I went back to my chair and continued to read out the entries from the notebook while Thorndyke laid off the lines of direction with the protractor, taking out the distances with the dividers from a scale of equal parts on the back of the instrument. As the work proceeded, I noticed, from time to time, a smile of quiet amusement spread over my colleague’s keen, attentive face, and at each new reference to a railway bridge he chuckled softly.

“What, again!” he laughed, as I recorded the passage of the fifth or sixth bridge. “It’s like a game of croquet. Go on. What is the next?”

I went on reading out the notes until I came to the final one:

“ ‘Nine twenty-four. Southeast. In covered way. Stop. Wooden gates closed.’ ”

Thorndyke ruled off the last line, remarking: “Then your covered way is on the south side of a street which bears northeast. So we complete our chart. Just look at your route, Jervis.”