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     Children enjoy shouting at a high wall and hearing the sound come back to them. These sounds are called echoes(回声). Echoes have given us a number of valuable tools. 
     Echo sounding devices were early used in making maps of the ocean floor. Sounds or ultrasonic(超声的) sounds make good tools for determining how deep the water is under ships. Sometimes echoes from ultrasonic distance finding devices were prevented from working by fish swimming past or by the presence of large objects. So ultrasonic devices have been replaced by other tools. 
     Radar is now a familiar tool. Like many others it was an unexpected discovery. It was first observed by two researchers, who were studying sound communication. They were sending signals from a station on one side of a river in Washington DC to a vehicle across the river. They discovered that their signals were stopped by passing ships. They recognized the importance of this discovery at once. 
     All this was of course just a start, from which our present radar has developed. The word “radar,” in fact, gets its name from the term “radio detection(检测)and ranging.” “Ranging” is the term for detection of the distance between an object and the radar set. Today, in our scientific age, it would be difficult to manage without radar. 
     One of the functions of radar is as a speed control device on highways. When a person in an automobile is driving faster than the speed limit, radar will show this clearly and the traffic police can take measures to stop him. 
     A pilot cannot fly a plane by sight alone. Many conditions such as flying at night and landing in dense fog require the pilot to use radar. Human eyes are not very good at determining speeds of approaching objects, but radar can show the pilot how fast nearby planes are moving.