During the winter months a remarkable series of safe robberies had taken place in the metropolis. In each case the safe had been blown open in the most scientific manner, and neither the public nor the police doubted that an exceptionally expert gang was at work; but it was a gang of which Scotland Yard had no knowledge, and a rumor had got about--how, I cannot say--that the thieves were Americans. Moreover, it was so evident that the thieves knew where and when they were likely to obtain the greatest haul that in one or two instances grave suspicions had fallen upon employees of the firms robbed, but there was not sufficient evidence to warrant arrest. As it happened, none of these cases had come into my hands, and I had told Christopher Quarles that I was disappointed. He suggested that I might fail, as others had done, which was possible, even probable, but somehow I had a lust to try my strength against this gang, and there was a conviction at the back of my mind that I should succeed. Well, I had got my chance, at any rate, and before I had finished my narrative the professor was just as keen as I was. At some time between the early closing on Saturday afternoon and nine o'clock on Sunday morning the head office of the City, Suburban and Provincial Bank, in Lombard Street, had been robbed of an immense sum in gold and valuables. The full amount of the loss had not yet been ascertained, but it was soon apparent that the first estimate was below the mark. Banks, as is well known, always keep a very large sum in gold upon the premises in case of emergency, and, naturally, extreme precaution is taken for its safety. At the City, Suburban and Provincial Bank this gold reserve, in sealed bags, containing definite sums, was in an inner strong-room. The steel doors of both the outer and inner rooms had been blown open with an explosive of immense strength but presumably making little noise. Several bags of gold had been taken from the inner safe, and in the outer safe two or three deed boxes belonging to clients had been forced open, and jewels stolen from them. On Saturday the night porter was a man named Coulsdon, who had been in the service of the bank for many years. It was his duty to visit every part of the premises at intervals during the night, and to register the time of each visit by the telltale clocks provided for the purpose. He was armed with a revolver, and by means of an electric bell in the entrance-hall could communicate, if necessary, with the porter who lived on the premises. His vigil ended at nine o'clock on the Sunday morning, when two clerks arrived to stay in the bank all Sunday. This was a special duty, especially paid for, and, as a general rule, each pair of clerks had the duty for four Sundays, when they were relieved by another pair. It was the custom for the resident porter to admit the clerks at a side door of the bank, opening into the narrow street turning at right angles to Lombard Street.