative, its
essential unity remains the same, these qualities serving only to
indicate a point, more or less in common, between the different
charges. The heights of the atmosphere are more powerfully electrified
than the surface of the globe, and the degree of electricity increases
in the atmosphere with the distance from the earth.
Atmospheric electricity undergoes, like warmth, and like atmospheric
pressure, a double fluctuation, yearly and daily, as well as
accidental fluctuations more considerable than the regular ones. The
maximum comes between six and seven in the morning in summer, and
between ten and twelve in winter; the minimum comes between five and
six in the afternoon in summer, and about three in the afternoon in
winter. There is a second maximum at sunset, followed by a diminution
during the night until sunrise. This fluctuation is connected with
that of the hygrometric condition of the air. In the annual
fluctuation the maximum comes in January, and the minimum in July; it
is due to the great atmospherical circulation; the winter is the time
when the equatorial currents are most active in our hemisphere, and
when the aurora borealis is to be seen most often.
On the other hand, the water of oceans and rivers is continually
evaporating under the influence of solar heat, and rises into the
atmosphere, where it remains in the form of an invisible gaseous
vapour. Soon it becomes cold again, and, in the process of
condensation, transparent gaseous molecules become transformed into
minute drops, which accumulate into a cloud.
Generally speaking, clouds are, like the atmosphere, charged with
positive electricity. Sometimes, however, there are negative clouds.
You may frequently see, on the summits of mountains, clouds which seem
to cling to the peaks for a while, as though drawn to them by some
force of attraction, and then move away to follow the general
direction of the winds. It often happens that in this case the clouds
have lost their positive electricity in thus coming in contact with
the mountains, and have derived from them in its place the negative
electricity which, instead of holding them, has a tendency to drive
them off. A mass of clouds lying between the negative earth and a mass
of positive clouds above is almost neutral; the positive electricity
accumulates towards its lower surface, and the first drops of rain
will make it disappear. This mass will, from that moment, become like
the surface of the soil--that is to say, it will become negative under
the influence of the mass above it, endowed with a strong positive
tendency.
The cloud remains suspended in space until the moment when, under the
influence of the ambient medium, it dissolves in rain.
The causes of the instability of clouds are very numerous. My readers
are aware that the atmosphere is being constantly agitated by vast
currents which pass from the equator to the poles, and from which the
different winds result.
The clouds take part in this universal whirl of atmospheric waves.
Transported from one point to another--often far beyond the region
where they came into existence--subjected to every vicissitude of
atmosphere, and blown about by contrary currents, they follow the
gigantic movements which take the form sometimes of cyclones and
tempests.
Under the influence of warmth, and probably also by its
transformation, these movements engender great masses of electricity,
and presently, when the clouds have become saturated with it, the
electricity breaks out, and there is a thunderstorm.
The electric fluid, escaped from the cloud in which it has been
imprisoned, flies to unite itself, either with the negative
electricity stored in the surface of the earth, or else with the
electricity in other neighbouring clouds. Almost always the cloud torn
open by the electric discharge dissolves in rain or hail.
Thus a storm is the outcome of violent movements produced by the force
of electricity when this has reached its maximum of intensity.
Thunderstorms are generally heralded by certain premonitory signs. The
barometer goes down steadily. The air, calm and heavy, is pervaded by
a bitter sulphurous odour. The heat is stifling. An abnormal silence
reigns over the land. All this has a remarkable effect upon certain
organisms, and produces nervous complaints, a buzzing in the ears, a
sense of painful oppression, a sort of good-for-nothingness that we
combat in vain.
In most cases storms come to us in France ready made, so to speak,
from the sea, borne in by the currents from the south-west; they are
the off-shoots of the cyclones, and are born in the tropics, moving in
lines from the south-west to the north-east. Ordinarily they lose part
of their strength _en route_ and come to an end suddenly with us.
There are, of course, home-made storms also, so to speak, especially
in France during our hot summers, when the sun is shining all the day,
and thus promoting the rapid evaporation of our seas and rivers.
