It’s now thought possible flight #370 flew for four hours after the transponder signal disappeared. It cruises at 84% of the speed of sound which at 35,000 feet is 560 miles and hour. If true the missing aircraft could be 2,000 miles or more from it’s last known position.
But it seems most probable the a/c crashed into the sea.
The transponder is a type of receiver and transmitter that sends a signal on a frequency set by the pilot. It has a simple on-off switch and four small knobs each of which set a number from zero to 9 in a small window. The transponder is said to “squawk” a signal meaning it’s sending and receiving. Ground control can “see” the signal on their radar and follow the flight path. The radar follows the transponder signal because the blip from the aircraft is very small on a radar screen. The disappearance is a mystery and we hope the aircraft is found safe but that seems unlikely.