Finally the man figures out the explanation. When he made his first experiment, with primitive equipment, he had given one sharp shout and then waited for a long time for the signal to return; thus there was never any uncertainty about the source of the echo. The time that elapsed between shout and return had clearly indicated the distance of the echo-producing object. But the improved automatic equipment of the second experiment produced a train of signals going out continuously at regular intervals, twenty seconds apart. Therefore when the sound waves encountered a definite object, a train of echoes began coming back, twenty seconds apart. An object at a distance of 10,000 feet would return an echo in twenty seconds; another object at a distance of 11,000 feet would return an echo in twenty-two seconds. But an echo from this second object would reach the listener at exactly the same time as an echo from an object only 1000 feet away. He now understands why he seemed to detect a structure at a distance of 1000 feet which disappeared as he approached and then reappeared 10,000 feet farther away. In fact, the object that returned the misinterpreted echo could have been 20,000 or 30,000 or 40,000 feet farther away--any multiple of 10,000 feet. Large numbers of signals were returning every twenty seconds. The man had no way of deciding for certain whether a particular echo came from the most recent signal and therefore indicated a relatively close object, or whether it came from an earlier signal and therefore from a more distant object. Broadening his experiment our man eventually learned other characteristics of these echoes. He found that on the average day he was rarely plagued by this uncertainty in identifying the returns. The second-round echoes were very weak, almost undetectable, and therefore caused no major problem. But on other days, under different weather conditions, sound tended to travel long distances without losing much in intensity. On such days the echoes were often confusing. _Weather and Radar Echoes_ Radar is an echo machine that reflects radio waves instead of sound waves. Instead of traveling at the speed of sound, about 1000 feet a second, radio waves travel at the velocity of light, 186,000 miles a second. Successive pulses go out at very short intervals, perhaps one one-thousandth of a second apart, so that each pulse is followed by another just 186 miles behind it. If the operator gets a return from an object that is apparently at a distance of 25 miles, he must sometimes allow for the possibility that he is getting a secondary echo and that the actual distance may be different. The object that produces the echo may be at a distance of 25 plus 186 miles, or 25 plus twice 186 miles, or 25 plus any other whole-number multiple of 186 miles.