The Professor climbed slowly up the steep slopes of the natural earthwork, which stands like a fortress guarding the only gap in the long range of the Purbeck hills, and listened attentively while the guide, delighted at finding a sympathetic audience so early in the year, recounted the history of the stronghold and indicated its more prominent features. Then, his inspection over, he sat down on the grass on the north-eastern slope of the hill, and drew his ordnance map from his pocket. His horizon was bounded by the high land on the southern side of the River Stour. At his feet was a rolling, sandy plain, diversified here and there by clumps of trees, but mostly covered with gorse and heather. This plain stretched away into the distance, and beyond it an occasional silvery gleam betrayed the winding channels of Poole harbour. Here and there across the plain he could see evidences of human occupation, the white surface of a clay-pit or the roof of a tiny isolated cottage. But, apart from these, the tract of country over which he looked appeared utterly deserted, abandoned to a great silence disturbed only by the note of some hovering bird. His map told him that no road traversed this expanse. Only a few winding paths, trodden by the feet of rare wayfarers, led away from where he stood in the direction of the thin whisps of smoke which indicated human habitation. Here and there he could see traces of the railway of which the landlord had spoken, which led from the hidden quarries behind him to a pier built upon one of the secluded arms of the harbour. It seemed to be but little used. The Professor, enjoying the quiet of the fine afternoon, sat for hour after hour, his eyes absorbing the subtle beauty of this unharassed land. And, except for the solitary figure of a labourer, tramping stolidly across the waste towards some unknown destination, he saw no sign of life in all the wide extent of country laid out before his eyes. Yet, somewhere, lost among the lonely solitudes of gorse and heather, lay the solution of the mystery which he had set himself to solve. Dr. Priestley smiled as the conviction that this was so came back to him with redoubled force. Utterly contrary to his usual methods of procedure, he had formed a theory based entirely upon conjecture. So far, these facts by which it must be tested seemed to point to the incorrectness of this theory. Yet, were they facts? Was it not even yet possible that some master brain had arranged these seeming facts in order purposely to mislead an investigator like himself? And, if so, what unknown dangers might he not be incurring by the attempt to prove them false?