“I don’t want to talk here,” Sir Clinton said to Cecil. “Get a coat and walk with us down to the car. We’ve done our work for the night.”
The Chief Constable waited until they were well away from the house before beginning his explanation.
“That otophone is—as I expect you saw—simply a microphone for picking up sound, plus a two-valve amplifier for magnifying it. The sounds that reach the microphone are amplified by the valves set to any extent, within limits, that you like to set it for. You can make the crumpling of a piece of paper sound like a small thunderstorm if you choose; and it’s especially sensitive to clicks and sounds of that sort. The mere involuntary shifting of your feet on that parquet floor made a lot of disturbance.
“Now in the older type of combination locks, if the dial was carefully manipulated, a person with sharp hearing might just be able to detect a faint click when a tumbler fell into place in the course of a circuit; and by making a note of the state of the dial corresponding to each click the combination could finally be discovered. In the modern patterns of locks this has been got round. They’ve introduced a thing called a balanced fence arbour, which is lifted away from the tumblers as soon as the lock spindle is revolved; so in this new pattern there’s no clicking such as the older locks give.”
“I see now,” said the Inspector. “That’s an old pattern lock; and you were using the otophone to magnify the sound of the clicks?”
“Exactly,” Sir Clinton agreed. “It made the thing mere child’s play. Each click sounded like a whip-crack, almost.”
“So that’s why Foss brought the otophone along? He meant to pick the lock of the safe and get the medallions out of it?”
“That was one possibility, of course,” Sir Clinton said, with a grave face. “But I shouldn’t like to say that it was the only possibility.”
He smoked for a few moments in silence, then he turned to Cecil.
“Now I’ve a piece of work for you to do; and I want you to do it convincingly. First thing to-morrow morning you’re to find some way of spreading the news that you’ve recovered all the genuine medallions and that they’re in the safe. Don’t give any details; but see that the yarn gets well abroad.”