[ti:Lesson7]
[ar:新概念四]
[al:]

[00:01.65]Lesson 7
[00:02.88]Bats
[00:04.88]First listen and then answer the following question.
[00:08.34]In what way does echo-location in bats play an utilitarian role?
[00:14.89]Not all sounds made by animals serve as language,
[00:21.10]and we have only to turn to that extraordinary discovery of echo-location in bats
[00:28.44]to see a case in which the voice plays a strictly utilitarian role.
[00:32.76]To get a full appreciation of what this means we must turn first to some recent human inventions.
[00:40.90]Everyone knows that if he shouts in the vicinity of a wall or a mountainside,
[00:47.45]an echo will come back.
[00:49.86]The further off this solid obstruction,
[00:52.69]the longer time will elapse for the return of the echo.
[00:55.79]A sound made by tapping on the hull of a ship will be reflected from the sea bottom,
[01:02.43]and by measuring the time interval between the taps and the receipt of the echoes,
[01:08.22]the depth of the sea at that point can be calculated.
[01:11.62]So was born the echo-sounding apparatus,
[01:15.67]now in general use in ships.
[01:18.35]Every solid object will reflect a sound,
[01:22.61]varying according to the size and nature of the object.
[01:26.35]A shoal of fish will do this.
[01:29.56]So it is a comparatively simple step from locating the sea bottom to locating a shoal of fish.
[01:37.23]With experience,
[01:39.44]and with improved apparatus,
[01:41.81]it is now possible not only to locate a shoal but to tell if it is herring, cod,
[01:48.60]or other well-known fish, by the pattern of its echo.
[01:52.61]It has been found that certain bats emit squeaks and by receiving the echoes,
[02:00.57]they can locate and steer clear of obstacles --
[02:03.06]or locate flying insects on which they feed.
[02:07.08]This echo-location in bats is often compared with radar,
[02:11.32]the principle of which is similar.