been tied by a stranger; therefore he has had them off in a Turkish bath or a boot shop, and since the newness of the boots makes it unlikely that he has been buying another pair, therefore he must have been to a Turkish bath. "Holmes," says Watson, "this is marvellous!" Marvellously lucky, anyway. For, however new his boots, poor old Watson might have been buying a pair of pumps, or bedroom slippers, or tennis shoes that morning, or even, if the practice allowed such extravagance, a second pair of boots. And there was, of course, no reason whatever why he should not have sat at the side of his hansom, even if alone. It is much more comfortable, and is, in fact, what one always did in the hansom days, and still does in a taxi. So if Holmes was right on this occasion, he was right by luck and not by deduction. But that must be the best of writing a detective story, that you can always make the lucky shots come off. In no other form of fiction, I imagine, does the author feel so certainly that he is the captain of the ship. If he wants it so, he has it so. Is the solution going to be too easy! Then he puts in an unexpected footprint in the geranium bed, or a strange face at the window, and makes it more difficult, Is the reader being kept too much in the dark? Then a conversation overheard in the library will make it easier for him. The author's only trouble is that he can never be certain whether his plot is too obscure or too obvious. He knows himself that the governess is guilty, and, in consequence, she can hardly raise her eyebrows without seeming to him to give the whole thing away. There was a time when I began to write a detective story for myself. My murder, I thought, was rather cleverly carried out. The villain sent a letter to his victim, enclosing a stamped addressed envelope for an answer. The gum of the envelope was poisoned. I did not know, nor did I bother to find out, whether it was possible, but this, as I said just now, is the beauty of writing a detective story. If there is no such quick-working poison, then you invent one. If up to the moment when the doubt occurs to you, your villain had been living in Brixton, you immediately send him to Central Africa, where he extracts a poison from a "deadly root" according to the prescription of the chief medicine-man. ("It is the poison into which the Swabiji dip their arrows," you tell the reader casually, as if he really ought to have known it for himself.) Well, then, I invented my poison, and my villain put it on the gum of a self-addressed envelope, and enclosed it with a letter asking for his victim's autograph. He then posted the letter, whereupon a very tragic thing happened.