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.

"You can read a compass pretty quickly, I think?" Thorndyke said.

"Of course I can. Used we not to sail a yacht together when we were students?"

"To be sure we did; and we will again before we die. And now as to your method of locating this house. Here is a pocket reading-lamp which you can hook on the carriage lining. This notebook can be fixed to the board with an india-rubber band—thus. You observe that the thoughtful Polton has stuck a piece of thread on the glass of the compass to serve as a lubber's line. This is how you will proceed. As soon as you are locked in the carriage, light your lamp—better have a book with you in case the light is noticed—take out your watch and put the board on your knee, keeping its long side exactly in a line with the axis of the carriage. Then enter in one narrow column of your notebook the time, in the other the direction shown by the compass, and in the broad column any particulars, including the number of steps the horse makes in a minute. Like this."

He took a loose sheet of paper and made one or two sample entries on it in pencil, thus—

"9.40. S.E. Start from home.
9.41 S.W. Granite setts.
9.43. S.W. Wood pavement. Hoofs 104.
9.47. W. by S Granite crossing. Macadam—

and so on. Note every change of direction, with the time; and whenever you hear or feel anything from outside, note it, with the time and direction; and don't forget to note any variations in the horse's pace. You follow the process?"

"Perfectly. But do you think the method is accurate enough to fix the position of a house? Remember, this is only a pocket-compass with no dial, and it will jump frightfully. And the mode of estimating distance is very rough."

"That is all perfectly true," Thorndyke answered. "But you are overlooking certain important facts The track-chart that you will produce can be checked by other data. The house, for instance, has a covered way by which you could identify it if you knew approximately where to look for it. Then you must remember that your carriage is not travelling over a featureless plain. It is passing through streets which have a determined position and direction and which are accurately represented on the ordnance map. I think, Jervis, that, in spite of the apparent roughness of the method, if you make your observations carefully, we shall have no trouble in narrowing down the inquiry to a quite small area. If we get the chance, that is to say."

"Yes, if we do. I am doubtful whether Mr. Weiss will require my services again, but I sincerely hope he will. It would be rare sport to locate his secret burrow, all unsuspected. But now I must really be off."