Not all sounds made by animals serve as language, and we have only to turn to that extraordinary（非凡的） discovery of echolocation（回波定位） and bats（球拍） to see a case in which the voice plays a strictly utilitarian role. To get a full appreciation（欣赏） of what this means, we must turn first to some recent human inventions. Everyone knows that if he shouts in the vicinity（邻近） of a wall or a mountainside（山腹）, an echo（回音） will come back. The further off this solid obstruction（阻塞）, the longer time will elapse（(时间)过去） for the return of the echo（回音）. A sound made by tapping（轻敲） on the hull（(果实等的)外壳） of a ship will be reflected from the sea bottom, and by measuring the time interval（间隔） between the taps（轻敲） and the receipt of the echoes（回音）, the depth（[海洋] 深度） of the sea at that point can be calculated（计算）. So was born（承担） the echo（回音）-sounding apparatus（装置）, now in general use in ships.

  Every solid object will reflect a sound, varying（改变） according（使一致） to the size and nature of the object. A shoal（浅滩） of fish will do this. So it is a comparatively simple step from locating the sea bottom to locating a shoal of fish. With experience and with improved apparatus, 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. It has been found that certain bats emit（发出） squeaks（短促的尖叫声）, and by receiving the echoes, they can locate and steer（驾驶） clear of obstacles（障碍）, or locate flying insects on which they feed. This echo location in bats is often compared with radar（[雷达] 雷达）, the principle（原理） of which is similar.