CHAPTER XLII THE VELOCITY OF SOUND

“UNDER 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 serene¹ and the weather magnificent. Under the cumulus where the thunder is growling² 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 descend³ 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 thither⁴ in the vast spaces of the atmosphere. They travel, and on their way throw a shadow or precipitate⁵ 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 firearms 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 velocity⁶, 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 340 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 mallet⁷ tap the stone, and some time after we hear the sound.”

Bells Ringing

“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.”

“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 watchful⁸ eye and attentive⁹ 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.”

1 serene   

adj. 安详的，宁静的，平静的

参考例句：

• He has entered the serene autumn of his life.他已进入了美好的中年时期。
• He didn't speak much,he just smiled with that serene smile of his.他话不多，只是脸上露出他招牌式的淡定的微笑。

2 growling   

n.吠声, 咆哮声 v.怒吠, 咆哮, 吼

参考例句：

• We heard thunder growling in the distance. 我们听见远处有隆隆雷声。
• The lay about the deck growling together in talk. 他们在甲板上到处游荡，聚集在一起发牢骚。

3 descend   

vt./vi.传下来，下来，下降

参考例句：

• I hope the grace of God would descend on me.我期望上帝的恩惠。
• We're not going to descend to such methods.我们不会沦落到使用这种手段。

4 thither   

adv.向那里；adj.在那边的，对岸的

参考例句：

• He wandered hither and thither looking for a playmate.他逛来逛去找玩伴。
• He tramped hither and thither.他到处流浪。

5 precipitate   

adj.突如其来的；vt.使突然发生；n.沉淀物

参考例句：

• I don't think we should make precipitate decisions.我认为我们不应该贸然作出决定。
• The king was too precipitate in declaring war.国王在宣战一事上过于轻率。

6 velocity   

n.速度，速率

参考例句：

• Einstein's theory links energy with mass and velocity of light.爱因斯坦的理论把能量同质量和光速联系起来。
• The velocity of light is about 300000 kilometres per second.光速约为每秒300000公里。

7 mallet   

n.槌棒

参考例句：

• He hit the peg mightily on the top with a mallet.他用木槌猛敲木栓顶。
• The chairman rapped on the table twice with his mallet.主席用他的小木槌在桌上重敲了两下。

8 watchful   

adj.注意的，警惕的

参考例句：

• The children played under the watchful eye of their father.孩子们在父亲的小心照看下玩耍。
• It is important that health organizations remain watchful.卫生组织保持警惕是极为重要的。

9 attentive   

adj.注意的，专心的；关心(别人)的，殷勤的

参考例句：

• She was very attentive to her guests.她对客人招待得十分周到。
• The speaker likes to have an attentive audience.演讲者喜欢注意力集中的听众。