ere
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 <i>modus operandi</i> 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 whene