Part 4 (1/2)
There are several causes. The first is due to the length of lightning and the difference in speed between sound and light. Let us suppose, for example, a flash of lightning, AE, 11,000 metres long. The observer stationed at O, underneath extremity E of the lightning (which is one kilometre high), will see the lightning in its full length in one indivisible instant. The sound will form itself also at the same instant all along the line of lightning, but the sound-waves will only reach the ear of the observer at different times. That which starts at point E, the nearest, will arrive in 3 seconds, sound travelling about 337 metres a second. That which is formed at the same moment at point D, 2000 yards from point O, will take double the time to arrive. That which comes from point C will not arrive for 12 seconds. The sound formed at B will not arrive until the time necessary to cover 8 kilometres--that is to say, not for 23 seconds--and the sound formed at A will only reach after 32 seconds.
Thus the rolling will have lasted more than half a minute from start to finish.
[Ill.u.s.tration: DIAGRAM EXPLAINING THE DURATION OF THE SOUND OF THUNDER.]
And if, which is very often the case, the astronomer is not exactly under one of the extremities of the lightning, but at some other point in its course, he first hears a clap, then an increased noise, then a diminution. In fact, in this case, the sound which leaves point D just overhead, which is 1000 metres off, arrives alone in 3 seconds, but the sounds formed from D to E on one side, and from C to D on the other, arrive at the same time, having joined each other, taking 9 seconds, which is the necessary time to come from 1000 to 3000 metres.
The sounds beyond C arrive and depart according to distance, as in the preceding example, and the thunder has lasted 23 seconds instead of 32 seconds.
[Ill.u.s.tration: COMMENCEMENT, AUGMENTATION, AND DIMINUTION OF THE INTENSITY OF THUNDER.]
I must add that lightning is never straight, but always crooked.
The length of time the thunder rolls has nothing to do with the distance of the cloud where the phenomenon begins. It is proportionate to the length of the lightning with which it is a.s.sociated. The rolling is often still more prolonged by a succession of small discharges, which follow each other very rapidly between the stormy clouds; by the zigzags and ramifications of the lightning caused by the hygrometrical diversity of the different beds of air; by the echoes repeated by the mountains, the earth, the water, and the clouds themselves--to all which must be added also the interferences caused by the encounter of the different systems of sound-waves.
Its duration is extremely variable, however; it rarely exceeds 30 seconds, though the noise may sometimes seem to last much longer, so that an observation of this kind may have any value--one must take into consideration the echo, and isolate a single clap from the series of discharges which take place in the bosom of the storm. The longest verified duration of a single discharge is 45 seconds. That is tremendous if we think of the instantaneousness of the lightning, and reflect that the flash and the sound are produced in reality at the same moment, that they are dependent the one on the other, and that in their various manifestations there is only the difference of motion.
Sound moves like a tortoise behind the swift lightning, whose vibrations spread in the air with inconceivable rapidity.
Hence these 45 seconds correspond to a flash of lightning more than 15 kilometres in length, but we know that there are even longer ones.
I have already said that we can calculate the distance of the celestial cannon from which the fulminating discharge comes by counting the number of seconds which separate the apparition of the lightning from the first growls of the thunder. Thus the longest interval that has been proved between the appearance of the lightning and the noise it produces is 24 kilometres. This, however, is a maximum.
Numerous observations have proved that thunder is never heard beyond 20 or perhaps 25 kilometres. Lightning pierces the cloudy veil, but the voice of thunder does not carry so far. In this the great Jupiter shows himself inferior to the ingenuity of human pigmies, whose destructive and barbarous art has been able to invent infernal machines the noise of which can be heard much further.
Cannon can easily be heard at a distance of 40 kilometres. Sometimes, in sieges and big battles the cannonades can be heard muttering lugubriously more than 100 kilometres away.
During the siege of Paris, Krupp's cannon--that most expeditious of all vehicles of civilization in the eyes of the statesmen of this planet!--could be heard as far as Dieppe, 140 kilometres away, during the nights when they were bombarding. The cannonade of March 30, 1814, which crowned the First Empire, as it crowned the Second, was heard between Lisieux and Caen, a distance of 175 kilometres. Arago even alleges that the cannon at Waterloo could be heard as far as Creil, which is 200 kilometres away. Thus man's thunder can be heard at a greater distance than that of nature. It is true that it is incomparably more vicious, and that it has a great many more victims.
