"First: X is a person concerning whom I possess certain exclusive information.
"Second: He has some knowledge of my personal habits.
"Third: He is a man of some means and social position.
"Fourth: He is a man of considerable knowledge, ingenuity and mechanical skill.
"Fifth: He has probably purchased, quite recently, a second-hand 'Blick' fitted with a literary typewheel.
"Sixth: That machine, whether his own or some other person's property, can be identified by a characteristic mark on the small 'e.'
"If you will note down those six points and add that X is probably an expert cyclist and a fairly good shot with a rifle, you may possibly be able, presently, to complete the equation, X = ?"
"I am afraid," I said, "I do not possess the necessary data; but I suspect you do, and if it is so, I repeat that it is your duty to society—to say nothing of your clients, whose interests would suffer by your death—to have this fellow laid by the heels before he does any mischief."
"Yes; I shall have to interfere if he becomes really troublesome, but I have reasons for wishing to leave him alone at present."
"You do really know who he is, then?"