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.

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 circumstance, 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 accumulated either in another cloud or on the ground. An electric deflagration ensues, and a long ignited dart precipitates itself into space, 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 constitutes lightning.

Lightning is not always the same, and in order to classify the different forms it takes more easily, it can be divided into three groups—diffused lightning, linear lightning, and fireballs. This last is the most curious of the three. The variety and eccentricity of fireballs 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 space, 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 passage 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 principal branch, out of which a great many sparks burst forth.

These incandescent sheaves move through space 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 quantity of the electricity thrown on the atmosphere by the discharge; on the density of the air at the time of the passage 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