“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.”

“Good-bye, then,” said Thorndyke, slipping a well-sharpened pencil through the rubber band that fixed the notebook to the board.  “Let me know how the adventure progresses—­if it progresses at all—­and remember, I hold your promise to come and see me again quite soon in any case.”