“But there must be a hidden spring of some sort, unless you are to open it by a mere exercise of force.  I suppose the box is empty.”

“I thought it was at first, but now I am not so sure of that either.  It all depends on the position in which you hold it.  Hold it in this position—­like this—­close to your ear.  Have you a small hammer?” I took a small hammer.  “Tap it softly, with the hammer.  Don’t you notice a sort of reverberation within?”

Pugh was right, there certainly was something within; something which seemed to echo back my tapping, almost as if it were a living thing.  I mentioned this to Pugh.

“But you don’t think that there is something alive inside the box?  There can’t be.  The box must be air-tight, probably as much air-tight as an exhausted receiver.”

“How do we know that?  How can we tell that no minute interstices have been left for the express purpose of ventilation?” I continued tapping with the hammer.  I noticed one peculiarity, that it was only when I held the box in a particular position, and tapped at a certain spot, there came the answering taps from within.  “I tell you what it is, Pugh, what I hear is the reverberation of some machinery.”

“Do you think so?”

“I’m sure of it.”

“Give the box to me.”  Pugh put the box to his ear.  He tapped.  “It sounds to me like the echoing tick, tick of some great beetle; like the sort of noise which a deathwatch makes, you know.”

Trust Pugh to find a remarkable explanation for a simple fact; if the explanation leans toward the supernatural, so much the more satisfactory to Pugh.  I knew better.

“The sound which you hear is merely the throbbing or the trembling of the mechanism with which it is intended that the box should be opened.  The mechanism is placed just where you are tapping it with the hammer.  Every tap causes it to jar.”

“It sounds to me like the ticking of a deathwatch.  However, on such subjects, Tress, I know what you are.”

“My dear Pugh, give it an extra hard tap, and you will see.”

He gave it an extra hard tap.  The moment he had done so, he started.

“I’ve done it now.”

“What have you done?”

“Broken something, I fancy.”  He listened intently, with his ear to the box.  “No—­it seems all right.  And yet I could have sworn I had damaged something; I heard it smash.”

“Give me the box.”  He gave it me.  In my turn, I listened.  I shook the box.  Pugh must have been mistaken.  Nothing rattled; there was not a sound; the box was as empty as before.  I gave a smart tap with the hammer, as Pugh had done.  Then there certainly was a curious sound.  To my ear, it sounded like the smashing of glass.  “I wonder if there is anything fragile inside your precious puzzle, Pugh, and, if so, if we are shivering it by degrees?”

II