“This is the microphone,” he explained, drawing out a thick ebonite disk mounted on the false bottom of the compartment. “It’s attached to a longish wire so that you can take it out and put it on a table while the case with the valves and batteries lies on the floor out of the way. Now we’ll tune up.”
He brought microphone and ear-phone together, when a faint musical note made itself heard. Then he handed the microphone to Cecil.
“Hold that tight against the safe door, Cecil. Get the base in contact with the metal of the safe and keep the microphone face downwards. It’s essential to hold it absolutely steady, for the slightest vibration will put me off.”
He fitted on the head-band and moved the two tiny levers of the otophone until the adjustment of the instrument seemed to satisfy him. Then, very cautiously, he began to work the mechanism of the combination lock. For some time he seemed unable to get what he wanted; but suddenly he made a slight gesture of triumph.
“It’s an old pattern, as I thought. There’s no balanced fence arbour. This is going to be an easy business.”
Easy or not, it took him nearly a quarter of an hour to accomplish his task; for at times he obviously went astray in the work.
“Try to keep your feet still,” he said. “Every movement you make is magnified up to the noise of a pocket avalanche.”
At last the thing was done. The safe door swung open. Sir Clinton took off the head-band, received the microphone from Cecil, and packed it away in the case of the otophone along with the ear-phone.
“You’d better jot down the number of the combination, Cecil,” he suggested. “It’s on the dial at present.”
While Cecil was busy with this, the Chief Constable switched off the otophone and put it in a place of safety.
“Now we’ll see what’s inside the safe,” he said.
He swung the door full open and disclosed a cavity more like a strong-room than a safe.
“Have you any idea where the medallions were usually kept?” he inquired.
Cecil went over to one of the shelves and searched rapidly.
“Why, there are only two of them here!” he exclaimed in dismay.
“Hush!” Sir Clinton warned him sharply. “Don’t make a row. Have a good look at the things.”
Cecil picked up the medallions and scanned them minutely. His face showed his amazement as he turned from one to another.
“These are the replicas! Where have the genuine Leonardos gone?”
“Never mind that for the present. Put these things back again. I’m going to close the safe. We mustn’t risk talking too much here; and the sooner we’re gone the better.”
He picked up the otophone and led the way out of the museum.
“You might bring Froggatt back to his post here,” he said. “We don’t need him at the stair any longer. I must go upstairs again for a moment with this machine.”
Cecil piloted Froggatt back to his original post just as the Chief Constable rejoined them.
“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 valve 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.
