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 ma.s.ses of cotton-wool. These are termed _c.u.muli_. 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 s.p.a.ce. The heavens, darkened over, seem to have sunk quite low, and to have become a great black ma.s.s, 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 ma.s.ses, differently electrified--the lower one giving out negative electricity, the higher positive electricity.

The flashes of lightning occur generally between these two ma.s.ses, though also, less frequently, between the lower ma.s.s and the earth.

It may be said that, generally speaking, storms are the result of the meeting of two ma.s.ses 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 lat.i.tudes, 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 lat.i.tude. 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 lat.i.tudes they come most often in the afternoon.

CHAPTER III

THE FLASH AND THE SOUND

The Romans attributed a mysterious influence to each manifestation of electricity. They divided lightning into individual and family lightning, lightning of advice, monitory, explanatory, expostulatory, confirmatory, auxiliary, disagreeable, perfidious, pestiferous, menacing, murderous, etc., etc.

They adapted it to every taste and circ.u.mstance, but modern science has come to put order into this Capharnaum.

When a cloud is superabundantly charged with electricity, this electricity, which is compressed in the cloudy envelope, tries to escape in order to join the electricity acc.u.mulated either in another cloud or on the ground. An electric deflagration ensues, and a long ignited dart precipitates itself into s.p.a.ce, showing us on a large scale what our experience of physics has taught us in a small way in our laboratories. This luminous and often dazzling trail const.i.tutes lightning.

Lightning is not always the same, and in order to cla.s.sify the different forms it takes more easily, it can be divided into three groups--diffused lightning, linear lightning, and fireb.a.l.l.s. This last is the most curious of the three. The variety and eccentricity of fireb.a.l.l.s are celebrated in the history of lightning, and I propose to devote the following chapter to their vagaries.

Diffused lightning is the commonest of all. You can count hundreds of flashes on a stormy night. Occasionally they succeed one another with such rapidity that the sky is momentarily entirely illumined with a fantastic brightness. At these times great sombre clouds can be seen surging from the darkness of the night, to shine suddenly with an ephemeral brightness of a diffused red, blue, or violet tinge. Their irregular shapes, with their jagged edges of light, are visible against the dark background of the heavens, and the thunder growls monotonously. Whether the exchange of electricity is produced on a vast stretch between two rows of clouds, or whether it is manifested by a long thin spark launched like an arrow and veiled by the curtain of clouds, all that can be seen is a strange light, vague, diaphanous, instantaneous, which sometimes spreads itself like a sheet of fire all over the horizon.

It is diffused lightning which gives us the finest storm effects on those heavy summer evenings when the air is breathless and saturated with electricity. Suddenly the clouds are illumined, nebulous veils of light on which can be seen, in sombre fantastic, fugitive vision, the outlines of the trees, houses, and other landmarks. Then, all at once, heaven and earth fall back into a darkness deeper than before, owing to the contrast.

Linear lightning is more terrible. It is regarded by astronomers as the most perfect form of destructive lightning. It is a strong flash--a thin trail of light--very clear, and extraordinarily rapid, which shoots from an electric cloud to the earth, or from one cloud to another.

Like a supple and undulating serpent of fire, it twists itself luminously into s.p.a.ce, spreading itself menacingly in the heavens with its long spirals of light.

Sometimes--in a hurry, no doubt, to reach its prey--it effects its pa.s.sage in a straight line, but as a rule it follows a sinuous track, and forms itself into a zigzag at an obtuse angle. The different forms which this lightning takes are no doubt attributable to various causes. One of the chief of these seems to be the unequal distribution of humidity in the air, which renders it a more or less good conductor. In fact, fulminic matter is strongly attracted towards damp regions, and goes quickly from one point to another, guided in its chosen way by the hygrometrical conditions of the atmosphere; and it is these constant changes of direction which determine the meanderings of its course. Thus the lightning would trace a sort of plan of the hygrometrical state of the air for a certain portion of the atmosphere. For it, the short road is hardly ever the straight line.

On the other hand, the variability of the overloading of electricity has something to say to the form it takes.

Sometimes lightning forms itself into two or three branches, and becomes forked lightning. Or it even divides itself into a number of points from a princ.i.p.al branch, out of which a great many sparks burst forth.

