much less reliable.  Now, this man had an artificial foot, and he evidently distrusted his knee-joint, as is shown by his steadying it with his stick on the same side.  If he had merely had a weak leg, he would have used the stick with his right hand—­with the natural swing of the arm, in fact—­unless he had been very lame, which he evidently was not.  Still, it was only a question of probability, though the probability was very great.  Of course, you understand that those particles of woody fibre and starch granules were disintegrated snuff-grains.”

This explanation, like the others, was quite simple when one had heard it, though it gave me material for much thought as we pedalled on along the dark road, with Thorndyke’s light flickering in front, and the dogcart pattering in our wake.  But there was ample time for reflection; for our pace rather precluded conversation, and we rode on, mile after mile, until my legs ached with fatigue.  On and on we went through village after village, now losing the trail in some frequented street, but picking it up again unfailingly as we emerged on to the country road, until at last, in the paved High Street of the little town of Horsefield, we lost it for good.  We rode on through the town out on to the country road; but although there were several tracks of motors, Thorndyke shook his head at them all.  “I have been studying those tyres until I know them by heart,” he said.  “No; either he is in the town, or he has left it by a side road.”

There was nothing for it but to put up the horse and the machines at the hotel, while we walked round to reconnoitre; and this we did, tramping up one street and down another, with eyes bent on the ground, fruitlessly searching for a trace of the missing car.

Suddenly, at the door of a blacksmith’s shop, Thorndyke halted.  The shop had been kept open late for the shoeing of a carriage horse, which was just being led away, and the smith had come to the door for a breath of air.  Thorndyke accosted him genially.

“Good-evening.  You are just the man I wanted to see.  I have mislaid the address of a friend of mine, who, I think, called on you this afternoon—­a lame gentleman who walks with a stick.  I expect he wanted you to pick a lock or make him a key.”

“Oh, I remember him!” said the man.  “Yes, he had lost his latchkey, and wanted the lock picked before he could get into his house.  Had to leave his motor-car outside while he came here.  But I took some keys round with me, and fitted one to his latch.”

He then directed us to a house at the end of a street close by, and, having thanked him, we went off in high spirits.

“How did you know he had been there?” I asked.

“I didn’t; but there was the mark of a stick and part of a left foot on the soft earth inside the doorway, and the thing was inherently probable, so I risked a false shot.”