"Nothing but a pot-hole," judged Thorburn, bending his spare, angular body to examine the print. The Professor's vitality all seemed centered in his big-bulged skull that sheltered a marvellous thinking machine. Now he put his lean hand to the base of his brain, rubbing the back of his head as if it ached. Then, under what seemed some powerful compulsion, he ran his bony finger around the print in the rock.