As you are aware, such devices are common in old libraries. I observed that books were piled on the floor at all other points, but that one bookcase was left clear. This, then, might be the door. I could see no marks to guide me, but the carpet was of a dun colour, which lends itself very well to examination. I therefore smoked a great number of those excellent cigarettes, and I dropped the ash all over the space in front of the suspected bookcase. It was a simple trick, but exceedingly effective. I then went downstairs, and I ascertained, in your presence, Watson, without your perceiving the drift of my remarks, that Professor Coram’s consumption of food had increased⁠—as one would expect when he is supplying a second person. We then ascended to the room again, when, by upsetting the cigarette-box, I obtained a very excellent view of the floor, and was able to see quite clearly, from the traces upon the cigarette ash, that the prisoner had in our absence come out from her retreat. Well, Hopkins, here we are at Charing Cross, and I congratulate you on having brought your case to a successful conclusion. You are going to headquarters, no doubt. I think, Watson, you and I will drive together to the Russian Embassy.”