THE place to spot him was Brie, or Vincennes, or Berck-sur-Mer, or any place in France where, on certain days of the year, the sky was alive with kites. There André Cassagnes would mingle with the crowd, an unassuming elderly man in a tweed coat or a baseball cap. If your hands were cold, he might give you a pair of gloves. When your kite-line got tangled he would teach you his own noeud Cassagne, or slip-knot, which could free you in seconds. If a child’s kite was heading for disaster, “Dédé” would unhesitatingly ditch his own to save it.

His own kites, though, were masterpieces: great cellular wheels 13 feet in diameter, a double wheel called “the Crown”, clocks, castles, star-shapes made of tetrahedrons, and five interlocking rings in honour of the Olympic games. No kitemaker was more celebrated in France, and he was closely followed, too, in Germany and America. He had invented a plastic connection that allowed his floating constructions of nylon and aluminium to be assembled and disassembled in a trice. He had also devised a kite-ferry that could run up a line, fold the kite’s wings, slide down the line, reopen them, then climb again, in perpetual motion, like a butterfly. Few stunts were more impressive.

Lines, and how to control them, were his passion. He was no mathematician or engineer—ingénieux rather than ingénieur, as he liked to say—and his education was no finer than could be expected for the son of a baker from Vitry-sur-Seine. Had he not been allergic to flour, he might have stayed in the shop. But he loved symmetry and geometry, and the way that, if you plotted points on the static x and y axes, lines would start forming and growing. He got bitten by kites, at the ripe age of 50, as he watched the ever-changing lines they made, vertical against horizontal, above a windswept beach in Normandy. Mr Cassagnes employed himself for the next few decades making kites not only more geometrically sophisticated but also more pilotable, or delicately responsive to the flyer’s imagination.

His day-job, until 1987 when he retired, was as an electrician for an interior-decoration firm based near Paris. There too he was involved in laying, tracing and untangling lines, some live, some dead. But the culmination of his passion appeared elsewhere, in an invention he never boasted of: a rectangular red plastic box with a grey screen and two white knobs, called the Etch-a-Sketch.

This toy, licensed in 1960, sold more than 100m units worldwide in 50 years and earned a place in America’s National Toy Hall of Fame, alongside Barbie and Mr Potato Head. The principle was simple. The user drew on the screen by turning the left-hand knob, for a horizontal line, or the right-hand one, to get a vertical. The lines were drawn by a stylus moving through aluminium powder; but they seemed to appear by magic and take unexpected turns. They disappeared by magic, too, when the box was turned over and given a good shake. The artist, in theory (for the practice was so difficult that many an Etch-a-Sketch was thrown aside in despair), was free to take his line wherever he liked. Deliberately, the toy was designed to look like a TV set: but one where the viewer was given only a blank, inviting sky.

Mr Cassagnes had invented this ardoise magique at work. While fitting a plate over an electric-light switch, he noticed that pencil marks he had made on the protective decal were transferred to the other side when he took it off. After much messing around with aluminium powder, and the fitting of a joystick to operate the stylus, he took his invention to the Nuremberg toy fair in 1959. The chairman of Ohio Art, an American company, bought the rights for $25,000, and the rest was history.

The charm of this toy was not only its simplicity, and the way it freed imagination, but the ease with which everything vanished into thin air and could be begun again. Hence the unfortunate words of one of Mitt Romney’s advisers in the 2012 race, who said his candidate’s campaign could be shaken and restarted like an Etch-a-Sketch; and the crowing remark of a rival, Newt Gingrich, who handed an Etch-a-Sketch to a child at one rally and pronounced her now fully equipped to be president of the United States.

Lines converging

All this fame Mr Cassagnes barely registered. He never took full credit, or royalties, for his invention. He could not afford to buy the patent, so a man called Arthur Grandjean did the paperwork and got his name on it. His name, too, went into the Toy Hall of Fame. Mrs Cassagnes was intensely frustrated by this. Her husband however, never seemed to mind much.

He had his kites, and spent, all told, twice as many years on them as on his drawing box. He never tired of finding new geometrical shapes and combinations for them. Besides, it was not in his nature to keep good discoveries to himself. At festivals he sometimes even gave his kites away. Nothing made him happier, at Brie or Vincennes or Berck-sur-Mer, than the free, magical, unexpected, yet subtly controlled movement of their lines in space. And perhaps no epitaph was more apt than the remark of a French blogger: “All our lines converge towards you, and our kites salute you.”