When we are travelling along in a ship or an aeroplane at night or in fog, we are much happier if we know that the captain of the ship, or the pilot of the plane, knows where rocks or mountain tops are，so that he can keep away from them. Until the Second World War, travellers could never have this feeling of safety，because there was no way of "seeing" dangers through fog and cloud. But now there is "radar", a wonderful wartime invention , which has saved the lives of many thousands，both in war and in peace.
Radar is not too difficult a thing for the ordinary man in the street to understand. All of us have heard echoes：we speak in a large hall or before a wall of rock, and our voices come back to us, echoed by the hard substance which they hit, just as light reflected by a mirror, or a ball is thrown back by the wall it has hit. Scientists know the speed at which sound travels, so they can measure the distance of a wall of rock by making a loud sound and seeing how long it takes to reach the wall and return to the place from which it started. For example, at the same time as the noise is made (perhaps with a car horn) a special watch, which can measure time very accurately, is started. As soon as the echo comes back, the watch is stopped, and it is found that the sound has taken 20 seconds to go and return. We know that the speed of sound is about 1 mile in 5 seconds. So we know that the noise of our car horn has covered 4 miles, two miles from us to the rock, and two from the rock back to us. The rock, then, must be two miles away.
But sound travels too slowly and cannot go far enough to be useful over long distances or when something is moving very fast. It is therefore no use to an aeroplane, or to a ship which wants to discover a small enemy ten miles away.
Now, it has been known for many years that wireless waves travel at very great speed; it takes them less than four seconds to go from the earth to the moon and back. But the difficult thing is to measure the time they take to go a certain distance. If they take less than two seconds to reach the moon, you can imagine that you would not be able to measure (with a watch or a clock) how long they take to go from a ship to the shore, or from an aeroplane to a mountain near it. Radar was made possible by the use of a thing called a "cathode ray tube" (CRT) which can measure millionths of a second. With this tube, we can "see" things at a great distance, and it shows us how far away they are, in which direction they lie, and what movements they are making. On the radar screen we can "see" all around us. The captain of a ship can find his way between rocks and other ships, and the soldiers guarding a town can see enemy aeroplanes hundreds of miles away, and follow them as they approach, perhaps from many different directions at once.
That is radar, one of the wonders of modern science, which is making travel and trade between the nations easier and safer.