“What may be the meaning of this martial display?” I asked, as we turned down Fetter Lane.

“House protection,” he replied promptly.  “You will agree that a discharge of musketry, followed by a bayonet charge, would disconcert the boldest of burglars.”

I laughed at the absurd picture thus drawn of the strenuous house-protector, but nevertheless continued to speculate on the meaning of my friend’s eccentric proceedings, which I felt sure were in some way related to the murder in Brackenhurst Chambers, though I could not trace the connection.

After a late lunch, I hurried out to transact such of my business as had been interrupted by the stirring events of the morning, leaving Thorndyke busy with a drawing-board, squares, scale, and compasses, making accurate, scaled drawings from his rough sketches; while Polton, with the brown-paper parcel in his hand, looked on at him with an air of anxious expectation.

As I was returning homeward in the evening by way of Mitre Court, I overtook Mr. Marchmont, who was also bound for our chambers, and we walked on together.

“I had a note from Thorndyke,” he explained, “asking for a specimen of handwriting, so I thought I would bring it along myself, and hear if he has any news.”

When we entered the chambers, we found Thorndyke in earnest consultation with Polton, and on the table before them I observed, to my great surprise, the dagger with which the murder had been committed.

[Illustration:  THE ALUMINIUM DAGGER.]

“I have got you the specimen that you asked for,” said Marchmont.  “I didn’t think I should be able to, but, by a lucky chance, Curtis kept the only letter he ever received from the party in question.”

He drew the letter from his wallet, and handed it to Thorndyke, who looked at it attentively and with evident satisfaction.

“By the way,” said Marchmont, taking up the dagger, “I thought the inspector took this away with him.”

“He took the original,” replied Thorndyke.  “This is a duplicate, which Polton has made, for experimental purposes, from my drawings.”

“Really!” exclaimed Marchmont, with a glance of respectful admiration at Polton; “it is a perfect replica—­and you have made it so quickly, too.”

“It was quite easy to make,” said Polton, “to a man accustomed to work in metal.”

“Which,” added Thorndyke, “is a fact of some evidential value.”

At this moment a hansom drew up outside.  A moment later flying footsteps were heard on the stairs.  There was a furious battering at the door, and, as Polton threw it open, Mr. Curtis burst wildly into the room.

“Here is a frightful thing, Marchmont!” he gasped.  “Edith—­my daughter—­arrested for the murder.  Inspector Badger came to our house and took her.  My God!  I shall go mad!”

Thorndyke laid his hand on the excited man’s shoulder.  “Don’t distress yourself, Mr. Curtis,” said he.  “There is no occasion, I assure you.  I suppose,” he added, “your daughter is left-handed?”