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.