Death out of Thin Air by Clayton Rawson

Wizard’s private scientific wizard, was the man who designed and built, to Don’s order, all the curious gadgets and offstage mechanisms which were the secret of many of the Diavolo illusions. He locked the door behind the dicks and waved an “All clear” signal toward the mirror.
    He had known that Diavolo and the girls had returned from the theater and were next door. One of the skulls that served as bookends on the library table had told him that. He had seen it grin at him when its lower jaw moved ever so slightly, downward. The grinning skull always meant that the Reverend’s house was occupied.
    Karl is a stoop-shouldered little man with a great bushy shock of white hair and thick-lensed glasses. After his graduation from a famous European University Karl’s eyesight had failed him so that he was unable to pursue his chosen profession of construction engineer.
    Instead, for many years, he had toured the vaudeville stages of the world as Prof. Memo, lightning calculator and memory expert. Karl, hearing their names called off but once, can immediately recite the names of the playing cards in a shuffled deck, forwards, backwards or both ways from the center.
    Ask him to give you the square of 1,684 or add ten four-digit figures in his head and he gives you the correct answer before an adding machine can make up its mind. For amusement. Karl plays chess, blindfolded.
    Diavolo had met Karl in Budapest at a time when the latter was being treated by a famous eye surgeon. The treatments were eventually successful enough so that Karl, aided by his thick-lensed glasses, could see once again. But now the profession of engineering wanted younger men, and so Karl had turned to contriving whatever mechanical hocus-pocus was necessary for the Mysteries of Diavolo. The House of Magic was almost entirely the product of his ingenuity.
    When Pat saw his signal she turned to a large three-sheet poster on the wall that bore a picture of Hermann the Great. She made a simple pass before it with her hand. The poster slid upward out of sight. She walked through the opening into the Diavolo living room and the doorway closed automatically behind her. No hint of such an entrance showed in the paneled wall of the living room.
    â€œHello, Pat,” Karl said. “Where’s Don? And what has he been up to now? Why did those detectives—?”
    Pat’s frown was worried. “They want him for murder, Karl. They’ve already arrested Chan.”
    Karl, whose ingenious surprises had astonished thousands, now took a big dose of his own medicine. “But … but what—” Karl floundered, baffled by Pat’s announcement.
    Pat sat down and rapidly gave him the whole story. “Karl,” she said, finishing, “can a — a man climb straight up the side of a sheer wall?”
    Karl frowned, thinking over her story. “Nothing’s impossible, Pat,” he answered after a moment. “You should know that by now. Give me time and money enough and I’ll deliver any miracle you name — or something that looks a lot like it, at any rate.”
    â€œYes, I know,” Pat replied. “A magician can appear to do almost anything. But, Karl, the Bat didn’t just look as if he was climbing that wall. He actually did. He got into Don’s dressing room and out again by the window! There was no other way—”
    Pat stopped abruptly and Karl sprang to his feet.
    A voice behind them — a hard, insistent, steely voice commanded, “ Restez en repos! ”
    Beyond the archway to the hall were the stairs. A figure stood on the steps halfway down, a grim and evil monstrosity whose long black cloak fused with the inky shadows around it and seemed part of them.
    A black-gloved hand protruded from the folds of the flowing cape. It held an ugly square-nosed automatic.
    Pat knew enough French to know that the voice had said, “Stand still!” But the phrasing