the garage out of the wind, where I laid a piece of paper on the bonnet of Moira’s car and did my best.
‘The sitting-room stretched all the way between the two thick walls, as you know,’ I said. ‘About thirty feet. Above that…’ I sketched, ‘there was my room, about eight feet wide, twelve deep, with a window on the short side looking out to the garden. Malcolm’s bedroom came next, I suppose about fifteen feet wide and much deeper than mine. The passage outside bent round it… and then his bathroom, also looking out to the garden, with a sort ofdressing-room at the back of it which also led out of the bedroom …’ I drew it. ‘Malcolm’s whole suite would have been about twenty-two feet wide facing the garden, by about seventeen or eighteen feet deep.’
Yale studied the drawing. ‘Your room and the suite together were more or less identical with the sitting-room, then?’
‘Yes, I should think so.’
‘A big house,’ he commented.
‘It used to be bigger. The kitchen was once a morning-room, and where the garage is now there were kitchens and servants’ halls. And on the other side, where the passage now goes out into the garden, there were gun-rooms and flower-rooms and music-rooms, a bit of a rabbit warren. I never actually saw the wings, only photographs of them. Malcolm had them pulled down when he inherited the house, to make it easier to deal with without the droves of servants his mother had.’
‘Hm,’ he said. ‘That explains why there are no sideways-facing windows on the ground floor.’
‘Yes,’ I agreed.
He borrowed my pen and did some calculations and frowned.
‘Where exactly was your father’s bed?’
I drew it in. ‘The bed was against the wall between his room and the large landing, which was a sort of upstairs place to sit in, over the hall.’
‘And your bed?’
‘Against the wall between my room and Malcolm’s.’
Smith considered the plan for some time and then said, ‘I think the charge here was placed centrally. Did your father by any chance have a chest, or anything, at the foot of his bed?’
‘Yes, he did,’ I said, surprised. ‘A long box with a padded top for a seat. He kept his tennis things in it, when he used to play.’
‘Then I’d think that would be where the explosion occurred. Or under your father’s bed. But if there was a box at the foot, I’d bet on that.’ Smith borrowed the pen again for some further calculations and looked finally undecided.
‘What’s the matter?’ I asked.
‘Mm… well, because of your tree roots, I was thinking of an explosive that farmers and landowners use sometimes which is safer than cordite. They blow up tree trunks, clear blocked ditches, that sort of thing. You can buy the ingredients anywhere without restrictions and mix it yourself.’
‘That sounds extraordinary,’ I said.
He smiled slightly, it’s not so easy to get the detonators to set it off.’
‘What is it, then?’ I asked.
Yale, too, was listening with great interest.
‘Fertiliser and diesel oil,’ Smith said.
‘What?’ I sounded disappointed and Smith’s smile expanded.
‘Ammonium nitrate,’ he said. ‘You can buy it in fine granules from seed merchants and garden centres, places like that. Mix it with fuel oil. Dead simple. As far as I remember, but I’d have to look it up to be sure, it would be sixteen parts fertiliser to one part oil. The only problem is,’ he scratched his nose, i think you’d need a good deal of it to do the sort of damage we have here. I mean, again I’d have to look it up, but I seem to remember it’ll be volume in cubic metres over three, answer in kilos.’
‘What volume?’ I asked.
‘The volume of the space you want cleared by the explosion.’
He looked at the mixed emotions I could feel on my face and dealt at least with the ignorance.
‘Say you want effective destruction of everything within a space three metres by three metres by three metres. Twenty-seven cubic metres, OK? Volume of your bedroom, near enough. Divide by three, equals nine. Nine kilos of explosive needed.’
‘Is that,’ I said slowly, ‘why reports of terrorist attacks are often so definite about the weight of the bomb used?’
‘Absolutely. The area cleared directly relates to the size of the… er… bomb. If you can analyse the type of explosive and measure the area affected, you can tell how much explosive was needed.’
Superintendent Yale was nodding as if he knew all that.