"Lady Yardly," I said, "I know why you have come here. You have received blackmailing letters about the diamond."

There was no doubt as to my bolt having shot home. She stared at me open-mouthed, all color banished from her cheeks.

"You know?" she gasped. "How?"

I smiled.

"By a perfectly logical process. If Miss Marvell has had warning letters -"

"Miss Marvell! She has been here?"

"She has just left. As I was saying, if she, as the holder of one of the twin diamonds, has received a mysterious series of warnings, you, as the holder of the other stone, must necessarily have done the same. You see how simple it is? I am right, then, you have received these strange communications also?"

For a moment she hesitated, as though in doubt whether to trust me or not, then she bowed her head in assent with a little smile.

"That is so," she acknowledged.

"Were yours, too, left by hand - by a Chinaman?"

"No, they came by post; but, tell me, has Miss Marvell undergone the same experience, then?" I recounted to her the events of the morning. She listened attentively.

"It all fits in. My letters are the duplicates of hers. It is true that they came by post, but there is a curious perfume impregnating them - something in the nature of joss-stick - that at once suggested the East to me. What does it all mean?"

I shook my head.

"That is what we must find out. You have the letters with you? We might learn something from the postmarks."

"Unfortunately I destroyed them. You understand, at the time I regarded it as some foolish joke. Can it be true that some Chinese gang are really trying to recover the diamonds? It seems too incredible."

We went over the facts again and again, but could get no further towards the elucidation of the mystery. At last Lady Yardly rose.

"I really don't think I need wait for Monsieur Poirot. You can tell him all this, can't you? Thank you so much, Mr -"

She hesitated, her hand outstretched.

"Captain Hastings."

"Of course! How stupid of me. You're a friend of the Cavendishes, aren't you? It was Mary Cavendish who sent me to Monsieur Poirot."

When my friend returned, I enjoyed telling him the tale of what had occurred during his absence. He cross-questioned me rather sharply over the details of our conversation and I could read between the lines that he was not best pleased to have been absent. I also fancied that the dear old fellow was just the least inclined to be jealous. It had become rather a pose with him to consistently belittle my abilities, and I think he was chagrined at finding no loophole for criticism. I was secretly rather pleased with myself, though I tried to conceal the fact for fear of irritating him. In spite of his idiosyncrasies, I was deeply attached to my quaint little friend.

"Bien!" he said at length, with a curious look on his face. "The plot develops. Pass me, I pray you, that 'Peerage' on the top shelf there." He turned the leaves. "Ah, here we are! 'Yardly... 10th viscount, served South African War'... tout ça n'a pas d'importance... 'mar. 1907 Hon. Maude Stopperton, fourth daughter of 3rd Baron Cotteril'... um, um, um,... 'has iss. two daughters, born 1908, 1910... Clubs... esidences.'... Voilà, that does not tell us much. But tomorrow morning we see this milord!"