Radar is an electronic assembly far too complex for detailed description here, but its basic principle is simple. It is merely an echo machine that reflects radio waves instead of sound waves. To illustrate by a rough analogy, let us imagine that a man is standing in the middle of an open field on a very dark night. He wants to find out something of the contours of the surrounding country but his only tools are a compass, a watch with luminous dial and hands, and a large megaphone. He raises the megaphone to his lips, points it directly north, and gives a sharp and piercing call: “Hi!” He now cups his hand to his ear and listens for an echo. Hearing no reply, he deduces that in the north there are no hills, tall buildings, or other obstructions that might have produced an echo. Changing his position, he turns to the east and tries the experiment again. After an interval his call returns as a faint echo: “Hi!” The time elapsed between call and echo, according to his watch, is ten seconds. His call has taken five seconds to reach the object and five seconds more to return. Since he knows that sound travels at the rate of about 1000 feet a second, he deduces that an obstruction lies in the east, about 5000 feet away. Slowly changing position, he repeats his call at various points around the compass. Some echoes take longer to return than others, indicating more distant objects. Other echoes come back in a fraction of a second, showing an object very close. Thus he gradually constructs a mental map of the surrounding terrain. Radar detects and locates objects in a similar way, by reflecting sharp pulses of radio waves. But spurious echoes, which sometimes deceive the operator, can also appear on the scope. These “anomalous” or abnormal returns may have one of several causes, including the nature of the radar mechanism itself. To help explain this, let us go back to our analogy of the man in the open field. Let us suppose that the man has mechanized his device. To ease the strain on his vocal cords, he has built a megaphone with a record-playing device. The megaphone rotates automatically and sends out a recorded “Hi!” once every twenty seconds, as regular as clockwork. To increase the sensitivity of his hearing, he wears ear trumpets that point in the same direction as the megaphone. This procedure is more effective than cupping his ears and eliminates some of the extraneous noise that might come in from the rear and the sides. With this improved equipment the man now repeats his experiment. As before, he gets no signal from the north. When he turns to the east he gets an echo after ten seconds, just as he did during his first experiment. As he continues to turn slowly, like a minute hand on a clock dial, he mentally maps the positions of the echoes as distances along the hand from the center of the dial, and compares this new map with the crude one he constructed earlier. Basically the two agree.