In its natural state, if we might explain it thus--left to itself--it comes directly to us from the high regions of the atmosphere, and is the most terrible of aerial messengers--a subtle messenger, malicious and violent, it is the terror of the human race. But ruled by the genius of man, it becomes a powerful agent towards modern civilization, and we cannot sufficiently admire its many advantages.
If we could tame lightning and guide it safely, its services would probably become innumerable. Lightning as man's right hand! Why not?
Was it not the auxiliary of the G.o.ds in the dark ages? To-day, is it not regarded by astronomers as one of the most important forces of nature? Why should it not be the collaborator of man's intelligence to-morrow?
CHAPTER IV
FIREb.a.l.l.s
Here we penetrate into what is, perhaps, the most mysterious, and certainly the least understood domain of thunder and lightning.
Among all the electrical phenomena to be observed in the atmosphere, there is nothing stranger than those fireb.a.l.l.s of which we have already spoken, and which in form and size recall the electric lights in our Paris boulevards. Curious the contrast between electricity tamed and civilized and electricity running wild! Between the arc lights fulfilling their peaceful and useful function as subst.i.tutes for the sun, and these dread engines of destruction sowing death and havoc!
It is not long since the existence of these fireb.a.l.l.s has been acknowledged by scientists as an actual fact. Until quite recent times they were regarded as the figment of excited imaginations, and wise men smiled at the wild stories of their ravages. Their reality has now been established, however, beyond the possibility of doubt.
In shape they are not always quite spherical, though this is their normal appearance; and although their contours are usually clearly defined, they are sometimes encircled by a kind of luminous vapour, such as we often see encircling the moon. Sometimes they are furnished with a red flame like a fuse that has been lit. Sometimes their course is simply that of a falling star. Sometimes they leave behind them a luminous trail which remains visible long after they themselves have disappeared. They have been described as looking like a crouching kitten, an iron bar, a large orange--so harmless apparently, that you were tempted to put out your hand to catch it. There is record of one being seen as large as a millstone.
One remarkable thing about them is the slowness with which they move, and which sometimes enables their course to be watched for several minutes. In our first chapter we gave several instances of the occurrence of fireb.a.l.l.s. Let us look at some more. Here is one taken from Arago's learned treatise upon thunder. The record is from the pen of Batti, a marine painter in the service of the Empress of Austria and resident at Milan.
”In the month of June, 1841, I was staying at the Hotel de l'Agnello in a room on the second floor, overlooking the Corso dei Servi. It was about six in the afternoon. The rain was coming down in torrents, and the darkest rooms were lit up by the lightning flashes better than our rooms generally are by gas. Thunder broke out every now and again with appalling violence. The windows of the houses were closed, and the streets were deserted, for, as I have said, there was a steady downpour, and the main road was turned into a torrent. I was sitting quietly smoking, and looking out at the rain, which an occasional ray of sunlight set flas.h.i.+ng like threads of gold, when I suddenly heard voices in the street calling out 'Guarda, guarda!'--'Look, look!' and at the same moment a clatter of hob-nailed boots. After half an hour of absolute silence, this noise attracted my attention. I ran to the window, and looking to the right, in the direction of the clamour, I saw a fireball making its way down the middle of the road on a level with my window, in a noticeably oblique direction, not horizontally.
Eight or ten persons, continuing to call out 'Guarda, guarda!' kept pace with it, walking down the street, stepping out quickly. The meteor pa.s.sed my window, and I had to turn to the left to see what would be the end of its caprice. After a moment, fearing to lose sight of it behind some houses which jutted out beyond my hotel, I went quickly downstairs and into the street, and was in time to see it again and to join those who were following its course. It was still going slowly, but it was now higher up, and was still ascending--so much so that after a few minutes it hit the cross upon the clock tower of the Chiesa dei Servi and disappeared. Its disappearance was accompanied by a dull report like that of a big cannon twenty miles away when the wind carries the sound.