“How do you know that?” I asked.

“Well, to begin with, it would be useless to fit a needle to the projectile unless the latter was made to travel with the point forwards; but there is direct evidence that the barrel was rifled.  You notice the little square projection on the back surface of the cylinder.  That was evidently made to fit a washer or wad—­probably a thin plate of soft metal which would be driven by the pressure from behind into the grooves of the rifling and thus give a spinning motion to the bullet.  When the latter left the barrel, the wad would drop off, leaving it free.”

“I see.  I was wondering what the square projection was for.  It is, as you say, extremely ingenious.”

“Highly ingenious,” said Thorndyke, enthusiastically, “and so is the whole device.  See how perfectly it would have worked but for a mere fluke and for the complication of your presence.  Supposing that I had been alone, so that he could have approached to a shorter distance.  In that case he would not have missed, and the thing would have been done.  You see how it was intended to be done, I suppose?”

“I think so,” I answered; “but I should like to hear your account of the process.”

“Well, you see, he first finds out that I am returning by a late train—­which he seems to have done—­and he waits for me at the terminus.  Meanwhile he fills the cylinder with a solution of a powerful alkaloidal poison, which is easily done by dipping the needle into the liquid and sucking at the small hole near the back end, when the piston will be drawn up and the liquid will follow it.  You notice that the upper side of the piston is covered with vaseline—­introduced through the hole, no doubt—­which would prevent the poison from coming out into the mouth, and make the cylinder secure from leakage.  On my arrival, he follows me on his bicycle until I pass through a sufficiently secluded neighbourhood.  Then he approaches me, or passes me and waits round a corner, and shoots at pretty close range.  It doesn’t matter where he hits me; all parts are equally vital, so he can aim at the middle of my back.  Then the bullet comes spinning through the air point foremost; the needle passes through the clothing and enters the flesh, and, as the bullet is suddenly stopped, the heavy piston flies down by its own great momentum and squirts out a jet of the poison into the tissues.  The bullet then disengages itself and drops on to the ground.

“Meanwhile, our friend has mounted his bicycle and is off, and when I feel the prick of the needle, I turn, and, without stopping to look for the bullet, immediately give chase.  I am, of course, not able to overtake a man on a racing machine, but still I follow him some distance.  Then the poison begins to take effect—­the more rapidly from the violent exercise—­and presently I drop insensible.  Later on, my body is found.  There are no marks of violence, and probably the needle-puncture escapes