him the photographs of the road. He took them and laid them down
in a long line on the library floor. They seemed to consist of
little ridges of dirt on either side of a series of regular round
spots, some of the spots very clear and distinct on the sides,
others quite obscure in the centre. Now and then where you would
expect to see one of the spots, just for the symmetry of the
thing, it was missing. As I looked at the line of photographs on
the floor I saw that they were a photograph of the track made by
the tire of an automobile, and I suddenly recalled what the
gardener had said.

Next Craig produced the results of his morning's work, which
consisted of several dozen sheets of white paper, carefully
separated into three bundles. These he also laid down in long
lines on the floor, each package in a separate line. Then I began
to realise what he was doing, and became fascinated in watching
him on his hands and knees eagerly scanning the papers and
comparing them with the photographs. At last he gathered up two
of the sets of papers very decisively and threw them away. Then
he shifted the third set a bit, and laid it closely parallel to
the photographs.

"Look at these, Walter," he said. "Now take this deep and sharp
indentation. Well, there's a corresponding one in the photograph.
So you can pick them out one for another. Now here's one missing
altogether on the paper. So it is in the photograph."

Almost like a schoolboy in his glee, he was comparing the little
round circles made by the metal insertions in an "anti-skid"
automobile tire. Time and again I had seen imprints like that