"These points, however, were of secondary importance, merely tending to shake slightly what lawyers term the probability of the case against Penreath. But a point of more importance was my discovery that the candle-grease dropped on the carpet was of two different kinds--wax and tallow--suggesting that two different persons were in the room on the night of the murder. Mr. Glenthorpe did not use a candle, but a reading lamp. Neither did Mr. Glenthorpe use the gas globe in the middle of the room. Yet that gas tap was turned on slightly when we examined the room, and the globe and the incandescent burner smashed. Who turned on the tap, and who smashed the globe? Penreath is not tall enough to have struck it with his head. Superintendent Galloway's theory was that it might have been done by the murderer when throwing the body of his victim over his shoulder. "An ideal case of circumstantial evidence may be weakened, but not destroyed, by the destruction of one or more of the collateral facts which go to make it up. There are two kinds of circumstantial evidence. In one kind presumption of guilt depends on a series of links forming a chain. In the other, the circumstances are woven together like the strands of a rope. That is the ideal case of circumstantial evidence, because the rope still holds when some of the strands are severed. The case against Penreath struck me as resembling a chain, which is no stronger than its weakest link. The strongest link in the chain of circumstances against Penreath was the footprints leading to the pit. They had undoubtedly been made by his boots, but circumstances can lie as well as witnesses, and in both cases the most plausible sometimes prove the greatest liars. Take away the clue of the footprints, and the case against Penreath was snapped in the most vital link. The remaining circumstances in the case against him, though suspicious enough, were open to an alternative explanation. The footprints were the damning fact--the link on which the remaining links of the chain were hung. "But the elimination of the clue of the footprints did not make the crime any easier of solution. From the moment I set foot in the room it struck me as a deep and baffling mystery, looking at it from the point of view of the police theory or from any other hypothesis. If Penreath had indeed committed the murder, who was the second visitor to the room? And if Penreath had not committed the murder, who had?