In one direction it led towards the railway line by which the Professor had approached the place, and rejoined the line lower down than the point at which he had left it. He started to walk down the track, with a muttered exclamation of satisfaction. By following it he might add a yard or two to his journey, but it would save him a slow and tiresome trudge through the heather in the gathering darkness. Of his ability to find his way back to the inn he had no doubt. He had only to follow the railway line backwards until the lights of the village came in sight, and then strike across country till he reached them, even if he missed the lane by which he had come. The Professor had not gone very far before he stopped dead, and bent down to examine the narrow white strip beneath his feet. He felt in his pocket, and struck a match, which he held close to the surface of the path. It was a still, windless evening, and the flame of the match burnt clear and unflickering, illuminating a narrow circle of fine, white sand. Across the centre of this patch, following the path upon which the Professor stood, ran the sharp and unmistakable imprint of a bicycle tyre. Somehow the discovery made the Professor’s pulses beat quicker than before. This apparently deserted track was in use, then, despite its overgrown appearance. No doubt it formed a short cut between two spots upon the heath, such as a pit and a clay-worker’s cottage. This was the obvious explanation, but to the Professor’s mind it seemed unsatisfactory. He had studied the map very carefully that afternoon, and knew roughly the location of every human habitation marked upon it. And this particular track seemed to run in the wrong direction to connect up with any of these. Was it possible that somebody beside himself was interested in the ruined cottage which he had just left? He threw the match away, and strode forward briskly in the direction of the railway line, which he calculated must now be less than a quarter of a mile ahead. He had an uncanny feeling that the rider of the bicycle might have been concealed close at hand while he was investigating the ruins, that he himself might have been the object of close observation all the time he had believed himself to be alone in the vast emptiness of the heath. It was an uncomfortable and a disturbing thought, and the Professor pushed on rapidly, grasping his stick tightly. Of course it was absurd, he was not the least likely to be molested. But he could not keep his mind from drawing ugly pictures of the dark thickets, of those deep, black pools, so admirably adapted as a hiding place for a murdered body——