Pool Watch Swimmers can drown in busy swimming pools when lifeguards fail to notice that they are in trouble. The Royal Society for the Prevention of Accidents says that on average 15 people drown in British pools each year, but many more suffer major injury after getting into difficulties. Now a French company has developed an artificial intelligence system called Poseidon that sounds the alarm when it sees someone in danger of drowning. When a swimmer sinks towards the bottom of the pool, the new system sends an alarm signal to a poolside monitoring station and a lifeguard's pager. 'In trials at a pool in Ancenis, near Nantes, it saved a life within just a few months, ' says Alistair MeQuade, a spokesman for its maker, Poseidon Technologies. Poseidon keeps watch through a network of underwater and overheard video cameras. AI software analyses the images to work out swimmers trajectories(轨道，轨线). To do this reliably, it has to tell the difference between a swimmer and the shadow of someone being cast onto the bottom or side of the pool. 'The underwater environment is a very dynamic one, with many shadows and reflections dancing around. ' says McQuade. The software does this by 'projecting' a shape in its field of view onto an image of the far wall of the pool. It does the same with an image from another camera viewing the shape from a different angle. If the two projections are in the same position, the shape is identified as a shadow and is ignored. But if they are different, the shape is a swimmer and so the system follows its trajectory. To pick out potential drowning victims, anyone in the water who starts to descend slowly is added to the software's 'pre-alert' list, says McQuade. Swimmers who then stay immobile on the pool bottom for 5 seconds or more are considered in danger of drowning. Poseidon double-checks that the image really is of a swimmer, not a shadow, by seeing whether it obscures the pool's floor texture when viewed from overhead. If so, it alerts the lifeguard, showing the swimmer's location on a poolside screen. The first full-scale Poseidon system will be officially opened next week at a pool in High Wycombe. Buckinghamshire. One man who is impressed with the idea is Travor Baylis, inventor of the clockwork radio. Baylis runs a company that installs swimming pools -- and he was once an underwater escapologist (表演脱身术的人)with a circus. 'I say full marks to them if this works and can save lives, ' he says. But he adds that any local authority spending 230,000 -- plus on a Poseidon system ought to be investing similar amounts in teaching children to swim. AI means the same as