"Yes," he replied, reminiscently, when I reminded him of the alliterative title and inquired to what it might refer, "that was one of the branch cases which grew out of the von Ewald chase--you remember Mary McNilless and the clue of Shelf Forty-five? Well, Dick Walters, the man who landed von Ewald, wasn't the only government detective working on that case in New York--not by some forty-five or fifty--and Mary wasn't the only pretty woman mixed up in it, either. There was that girl at the Rennoc switchboard.... "That's another story, though. What you want is the taxicab clue." * * * * * If you remember the incidents which led up to the von Ewald affair [continued Quinn, as he settled comfortably back in his chair] you will recall that the German was the slipperiest of slippery customers. When Walters stumbled on his trail, through the quick wit of Mary McNilless, there wasn't the slightest indication that there was such a man. He was a myth, a bugaboo--elusive as the buzz of a mosquito around your ear. During the months they scoured New York in search for him, a number of other cases developed. Some of these led to very interesting conclusions, but the majority, as usual, flivvered into thin air. The men at headquarters, the very cream of the government services, gathered from all parts of the country, were naturally unable to separate the wheat from the chaff in advance. Night after night they went out on wild-goose chases and sometimes they spent weeks in following a promising lead--to find only blue sky and peaceful scenery at the end of it. Alan Whitney, who had put in two or three years rounding up counterfeiters for the Service, and who had been transferred to the Postal Inspection Service at the time of those registered mail robberies in the Middle West--only to be detailed to Secret Service work in connection with the von Ewald case--was one of the bitterest opponents of this forced inaction. "I don't mind trouble," Whitney would growl, "but I do hate this eternal strain of racing around every time the bell goes off and then finding that some bonehead pulled the alarm for the sheer joy of seeing the engines come down the street. There ought to be a law against irresponsible people sending in groundless 'tips'--just as there's a law against scandal or libel or any other information that's not founded on fact." But, just the same, Al would dig into every new clue with as much interest and energy as the rest of the boys--for there's always the thrill of thinking that the tip you're working on may be the right one after all. Whitney was in the office one morning when the phone rang and the chief answered it. "Yes," he heard the chief say, "this is the right place--but if your information is really important I would suggest that you come down and give it in person. Telephones are not the most reliable instruments in the world." A pause followed and the chief's voice again: