What else might the engineer think of? Well, blind humans sometimes seem to have an uncanny sense of obstacles in their path. It has been given the name ' facial vision, because blind people have reported that it feels a bit like the sense of touch, on the face, One report tells of a totally blind boy who could ride his tricycle at good speed round the block near his home, using facial vision. Experiments showed that, in fact. facial vision is nothing to do with touch or the front of the face, although the sensation may be referred to the front of the face, like the referred pain in a phantom limb. The sensation of facial vision, it turns out, really goes in through the ears. Blind people, without even being aware of the fact, are actually using echoes of their own footsteps and of other sounds, to sense the presence of obstacles. Before this was discovered, engineers had already built instruments to exploit the principle, for example to measure the depth of the sea under a ship. After this technique had been invented, it was only a matter of time before weapons designers adapted it for the detection of submarines. Both sides in the Second World War relied heavily on these devices, under such codenames as Asdic(British) and Sonar(American), as well as Radar(American) or RDF(British), which uses radio echoes rather than sound echoes.
Complete the summary below.
Choose ONE WORD ONLY from the passage for each answer.
Write your answers in boxes 31-34 on your answer sheet.
Facial Vision
Blind people report that so-called ＇facial vision＇ is comparable to the sensation of touch on the face. In fact，the sensation is more similar to the way in which pain from a _____31______ arm or leg might be felt. The ability actually comes from perceiving ______32______through the ears. However，even before this was understood，the principle had been applied in the design of instruments which calculated the _____33______of the seabed. This was followed by a wartime application in devices for finding _____34______
