The Story Book of Science  by Jean Henri Fabre

The Velocity of Sound

"U NDER that big white cloud that you call cumulus," said Emile, "there is at this very moment a storm. We have just seen the lightning and heard the thunder. Here, on the contrary, the sky is blue. So it does not rain everywhere at the same time. When rain is falling in one country, it is fine in others. And yet, when it rains here the whole sky is covered with clouds."

"You need only put your hand over your eyes to hide the sky," his uncle explained. "A cloud much farther off, but also much larger, produces the same effect: it veils what is surrounding us and makes it all cloudy. But that is only in appearance; beyond the region covered by the cloud the sky may be serene and the weather magnificent. Under the cumulus where the thunder is growling now, it rains, you may be sure, and the sky looks black. To the people in that region the surroundings present only a rainy appearance, because they are wrapped in clouds; if they were to go elsewhere, beyond the clouds, they would find the sky as serene as we have it here."

"With a fast horse they could, then," suggested Emile, "get from under the clouds, leave the rain, and come into fine weather; as also they could leave the sunshine and get into the rain under the clouds."

"Sometimes that would be possible, but more often not, because clouds can cover large areas. Besides, they travel, they go from one country to another, with such speed that the best horseman could not follow them in their course. You have all seen the shadow of the clouds run over the ground when the wind blows. Hills, valleys, plains, water-courses, all are crossed in less than no time. The shadow of a cloud passes over you at the moment you reach the top of a hill. Before you have taken three steps to descend into the valley, the shadow, with giant strides, is mounting the opposite slopes. Who could flatter himself that he could follow the cloud and keep under its cover?

"If rain sometimes falls over great stretches of country, it is never general, absolutely. If it should rain at one time over a whole province, what is that compared with the earth? A clod compared with a large field. Chased by the wind, clouds run hither and thither in the vast spaces of the atmosphere. They travel, and on their way throw a shadow or precipitate rain. Where they pass there is rain; everywhere else, no. In the same place there can even be both rain and fine weather, according as one is below or above the clouds. You know that on a mountain-top the clouds are sometimes beneath one. The plain under the clouds may receive a hard shower, while on the summit the sun shines without a single drop of rain."

"All that is easily understood," said Jules. "It is my turn now, Uncle, to ask you a question. From the storm-cloud that we see from here, there first came a flash of lightning; then, after waiting some time, the sound of the thunder was heard. Why do not the sound and the lightning come together?"

"Two things tell us of the thunderbolt: light and noise. The light is the flash of the lightning, the noise is thunder. Likewise in the discharge of fire-arms there is the light produced by the ignition of the powder and the noise resulting therefrom. At the scene of the explosion light and noise are coincident; but for persons at a distance the light, which travels at an incomparably greater velocity, arrives before the sound, which moves more slowly. If you note the discharge of a gun a considerable distance away, you see first the flash and smoke of the explosion, and do not hear the report until some time after; the more distant the explosion, the longer the time. Light travels an immense distance in an exceedingly short time. The flash of the explosion, therefore, reaches the eye at the very instant of its occurrence. If the sound does not arrive until after, it is because it travels much less rapidly and, in order to cover a considerable distance, requires considerable time, which is easily measured.

"Suppose ten seconds pass between the flash of a cannon's discharge and the arrival of the sound. The distance is measured between the place where the explosion occurred and that where it was heard. It is found to be 3400 meters. Sound, therefore, moves through the air, in a single second, a distance of 3400 meters. That is a good rate of speed, comparable with that of the cannon-ball, but nothing, after all, in comparison with the inconceivable velocity of light.

"The unequal rapidity with which sound and light travel accounts for the following fact. From a distance a wood-cutter is seen chopping wood, or a mason cutting stone. We see the ax strike the wood, the mallet tap the stone, and some time after we hear the sound."

"One Sunday before church," interposed Jules, "I was watching from a distance the ringing of the bell. I saw the tongue strike and the sound did not come until later. Now I see the reason."

Bells Ringing

"If you count the number of seconds between the appearance of the flash and the instant the thunder begins to be heard, you can tell what distance you are from the storm-cloud."

"Is a second very long?" Emile asked.

"It is about the length of one beat of the pulse. All we have to do, then, is to count, one, two, three, four, etc., without haste, but not too slowly, to have about the number of seconds. Note the instant the flash lights up the stormy cumulus, and count slowly until you hear the thunder."

With watchful eye and attentive ear all began the observation. Finally a flash was seen. They counted, the uncle beating time. One—two—three—four—five— At twelve came the thunder, but so faint that they could only just hear it.

"It took twelve seconds for the sound of the thunder to reach us," said Uncle Paul. "From what distance does it come, if sound travels 340 meters a second?"

"You must multiply 340 by twelve," replied Claire.

"Well, Miss, do it."

Claire made the calculation. The result was 4080 meters.

"The flash of lightning was 4080 meters away; we are more than a league from the storm-cloud," said her uncle.

"How easy that is!" exclaimed Emile. "You count one, two, three, four, and without moving you know how far away the thunderbolt has just fallen."

"The longer the time between the flash and the noise, the farther away is the cloud. When the report comes at the same time as the flash, the explosion is quite near. Jules knows that well since the day of the storm in the pine woods."

"I have heard that there is no longer any danger after the lightning is seen," said Claire.

"A thunderbolt is as rapid as light. An electric explosion is, therefore, ended as soon as the flash appears, and all danger is then passed; for the thunder, however loud it may be, can do no harm."