What could be the explanation of that mystery? It was plain that the enemy were in possession of some facts and, further, that at all hazards, and regardless of all risks, they intended to discover our secret. I disregarded her remark, merely answering: "I fear the Zeppelin menace will be serious in the North Sea before many months. It is only the bad weather which protects us." The alterations to my machine were being carried on by a first-class firm at Willesden, therefore, at Teddy's suggestion, all three of us ran over in the car in order to inspect the work, which we found progressing most favourably. The foreman engineer, a big fat, elderly man, just as we were about to leave the premises, called me aside and, in a confidential tone, exclaimed: "Excuse me, sir. But did you send a gentleman named Hale here?" "Hale?" I repeated, looking at him in surprise. "I know nobody of that name!" "Well--here's his card," said the engineer. "He called yesterday afternoon, and told me that you'd sent him, and that he had your authority to look at your machine." I took the rather soiled card, and saw upon it the name: "Harold Hale-- National Physical Laboratory." I held it in my hand in surprise. "A Government official!" I exclaimed in wonder. "I gave no such permission!" I declared. "As I have repeatedly said, these alterations you are making are strictly in secret." "That's what I told him, sir." "You didn't let him see the work, I hope?" I asked anxiously. "Not very likely, sir," was the man's reply. "I asked him for a written authorisation, but he said he'd left it in his office. There was a good deal of swank about him, I thought. He seemed to have a swelled head." "Well--what happened?" I inquired. "Oh! He became very officious-like--said he was a Government inspector of aircraft, and demanded to see what alterations you were making in your machine. My reply was to tell him that when he brought a letter from you, I'd show him--and not before." "Excellent!" I said. "Then he didn't produce any credentials?" "None. But he argued with me for a long time--told me that I had no right to deny him access to information required for official purposes; that I was liable under the Defence of the Realm Act, and all sorts of other bunkum. In reply, I merely told him to go along to the office and see Mr Smallpiece, our manager--whom I knew to be up at the London office," added the foreman with a grin. "What kind of man was he? Describe him," I urged. "Well--he was about forty I should say--round-faced, with a little close-cropped black moustache. He was well dressed--a dark-blue overcoat with velvet collar, and a grey plush hat. He came in a taxi." "Ah! If we could find the driver, we might perhaps discover who he was," I exclaimed. "Well, sir, I suspected him, somehow. I didn't like him. So I took the number of the taxi. You'll see it on the back of the card."