07 Bats

名称：07 Bats
内容简介：
新概念英语第四册（英音）
[al:新概念英语（四）]
[ar:MP3 同步字幕版（英音）]
[ti:Bats]

[00:01.47]Lesson 7
[00:03.39]Bats
[00:11.30]In what way does echo-location in bats play an utilitarian role?
[00:19.78]Not all sounds made by animals serve as language,
[00:24.17]and we have only to turn to that extraordinary discovery of echo-location in bats
[00:30.34]to see a case in which the voice plays a strictly utilitarian role.
[00:36.35]To get a full appreciation of what this means
[00:39.73]we must turn first to some recent human inventions.
[00:44.50]Everyone knows that if he shouts in the vicinity of a wall or a mountainside,
[00:49.97]an echo will come back.
[00:52.57]The further off this solid obstruction,
[00:55.01]the longer time will elapse for the return of the echo.
[00:59.16]A sound made by tapping on the hull of a ship will be reflected from the sea bottom,
[01:05.35]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.
[01:15.11]So was born the echo-sounding apparatus, now in general use in ships.
[01:21.77]Every solid object will reflect a sound,
[01:25.65]varying according to the size and nature of the object.
[01:30.09]A shoal of fish will do this.
[01:32.95]So it is a comparatively simple step from locating the sea bottom to locating a shoal of fish.
[01:40.85]With experience, and with improved apparatus,
[01:44.48]it is now possible not only to locate a shoal but to tell if it is herring, cod, or other well-known fish, by the pattern of its echo.
[01:56.28]It has been found that certain bats emit squeaks and by receiving the echoes,
[02:02.42]they can locate and steer clear of obstacles -- or locate flying insects on which they feed.
[02:09.96]This echo-location in bats is often compared with radar,
[02:14.16]the principle of which is similar.