I looked, and there found a number scribbled in pencil. "By Jove!" I cried. "Most excellent. I'll soon find out what his movements were. Thank you very much," I added. "Remember nobody is to know anything whatever of the work in progress. That man may have been a Spy." "That's just exactly what I put him down to be, sir!" declared the foreman. "But trust me. Nobody shall know anything." When I rejoined Roseye and Teddy they were inquisitive--and very naturally--as to what the foreman had been telling me. But I kept my own counsel, determined to make investigations alone. We drove back to town and lunched in the restaurant at the Piccadilly Hotel. Teddy had suggested the Automobile Club, but I had overruled him, and we went to the Piccadilly instead. At the club there was far too much flying "shop"--and I wanted time to think. At three o'clock I ran Roseye home, dropping Teddy on the way, and then returned to Shaftesbury Avenue. As I entered, Theed told me that his father had been up to say that on the previous night there had been some strangers about the shed at Gunnersbury. He had heard footsteps around the place at about three o'clock in the morning, but on going out he could discover nobody. He had taken out his big heavy Browning pistol which I had bought for him, and he had told his own son that he regretted that he had not caught the intruders. Here was another source of suspicion! It confirmed my belief that the Invisible Hand had been laid once more upon us, and, further, that whoever directed it was alike most daring and unscrupulous. "That's most curious!" I said, in reply to Theed. "Your father seems to be having quite a lively time at night out there!" "Yes. He does, sir. He's convinced that somebody is watching to find out what's going on--spies, he declares." "No, no, Theed," I laughed, in order to hearten him. "There's far too much bunkum talked about spies, and far too many sensational rumours on every hand. Tell your father that he's becoming nervous. Surely he ought not to be after all his long police service!" I only uttered those words for effect. I knew that Theed would bully the old man, and tell him that he was suffering from nerves. Every son loves to jeer at his father, be he peer or peasant. I passed into my room and took up the telephone. In a few moments I was on to my friend Professor Appleton, the Director of the National Physical Laboratory, that department which, for years, had studied aeronautics. "I don't follow you, Mr Munro," he said, when I told him the facts. "What name do you say?" "Hale," was my reply. "H-a-l-e," and I spelt it. "We've nobody of that name. There must surely be some mistake!"