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in them. Smells of garlic, and smokes cigarettes all day long."

"A very good fancy portrait of a French forçat. But one can't go much by description. We shall know Antoine better by-and-by, no doubt. In the meantime, the first thing to do is to make sure that nobody effects an entry without our knowledge."

"How do you propose to prevent it?"

"That is an easy matter. I shall rig up an electric alarm across the piece of wall they are working on. The plant provided for our bell-hanging arrangements will be just the thing. If you will lend me a hand, I will have it fixed in no time."

We set to work accordingly. Our first task was to fix two bells, one in my private office and the other in my bedroom, and to carry wires from them to the strong room. So far, we were able to work at our ease, and to converse when necessary. Now, however, we had to deal with the very wall behind which the concealed workman was engaged in his felonious task. Still, with unfailing regularity, came, first, the tap, tap of the mallet, then the scrunch of the chisel, and the fall of the displaced material. As we could hear him so plainly, it was conceivable that he might hear us also, and we therefore had to work in absolute silence. I held the candle while Macpherson attached with sealing-wax a number of silk threads, crossing the wall in various directions, and connected in some way, which I was not electrician enough to appreciate, with the wires of the bells.

After half an hour of this work, Macpherson gave me a nod of satisfaction, indicating that all was complete; and we returned to the office above.

"Thank goodness, that's over!" I said. "Now, will you kindly explain how it works? I thought silk was a non-conductor."

"So it is," he replied. "The principle is just this. No part of that wall can be displaced without making a pull upon one or other of those threads. The moment that happens, the circuit is completed mechanically and the bell rings. It's not easy to explain, save on the spot, but I'll guarantee that it works all right. By Jove, it is half-past twelve. I'll have just one more glass of the Madeira, and then to bed, to think out my plan for catching the thieves."

"I stole downstairs."

We retired to rest, but I for my part could not sleep. At half-past two I got up, and partially dressing myself, stole downstairs and paid a visit of inspection to the strong room. All was quiet, the midnight excavator having apparently suspended his labours for the night. Thus satisfied that there was no immediate danger, I returned to my bed, and slept soundly till daylight.

Macpherson met me at the breakfast table with a triumphant air. "My plan is complete,' he said. "Electricity will tell us when our thieves break through the wall, and chemistry shall capture them for us. Did you ever hear of the Grotta del Cane?"

"The name sounds familiar. Somewhere in Italy, isn't it?"

"The Grotta del Cane is a cavern near Naples, the soil of which generates carbon dioxide, commonly called carbonic acid gas. This gas, being heavier than air, does not disperse, but lies at the bottom of the cave, to a depth of a couple of feet or so. If you send a dog into into the cavern he becomes