These incandescent sheaves move through s.p.a.ce with extraordinary agility. It has not been possible to measure their speed with absolute accuracy, but their rapidity is such that their transit appears to be instantaneous. The latest researches seem to have proved that their speed is superior to that of light, which is 300,000 kilometres a second.

Lightning is not always of a dazzling whiteness, it is often yellow, red, blue, violet, or green. Its colour depends on the quant.i.ty of the electricity thrown on the atmosphere by the discharge; on the density of the air at the time of the pa.s.sage of the ignited matter; on its hygrometrical state, and on the substances which it contains during suspension. It has been remarked in the study of physics that the electric spark is white in the open air, but that it gets a violet tinge in the vacuum of a pneumatic machine.

This proves that violet lightning comes from the far-off regions of the atmosphere. It traverses a bed of rarified air, and shows the great height of the storm-clouds from which it emanates.

The fulminating spark is so fugitive that it is difficult to form an idea of its length. One could easily take it to be a yard or so long, so illusory and deceptive are our impressions. As a matter of fact, it is proved that flashes of lightning cover a distance of several kilometres.

There are various methods to which one can have recourse in these scientific researches. The first, which gives the length of horizontal lightning, is based on a minute comparison between the trajectory described by the meteor and the known distance of the terrestrial points between which it travels. In order to gauge the extent of vertical lightning, you must estimate approximately the height of the clouds from which it comes, based on the irregularities of the earth of which the height is known.

But there is a still simpler method for approximate measurement within the reach of every one. It consists in multiplying 337 (the number of yards traversed by sound in a second) by the number of seconds during which the thunder lasts.

These methods all give the same result, and prove that lightning is often 1, 5, and 10 kilometres in length. The greatest length proved up to the present has been 18 kilometres. When one thinks of the instantaneousness of these flashes, one marvels at their incomparable agility, and we can only be lost in admiration of the magic force of the heavenly sling, which is capable of hurling these whole rivers of fire to roll in their sinuous course right through s.p.a.ce, and in a s.p.a.ce of time almost inappreciable to our senses.

Yet, in spite of the extreme rapidity of the lightning, it has been possible to determine that these meteors do not last the thousandth part of a second. To prove this, we take a circle of cardboard, divided from the middle into black and white sections. This circle can be turned like a wheel almost as quickly as one can wish. We know that luminous impressions remain on the retina the tenth part of a second; thus, if we imitate the childish game of turning a lighted coal--if the turn is made in the tenth of a second, each successive position of the coal remaining impressed on the retina for the same length of time--we have a continuous circle. In turning our cardboard wheel with the black and white spokes, if each spoke pa.s.ses before our eyes in less than the tenth of a second, we can no longer distinguish between the sections, but can only see a grey circle. But we can make it rotate a hundred turns or more in a second; this being done, if we continue to observe the circle, we can no longer see the lines, they succeed each other more quickly in our eyes than the impression they produce. But if the circle turns before us in the darkness, and it is suddenly lighted up and as suddenly darkened, the impression produced on our eyes by each of the sections would last less than the tenth of a second, and the circle would appear to us as if it were stationary.

In applying a calculated rotation to this contrivance, Charles Wheatstone has proved that some lightning does not last the thousandth part of a second. This measure is probably a minimum; in the majority of cases the duration of lightning is longer than this.

Often during the hot, transparent summer nights, we see a considerable number of flashes, which furrow the firmament with their gentle, bluish light. These fugitive gleams remind us in the sky of the will-o"-the-wisps, which come forth silently from marshy ground. The atmosphere is pure; there are no apparent traces of a storm, and yet the sky is glistening with thousands of small flames. The flashes succeed one another almost without interruption. These electric sparks are known as heat-lightning, but this is quite inaccurate, and has no meaning in the language of modern science.

In a great number of cases an astronomer would be able to discover certain characteristic signs indicating that a storm is taking place under the horizon at a very great distance from the point of observation. It is only at the moment when the sky is lighted up that one can see the ridge of clouds lying low on the horizon. At other times there is no sign of a storm, as far as the eye can see. The atmosphere is quite clear, and yet the sky is swept with a number of electric flashes. But afterwards you hear that a violent storm has devastated the region over which the gleams have appeared, and that it is to this that they are attributable. They are only reflected lights.