[al:新概念英语（四）] [ar:MP3 同步字幕版（美音）] [ti:Waves] [by:更多学习内容，请到yingyu.chazidian.com搜索“新概念”] [00:00.37]Lesson 40 [00:02.24]Waves [00:09.08]What false impression does an ocean wave convey to the observer? [00:15.37]Waves are the children of the struggle between ocean and atmosphere, the ongoing signatures of infinity. [00:23.26]Rays from the sun excite and energize the atmosphere of the earth, awakening it to flow, to movement, to rhythm, to life. [00:33.29]The wind then speaks the message of the sun to the sea [00:37.20]and the sea transmits it on through waves--an ancient, exquisite powerful message. [00:45.48]These ocean waves are among the earth's most complicated natural phenomena. [00:51.45]The basic features include a crest (the highest point of the wave), [00:56.13]a trough (the lowest point), a height (the vertical distance from the trough to the crest), [01:03.67]a wave length (the horizontal distance between two wave crests), [01:08.99]and a period (which is the time it takes awave crest to travel one wave length). [01:16.21]Although an ocean wave gives the impression of a wall of water moving in your direction, [01:22.46]in actuality waves move through the water leaving the water about where it was. [01:29.36]If the water was moving with the wave, [01:32.06]the ocean and everything on it would be racing in to the shore with obviously catastrophic results. [01:40.03]An ocean wave passing through deep water causes a particle on the surface to move in a roughly circular orbit, [01:48.49]drawing the particle first towards the advancing wave, then up into the wave, [01:54.41]then forward with it and then--as the wave leaves the particles behind--back to its starting point again. [02:03.30]From both maturity to death, a wave is subject to the same laws as any other 'living' thing. [02:10.15]For a time it assumes a miraculous individuality that, in the end, is reabsorbed into the great ocean of life. [02:19.11]The undulating waves of the open sea are generated by three natural causes: [02:25.25]wind, earth movements or tremors, and the gravitational pull of the moon and the sun. [02:31.95]Once waves have been generated, [02:34.14]gravity is the force that drives them in a continual attempt to restore the ocean surface to a flat plain.