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 cent res 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 in 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.

"But you don't think this bomb went off in my bedroom," I said.

"No, I don't. Nine kilos of ammonium nitrate in your bedroom would have annihilated it and made a nasty hole all round, but I wouldn't have thought it would bring half a house down. So if we locate the device in that foot-of-the-bed box, we are looking at something in the region of…" he did some more calculations "… say at least seventy-five cubic metres for your father's bedroom… that's twenty-five kilos of explosive."

"That's heavy," I said blankly.

"Yes. A large suitcase. But then you'd need a suitcase also if you were using cordite. For demolishing this whole house, you'd have needed four times that amount, placed in about four places on the ground floor right against the thickest walls. People often think a small amount of explosive will do a tremendous lot of damage, but it doesn't."

"What sets it off, then?" I asked.

"Ah." He smiled the professional smile that wasn't about to give away its secrets. "Let's just say fulminate of mercury, plus, I should say, an electrical circuit."

"Please do explain," I said.

He hesitated, then shrugged. "ANFO won't explode on its own, it's very stable."

"What's ANFO?" I interrupted.

"Ammonium nitrate fuel oil. The first letters. ANFO for short."

"Oh yes. Sorry."

"So you stick into it a package of something that explodes fast: the detonator, in fact. Then you arrange to heat the detonating substance, either with a burning fuse, or by an electrical circuit which can be achieved by ordinary batteries. The heat sets off the detonator, the detonator detonates the ANFO. And bingo."

"Bang, you're dead."

"Quite right."

"At four-thirty in the morning," I said, "it would probably be a time-bomb, wouldn't it?"

Mr Smith nodded happily. "That's what we're looking for. If it was an alarm clock, for instance, we'll probably find the pieces. We usually do if we look hard enough. They don't vaporise in the explosion, they scatter."

CHAPTER THIRTEEN

I drove unhurriedly to Epsom but as soon as I let myself into my flat, I knew I wouldn't stay there. It was too negative, too empty, too boring. I wouldn't live there much longer, I thought.

There were a few letters, a few bills, a few messages on the answering machine, but nothing of great interest. If I'd been blown up at Quantum along with Malcolm, it wouldn't have made any vital difference to anybody, and I didn't like that thought very much.

I went into the bedroom to see what I'd got left in the way of clothes and came to the white lace negligee. Well, maybe SHE would have been sorry for a while. I wished I could phone her, but it was forbidden: her husband would answer as he had once before when I'd tried, and too many "sorry, I've got the wrong numbers would raise the suspicions of the dimmest of men, which he reputedly wasn't.

Apart from her, I thought, making a mental inventory, I mostly knew a lot of racing people on the borderline between acquaintance and friend. Enough to be asked to parties, enough for contentment at work. I knew I wasn't in general unpopular. It was enough, I guessed. Or it had seemed enough, up to now.

I had enjoyed being with Malcolm more than I'd realised. I missed him already, and in the twelve days I'd spent with him, I'd developed a taste for spontaneity which made sitting around in my flat impossible. I packed a pair of breeches and a sweater added some limp old shirts to the new ones in the Simpson's suitcase, closed up the flat and went down to the car-park.

My own car stood there, but I took the hired one again, meaning to turn it in some time and return for my own by train. First stop was at the bank to drop through the letter box an envelope containing Malcolm's cheque, with a paying-in slip to lodge it in my account.

After that, I set off again in the overall direction of Quantum, but without really knowing where I was going.

I felt an awful aversion to the task of searching the psyches of the family, but I ended up in a place from where visiting them all would be easy, taking by impulse a turn onto the road to the village of Cookham and booking a room there in an old inn friendly with dark oak beams and log fires.