"But I never said ..." He smiled faintly, almost wearily. "And since I have the great honour to be associated with you, I trust you will allow me to suggest a plan of campaign." "I was going to try and think one out to-night." "It is a privilege to assist. I have come in contact with many crafty and malignant criminals during the last thirty years, but here one detects a master. It will be a pleasure indeed to hunt him down. Have I your honourable permission to smoke?" With one hand he produced a square of rice paper and a pinch of tobacco from his pocket, and rolled a cigarette on his knee like a conjuring trick. He had not raised his voice, but a sudden gleam came into the oblique black eyes which suggested the deep but hidden ferocity of his race. He resumed. "From all I have gathered, and I have talked much with Captain Pring, Mr. Rickaby and the passengers of the _Albatros_, we have to look for a man who is (1) an aviator in the first rank; (2) an inventor and mechanical genius, or able to command the services of such; (3) a person of some wealth or able to procure money." I followed him completely and said so. From what we already knew these deductions were perfectly fair ones. "I thank you. Now we come to the man himself. I believe him to be a person of education, and one who has held a good social position. He is also desperate in his circumstances, and a person to whom material pleasure is the highest good." "Rickaby said that the men who came aboard the _Albatros_ spoke like educated people." "Yes. Our field of search already begins to grow narrower. Am I right in saying that every aviator in this country must pass an examination and be licensed before he is allowed to fly?" "It is so. All aviators, professional or amateur, must have a licence from the Air Police. This is registered. I have already had the records for the past ten years searched at Whitehall. But this has yielded no result. There is no one who could possibly be our man." "It was well thought of, Sir John, if I may say so. But in my opinion we shall have to go back a good deal further than ten years. We now come to the question of the pirate airship itself and its peculiar qualities. Let us fix upon one--the silence of its engines. I am aware that the constructors of motor engines have been busy upon this problem for years." "And with little result. The problem has not been solved." "Except by our unknown friends. I have already examined all the recent patents of silencing devices at your patent office here. I spent yesterday morning there, and found nothing. The significance of that is obvious. Any ordinary inventor who had discovered something of such importance would protect it at once. We can therefore make up our minds that in no regular motor-engineering works throughout this country has the complete silencer been evolved. It would be impossible for the most brilliant inventor to keep such a thing entirely to himself." "Again the field shrinks?"