Moving noiselessly forward into the room, he went down on one knee, and proceeded to make a careful examination of the secret door. Then he took a measuring-tape out of his pocket, and proceeded to measure the exact distance of the opening from the upper end of the room. Then he took his penknife and cut away a couple of threads out of the carpet close to the book-case, at those points precisely where the secret door fitted into it when shut. Not less carefully did he examine the spring, and the mode by which it was acted on when the door was closed. There was nothing very complicated about it now that its mechanism was laid bare. A very slight examination sufficed to show Ducie its method of working, and where and how it was opened from without. A faint noise from below warned him that his time was up. He glided back as noiselessly as he had come, and disappeared behind the curtain just as M. Platzoff began to ascend the steps that led from the cavern. Captain Ducie stood with his hand on the door of the corridor for a full hour before he ventured to take another step in retreat. Then judging that Platzoff, who had gone to bed again, could not fail to be asleep, he went quietly back by the way he had come. CHAPTER XV. THE LITTLE PACKET FROM LONDON. Next morning, immediately after breakfast, Captain Ducie shut himself up in his own room on the plea of having several important letters to write. The letters resolved themselves into one note, of no great length, addressed to a friend in London--to the same friend, in fact, to whom he had applied for a translation of the stolen cryptogram. Although the note did not contain more than a dozen lines, Captain Ducie was unusually particular as to its composition. He corrected and re-wrote it several times before he was satisfied. Then he sealed and directed it, and went down into the village and posted it himself. Then he set himself to wait patiently for a reply. A reply came on the fifth day by post, in the shape of a tiny square packet. Captain Ducie received the packet from Jasmin with apparent indifference, but he did not open it till he was alone. The contents consisted of a brief note from his friend, inside which was a small square box made of very thin wood, which proved to be filled with some dark, fatty-looking substance, from which exhaled a faint, sickly odour that was far from pleasant. The following is a copy of the note:-- "My dear Ducie,--I send you a small quantity of the drug you ask for. I daresay there will be enough to serve your purpose. It is an exceedingly powerful narcotic, and very little of it must be used at one time. I greatly question the advisability of using it at all in the case of neuralgic pains such as you describe, but I presume you are acting under advice.