To the first I gave my attention, while he took out the contents of the second. The letters, of which there were seven or eight, secured by an elastic band, I took out and read, being puzzled greatly thereby. They were all type-written and bore the post-mark "London, S.E." The first had been received about three months before, the last as recently as a fortnight ago. They were very friendly, commencing "Dear Eva," and although the writer was apparently extremely intimate, there was, however, not a word of love, a fact which gave me some satisfaction. They all, without exception, contained a most mysterious reference to "the Silence," in terms extremely guarded and curious, one urging the utmost caution and declaring that a grave peril had unexpectedly arisen which must, at all hazards, be removed. The writer did not appear to be a very educated person, for in many places there were mistakes in spelling, while all were devoid of both address or signature, bearing only the single initial "Z." I passed them over to Boyd, asking his opinion, and as he sat at the writing flap reading them we were both suddenly startled by hearing a plaintive cry near us. It was a poor lean cat, who had accidentally been shut up there and was undoubtedly starving. "These letters are very strange," Boyd observed, looking up at me. "I wonder to what the silence refers?" "I don't know," I said. "There's evidently some very good reason that they've been concealed here." As I was speaking I took from beneath some letters, still remaining in the secret drawer Boyd had opened, a wooden pill-box, from which I removed the lid, there being disclosed a small quantity of a peculiar greyish-blue powder. "Hulloa!" Boyd exclaimed, with a quick glance at it. "What's that, I wonder? No label on the box. It looks suspicious!" "Yes," I agreed. "I wonder what it is, that it should be so carefully concealed?" "Leave it aside for a moment," he said. Then taking up a large envelope which, while I had been reading the letters, he had been carefully examining, he drew from it two photographs. "Do you recognise the originals of these?" he inquired with a grave smile. "Great Heavens!" I gasped. "Why, they are the man and the woman whom we found at Phillimore Place!" "Exactly," he said, in a voice of satisfaction, just as his assistant re-entered. Then, before I could recover from my bewilderment, he took up the little wooden box, exclaiming-- "This powder here is a very suspicious circumstance, but we'll test it at once." Turning to the local officer he said-- "I saw you eating something when you met us and you put part of it in your pocket. What was it?" "A sandwich. My wife always makes me one when I go out on night-duty," the man explained. "Have you any of it left?"