“Look here,” he said, and seizing a scrap of paper, he drew the two sketches which I reproduce. “The flask is divided down the middle by a diaphragm C, so as to form two chambers, A and B. In these chambers are put two liquids, of which one is drugged and the other isn’t. E and F are two half diaphragms, and D is a very light and delicately fitted flap valve which will close the passage to either chamber. When you invert the flask, the liquid in the upper or B chamber runs out along diaphragm C, and its weight turns over valve D so that the passage to A chamber is closed. The liquid from B then pours out in the ordinary way. The liquid in A, however, cannot escape, because it is caught by the diaphragm F. If you want to pour out the liquid from A you simply turn the flask upside down, when the conditions as to the two liquids are reversed. You probably didn’t notice that I used the flask in this way at our lunch. You may remember that I poured out your liqueur first—it was drugged, of course. Then I got a convenient fit of coughing. That gave me an excuse to set down the flask and pick it up again, but when I picked it up I was careful to do so by the other side, so that undrugged liqueur poured into my own cup. I drank my coffee at once to reassure you. Simple, wasn’t it?”
“More than simple,” Cheyne answered with unwilling admiration in his tone. “A dangerous toy, but I admit, deuced ingenious. But I don’t follow even yet. That would have left the drugged remains in the cup.”
“Quite so, but you have forgotten my other two bottles and my cloth. I poured the dregs from your cup into the empty bottle, washed the cup with water from the other, wiped it with my cloth, poured out another cup of coffee and drank it, leaving harmless grounds for any inquisitive analyst to experiment with.”
“By Jove!” said Cheyne, then adding regretfully: “If we had only tried the handle of the cup for fingerprints!”
“I put gloves on after you went over.”
Cheyne smiled.
“You deserved to succeed,” he admitted ruefully.
“I succeeded in drugging you,” Blessington answered, “but I did not succeed in getting what I wanted. Now, Mr. Cheyne, you would like to see the tracing. Show it to him, Dangle, while I go back to the other house for Miss Merrill.”
Dangle left the room, returning presently with the blue-gray sheet which had been the pivot upon which all the strange adventures of the little company had turned. Cheyne saw at a glance it was the tracing which he had secured in the upper room in the house in Hopefield Avenue. There in the corners were the holes made by the drawing-pins which had fixed it to the door while it was being photographed. There were the irregularly spaced circles, with their letters and numbers, and there, written clockwise in a large circle, the words: “England expects every man to do his duty.” Cheyne gazed at it with interest, while Dangle and Sime sat watching him. What on earth could it mean? He pondered awhile, then turned to his companions.