The air is charged with a heavy mist which veils the horizon; the
barometer is going down, the thermometer going up. The sun looks
leaden though there are no clouds. When it approaches the meridian and
its rays are most scorching, columns of vapour ascend and become
condensed into the light clouds termed _cirri_. At the end of some
hours these clouds become attracted to each other, descend a little,
and become grouped together into what look like great masses of
cotton-wool. These are termed _cumuli_. Presently a small grey cloud
joins the others. It looks innocent and harmless, but very often this
is the beginning of the battle. First there ensues, perhaps, a
discharge or two of lightning without casualties, but soon the
bombardment becomes general, and long blinding fusillades flash
through space. The heavens, darkened over, seem to have sunk quite
low, and to have become a great black mass, from which the lightning
escapes in sudden jets. Rain and hail pelt down upon the earth to an
accompaniment of the rumbling of thunder. Confusion has fallen upon
the entire universe.
Then, finally, the fight comes to a close. The clouds disperse and
allow us to see once again a wide expanse of sunlit blue. The birds,
their hearts freed again from terror, begin to sing again. Flowers and
foliage and soil, refreshed by the rain, give out sweet perfumes. An
immense joy takes the place of the sense of melancholy and oppression.
It is good to see the sun again! Alas, though, there are grim
realities to be faced presently. The hailstones have destroyed the
crops and begotten famine--the lightning has sown death and plunged
whole families into mourning. It is with these misfortunes before us
that we make up our minds to do what in us lies to diminish the
destructiveness of this terrible force.
How are storm-clouds to be detected?
Generally speaking, their shape is very clearly defined, and they have
a look of solidity about them.
Their lower surface is often unbroken, presenting a level plain from
which there rise huge ragged protuberances like great plumes.
Sometimes, on the other hand, they have great projections underneath,
trailing quite near the ground.
Storm-clouds move generally in large numbers, and are generally
composed of two separate masses, differently electrified--the lower
one giving out negative electricity, the higher positive electricity.
The flashes of lightning occur generally between these two masses,
though also, less frequently, between the lower mass and the earth.
It may be said that, generally speaking, storms are the result of the
meeting of two masses of clouds differently electrified.
For long, physicists refused to admit the validity of any other
theory, and combated in particular the idea that lightning could issue
from a single isolated cloud.
This has, however, been established now as a fact, and in such cases
the flashes have always, of course, taken place between the cloud and
the earth.
Marcorelle, of Toulouse, reports that on September 12, 1747, the sky
being then pure and cloudless but for one round speck, there was
suddenly a thunderclap and a flash which killed a woman on the spot,
burning her breast but doing no damage to her clothes.
Here is another interesting case. Two priests of the Cathedral of
Lombey, who were standing in the area of their chapter-house, busy
winnowing, saw a small cloud approaching them little by little. When
it was immediately above them a flash of lightning broke out and
struck a tree just beside them, splitting it from top to bottom. They
heard no thunderclap. The weather was quite fine. There was no wind,
and this was the only cloud in the sky.
Storms are far more prevalent in some countries than in others.
According to Pliny, thunder was unknown in Egypt, and, according to
Plutarch, in Abyssinia. This could not be said now, however, perhaps
because these lands have grown unworthy of their exemption. It might
be said, however, of Peru, whose pure and limpid skies are never
troubled by tempest. _Jupiter tonans_ must be a myth indeed to a
people who know nothing of thunderclaps or wet days.
Storms diminish in number in high latitudes, but there are local
conditions which affect their distribution. Then they are particularly
frequent in countries that are thickly wooded and in mountainous
districts.
Arago came to the conclusion, after a considerable number of
observations, that, out in the open sea or among islands, there is no
thunder in the north beyond the 75th degree of latitude. This is not
absolutely so, but it is a fact that storms are very much rarer in the
polar regions. They become more and more frequent towards the equator,
and are very numerous in the tropics.
On either side of the equator storms come year after year with
remarkable regularity in the wet season, and at the time of the
monsoons.
At Guadeloupe and Martinique there is never any thunder in December,
January, February, or March.
In temperate climates there are scarcely any storms in winter; they
begin in the spring, and attain their maximum of intensity in the heat
of summer.
In Italy there are thunderstorms at almost all times of the year.
In Greece they come chiefly in spring and autumn.
It is noticeable that in all latitudes they come most often in the
afternoon.