When Dr. Priestley settled down to writing a book, he concentrated his whole attention upon it, to the exclusion of everything else. He allowed nothing whatever to distract his mind, even for a few minutes. He lived entirely in his subject, refusing even to read the newspapers, except certain scientific periodicals which might happen to contain something relevant to the work he had in hand. As he expected his secretary to follow his example, it was hardly to be wondered at that both of them looked jaded and worn out. “I took the manuscript to the Post Office myself, sir,” said Harold Merefield listlessly. “Here is the registration receipt.” “Excellent, my boy, excellent,” replied the Professor, looking up. “So the work is finished at last, eh? I have been destroying such notes as we shall not require again. The rest you can file at your leisure. Dear me, you look as if you needed a change of occupation.” He stared at his secretary through his spectacles, as though he had seen him that morning for the first time for many months. “Yes, I think we both need a change of occupation,” he continued. “I feel that I should welcome some enticing problem, mathematical or human. It is time we stepped from our recent absorption back into the world. Let me see. What is the date?” “April 28th, sir,” replied Harold with a smile. He knew well enough that the Professor would have accepted any other day he chose to mention. “Dear me! Then the world is six months older than when we retired from it. No doubt many interesting problems have arisen in the interval, but I fear that their solutions lie in other hands than ours. By the way, when does our friend Inspector Hanslet return from America?” Harold turned to one of the big presses which lined the walls of the room, and took from it a folder marked “Inspector Hanslet.” He consulted this for a moment, then looked up towards his employer. “At the end of this month, sir. There is no definite date mentioned. I dare say he is in London already.” “Perhaps so,” agreed the Professor. “It does not really matter. My thoughts turned to him naturally, as to one who has in the past supplied us with some very satisfactory problems. Well, we must be patient, my boy. I have no doubt that we shall very soon succeed in finding some congenial work with which to occupy our minds.” He returned to the business of sorting his papers, while Harold sat down at the table reserved for his use, thankful to be able to do absolutely nothing for a few minutes. His idea of a change of occupation was not to plunge at once into some abstruse mathematical investigation which would involve him in the writing up of endless notes. If only Hanslet would come back and divert the Professor’s thoughts into some other channel! But of Hanslet, since he had departed for New York during the previous year to co-operate with the American police in running to earth a gang of international swindlers, nothing had been heard.