“Yes.  And now we will consider the very peculiar character of the weapon with which the crime was committed.”

He had placed his hand upon the knob of a drawer, when we were interrupted by a knock at the door.  I sprang up, and, opening it, admitted no less a person than the porter of Brackenhurst Chambers.  The man looked somewhat surprised on recognizing our visitors, but advanced to Thorndyke, drawing a folded paper from his pocket.

“I’ve found the article you were looking for, sir,” said he, “and a rare hunt I had for it.  It had stuck in the leaves of one of them shrubs.”

Thorndyke opened the packet, and, having glanced inside, laid it on the table.

“Thank you,” said he, pushing a sovereign across to the gratified official.  “The inspector has your name, I think?”

“He have, sir,” replied the porter; and, pocketing his fee, he departed, beaming.

“To return to the dagger,” said Thorndyke, opening the drawer.  “It was a very peculiar one, as I have said, and as you will see from this model, which is an exact duplicate.”  Here he exhibited Polton’s production to the astonished detective.  “You see that it is extraordinarily slender, and free from projections, and of unusual materials.  You also see that it was obviously not made by an ordinary dagger-maker; that, in spite of the Italian word scrawled on it, there is plainly written all over it ‘British mechanic.’  The blade is made from a strip of common three-quarter-inch tool steel; the hilt is turned from an aluminium rod; and there is not a line of engraving on it that could not be produced in a lathe by any engineer’s apprentice.  Even the boss at the top is mechanical, for it is just like an ordinary hexagon nut.  Then, notice the dimensions, as shown on my drawing.  The parts A and B, which just project beyond the blade, are exactly similar in diameter—­and such exactness could hardly be accidental.  They are each parts of a circle having a diameter of 10.9 millimetres—­a dimension which happens, by a singular coincidence, to be exactly the calibre of the old Chassepot rifle, specimens of which are now on sale at several shops in London.  Here is one, for instance.”

He fetched the rifle that he had bought, from the corner in which it was standing, and, lifting the dagger by its point, slipped the hilt into the muzzle.  When he let go, the dagger slid quietly down the barrel, until its hilt appeared in the open breech.

“Good God!” exclaimed Marchmont.  “You don’t suggest that the dagger was shot from a gun?”

“I do, indeed; and you now see the reason for the aluminium hilt—­to diminish the weight of the already heavy projectile—­and also for this hexagonal boss on the end?”

“No, I do not,” said the inspector; “but I say that you are suggesting an impossibility.”