But although I sat up until after two o'clock the next morning, she did not arrive and I received no word from her! What contretemps had occurred? I was seriously uneasy, for I had impressed upon her the vitally important nature of our task, and if she failed or met with any mishap we should be in a serious predicament, for we had no trace whatever of any of Shackleton's associations in London, and anything might happen before we could run him to earth again among the teeming millions of the metropolis, the safest hiding-place on earth. It was not until six o'clock the next evening that I received, by express messenger, a hastily scrawled note in which Madame Gabrielle said: "Be extremely careful! They have discovered me, and I am being watched, so cannot come near you. Great things are in progress. Get someone to watch Shackleton, who is at the address below. Some great plot is in progress--that is certain.--G." Without a moment's delay, I slipped round to Whitehall, and very soon an expert watcher was at the address given by Madame. Twelve hours later, I was filled with dismay by a telephone message which told me that neither Shackleton nor Norman had been to the address given. They had both disappeared! CHAPTER NINETEEN. SPY'S LETTER DECIPHERED. Back in Curzon Street, completely at a loss, I flung myself into a big arm-chair, and over a succession of pipes tried to piece our disconnected facts into a consecutive whole. Shackleton, or Schack, had moved from Newcastle to Bristol before the war, and I had little doubt that he had done so by express orders from the Koniggratzer-strasse. From this I argued mentally that Bristol would almost certainly be the seat of his main activities, and that his early return thither might be looked for with some degree of confidence. Added to this, we knew from the frank declaration of a high port official that the Bristol Channel towns were swarming with spies, and I felt little doubt that they were acting under Schack's direction. On the whole, now that we had apparently lost the two men in London, Bristol seemed the most promising base for our operations. I decided therefore to return, and, leaving Madame at the Grosvenor in London, I took Aubert with me, together with an English secret agent whom I will call Moore. Moore took lodgings opposite Shackleton's house in Bristol and at once opened an unwinking vigilance over the place. For a fortnight, however, there was no sign. In the meantime two letters arrived, addressed to Shackleton, at Soulsby's. These were opened by the authorities, photographed, and, after being resealed, were delivered in the ordinary way. In one of the letters, which had been posted in London and purported to be an ordinary business transaction, was the statement: "We are having great difficulty with our clients Johnson and Phillips, so we have placed the matter in the hands of our solicitors for advice."