“But, my dear Thorndyke,” I expostulated, “I have told you how I was conveyed to the house.  Now, will you kindly explain to me how a man, boxed up in a pitch-dark carriage, is going to identify any place to which he may be carried?”

“The problem doesn’t appear to me to present any serious difficulties,” he replied.

“Doesn’t it?” said I.  “To me it looks like a pretty solid impossibility.  But what do you suggest?  Should I break out of the house and run away up the street?  Or should I bore a hole through the shutter of the carriage and peep out?”

Thorndyke smiled indulgently.  “The methods proposed by my learned friend display a certain crudity inappropriate to the character of a man of science; to say nothing of the disadvantage of letting the enemy into our counsels.  No, no, Jervis; we can do something better than that.  Just excuse me for a minute while I run up to the laboratory.”

He hurried away to Polton’s sanctum on the upper floor, leaving me to speculate on the method by which he proposed that a man should be enabled, as Sam Weller would express it, “to see through a flight of stairs and a deal door”; or, what was equally opaque, the wooden shutters of a closed carriage.

“Now,” he said, when he returned a couple of minutes later with a small, paper-covered notebook in his hand, “I have set Polton to work on a little appliance that will, I think, solve our difficulty, and I will show you how I propose that you should make your observations.  First of all, we have to rule the pages of this book into columns.”

He sat down at the table and began methodically to rule the pages each into three columns, two quite narrow and one broad.  The process occupied some time, during which I sat and watched with impatient curiosity the unhurried, precise movements of Thorndyke’s pencil, all agog to hear the promised explanation.  He was just finishing the last page when there came a gentle tap at the door, and Polton entered with a satisfied smile on his dry, shrewd-looking face and a small board in his hand.

“Will this do, sir?” he asked.

As he spoke he handed the little board to Thorndyke, who looked at it and passed it to me.

“The very thing, Polton,” my friend replied.  “Where did you find it?  It’s of no use for you to pretend that you’ve made it in about two minutes and a half.”

Polton smiled one of his queer crinkly smiles, and remarking that “it didn’t take much making,” departed much gratified by the compliment.

“What a wonderful old fellow that is, Jervis,” Thorndyke observed as his factotum retired.  “He took in the idea instantly and seems to have produced the finished article by magic, as the conjurers bring forth rabbits and bowls of goldfish at a moment’s notice.  I suppose you see what your modus operandi is to be?”

I had gathered a clue from the little appliance—­a plate of white fret-wood about seven inches by five, to one corner of which a pocket-compass had been fixed with shellac—­but was not quite clear as to the details of the method.