练习材料：
Lesson 7-1 Bats
Not all sounds made by animals serve as language, and we have only to turn to that extraordinary discovery of echo-location in bats to see a case in which the voice plays a strictly utilitarian role.
To get a full appreciation of what this means we must turn first to some recent human inventions. Everyone knows that if he shouts in the vicinity of a wall or a mountainside, an echo will come back. The further off this solid obstruction, the longer time will elapse for the return of the echo. A sound made by tapping on the hull of a ship will be reflected from the sea bottom, and by measuring the time interval between the taps and the receipt of the echoes, the depth of the sea at that point can be calculated. So was born the echo-sounding apparatus, now in general use in ships.
应用配置：L0+L4
知识笔记：utilitarian/juˌtɪləˈtɛriən appreciation/əˌpriʃiˈeɪʃən obstruction/əbˈstrʌkʃən elapse/ɪˈlæps hull/hʌl interval/ɪntərvəl calculated/ˈkælkjəˌleɪtəd apparatus/æpəˈrætəs
练习感悟：用时43分。难读的单词稍多，多花了时间理顺。
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