The Grail Murders

Cosmas was killed: his door was locked but, before he retired, I suspect someone spread a coat of oil between the mattress and the bed support and then inserted a small bag of gunpowder.' 'How did they light that?' 'Oh, with a slow fuse.'
    'Oh, I see,' I replied. 'They just knocked on the door and asked Cosmas could they light a slow fuse under the bed?'
    'No, no, what they did was attach a slow fuse to the gunpowder and left it coiled under the bed.' 'And?'
    'When the poor man was asleep, someone came upstairs and began to pull the thread attached to the slow fuse which was left lying out on the gallery. The slow fuse uncoiled like a snake, the assassin pulling it slowly across the floor until the end appeared under the door. A tinder was struck, the fuse lit and Cosmas died.' 'But they couldn't do that! Cosmas would notice.'
    'No, he wouldn't. No more than you did. You came into the chamber, you were not looking for an almost invisible line of thread running from your bed underneath the door. Even if, in the poor light, you did see it, you would dismiss it. All the assassin had to do was take the piece of thread as I did, pull it very slowly, which hardly makes a sound, and murder is only a few seconds away. Like you, Cosmas would not hear the fuse. It's meant to burn slowly but very quietly. The only difference with you is that I used two grains of gunpowder and no oil. In Cosmas's case it was different.'
    'But we saw nothing. Surely, as the slow fuse burns, it would burn the floor beneath?'
    'No, it splutters not burns. And remember, Roger, the thread had been removed, the fuse destroys itself, and people coming in and out of the room, once the bed was on fire, would scarcely think it suspicious even if they saw the odd burn mark on the floor.' 'So how did you discover this?'
    'What really intrigued me was the scorch mark on the outside of the door facing the gallery, as well as the damage done to the heavy bedstead. A brilliant piece of murder, Roger. The assassin did not have to enter the room and, in killing Cosmas, destroyed all the evidence except for that small scorch mark on the other side of the door.' 'The murderer could have destroyed the whole house.'
    'No, as you noticed before, the floors in all our chambers are stone and there was no combustible material anywhere near. The bed would burn, its occupant die, but the fire would be discovered in time and the flames doused.' 'Why didn't Cosmas just get out of the bed?'
    'Ah, now, that did intrigue me until I remembered the gunpowder. There was probably sufficient to injure him badly. Do you remember the corpse? The bottom half of his legs had disappeared completely. The gunpowder either killed the poor man or caused such grievous wounds as to send him into a swoon from which he would never recover. Meanwhile, the oil was ignited. The bed is old wood and would burn as quickly as stubble in the driest summer.'
    I stared down at my own bed and accepted Benjamin's conclusions. The fuse would destroy itself, the gunpowder explode, poor Cosmas's legs would be shattered, and even if he wanted to, the fire spread so quickly as to prevent his escape.
    'Well,' Benjamin looked round the room, 'all is safe here, eh? No fire, no flames. Now let us go to the church, and I shall show you how Damien died.'
    We left the manor and went down to that silent tomb of a church. Benjamin pushed me in, locking the door behind us, and lit two sconce torches. The pitch spluttered and flared into life, making the place more eerie with dancing shadows.
    'Now,' Benjamin murmured, 'let's assume I am the assassin. I have come into this chapel to commit murder. Cosmas's body is laid out and his poor brother will come in for the death vigil. Unfortunately, others arrive: Mandeville, Southgate and finally us. Eventually we leave and the murderer, hiding on those steps leading to the tower, is granted an additional advantage by Damien locking the door.' Benjamin walked over, past the baptismal font and stood looking down into the sanctuary.
    'Now I have prepared everything well. The crossbow bolts and the arbalest have been hidden away. I have also lifted the catch, both on the outside and inside of one of the windows.' Benjamin, imitating the stance of an archer, pretended to fire a crossbow. 'Damien is killed. I make sure, then prepare to leave, but I want to make it appear that I entered and left the church like a ghost. This is how I do it.'
    Benjamin walked into one of the small