"But the cupronickel bullet shows smokeless powder and high-class criminal activity. I doubt if one of those little rods would take a modern steel-jacketed bullet. They're used in automatics." "But automatics have good rifling. That bullet was as smooth as before it was shot. Here it is!" Fosdick opened a drawer and pulled out a later-day projectile of the lesser-caliber. "This is smooth!" he repeated with heat. "It was cut from the old millionaire's brain. It ain't scratched. It never took the rifling it was intended for. My theory is, that it was fired from a gun of larger caliber. That is to say, it didn't fit the bore. A thirty-thirty rifle might be used to hold a twenty-two caliber bullet. It would not take the rifling of this." Drew shook his head. "That's hardly possible," he declared. "It's too vague and doesn't suit me. We're going to find that the deeper we get in this thing, the simpler will be the explanation. I remember any number of cases which have been solved in this city where the mystery was so wrapped up in speculation and the improbable that our minds failed to grasp the simple thing which was the solution." "Then you think the lack of rifling on the bullet might be the opening wedge to catching the man who shot Stockbridge?" "It could well be, Fosdick. The lack of a thing sometimes is just as important as the visible clue. Do you remember the Rajah case at Gramercy Park?" Fosdick leaned back in his chair and stared up at the ceiling. "Seems to me that I do," he said, thrusting out his lower lip. "There was a big jewel missing. Sort of an Idol's Eye case--wasn't it?" "Exactly! A white diamond was missing at a dinner. Lights went out as they were passing the stone around the table. Lights came on again and the diamond was gone. Everybody accused. A strange print was found on the sideboard. Servants knew nothing about it. The print didn't correspond to any which we took there. Seemed impossible and all that. Well, the very fact that the print didn't correspond was the means of finding the stone and the culprit. You remember it?" "Vaguely." "Simple! A Lascar who waited on the table slipped off his shoes, crept into the room, secured the diamond and climbed to the sideboard where he hid it on top of a picture. The thumbprint which we puzzled our heads over was a toe-print! We got the fellow!" "I recall it now," said Fosdick. "I think one of our men thought out the matter." "He didn't!" declared the detective. "We worked it out! The city department had given up the case. This may be the same. I'll venture to say that as soon as you get a good operative some private agency secures his services. Now, Commissioner, confess up. What manner of gun could fire a bullet, such as a cupronickel one, without leaving markings?" "Smooth bore. An old flint-lock--for instance."