After breakfast I improvised a dark room and developed the films, while Craig went down the back lane along the shore “looking for clues,” as he said briefly.  Toward noon he returned, and I could see that he was in a brown study.  So I said nothing, but handed 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 left in the dust and grease of an asphalted street or the mud of a road.  It had never occurred to me that they might be used in any way.  Yet here Craig was, calmly tracing out the similarity before my very eyes, identifying the marks made in the photograph with the prints left on the bits of paper.

As I followed him, I had a most curious feeling of admiration for his genius.  “Craig,” I cried, “that’s the thumb-print of an automobile.”

“There speaks the yellow journalist,” he answered merrily.  “’Thumb Print System Applied to Motor Cars’—­I can see the Sunday feature story you have in your mind with that headline already.  Yes, Walter, that’s precisely what this is.  The Berlin police have used it a number of times with the most startling results.”

“But, Craig,” I exclaimed suddenly, “the paper prints, where did you get them?  What machine is it?”

“It’s one not very far from here,” he answered sententiously, and I saw he would say nothing more that might fix a false suspicion on anyone.  Still, my curiosity was so great that if there had been an opportunity I certainly should have tried out his plan on all the cars in the Fletcher garage.