"And now you're as badly off as ever," she said, when I had finished. "I suppose all we can do now is to sit and wait to see what happens?"

"Wait for the axe to fall, you mean?" I said grimly. "Not quite. I haven't much hope from it, but I thought I might try the deductive reasoning act for a change. But before we can deduce, we have to have some facts we can deduce from. And we're very short on facts. That's why I asked you out here – to see if you could help me."

"I'll do anything I can, you know that." She lifted her head as the aurora swelled and flamed to the incandescent climax of its performance, and shivered violently as its unearthly beautiful colourings struck a million sparks of coloured light, red and green and yellow and gold, off the ice spicules in the sky. "I don't know why, that makes me feel colder than ever. . . . But I think I've already told you everything I know, everything I can remember, Dr Mason."

Tm sure you have. But you may have missed some things just because you couldn't see they mattered anyway. Now, as I see it, we have three big questions looking for an answer. How come the crash in the first place? How was the coffee spiked? How was the radio broken? If we can turn up anything that can throw a light on even one of these, we may be a long way towards finding out what we want to know."

Ten freezing minutes later we were still a long way from finding out anything. I'd taken Margaret Ross step by step from the Customs Hall, where she'd met her passengers, to the plane where she had settled them down, flown with them to Gander, watched them go through the same process again, flown them out of Gander, watched her as she'd served their evening meal, and still I'd learnt nothing, turned up nothing suspicious, off-beat or abnormal that could even begin to account for the crash. Then, slowly, just as she was describing the serving of the meal, her voice trailed away into silence, and she turned and stared at me.

"What's the matter, Miss Ross?"

"Of course," she said softly. "Of course! What a fool I am! Now I see.

"What do you see?" I demanded.

"The coffee. How it was tampered with. I'd just served Colonel Harrison – he was in the rear seat, so he was the last to be served -when he wrinkled his nose and asked if I could smell something burning. I couldn't, but I made some sort of joke about something burning on the galley hotplate and I'd just got back there when I heard the Colonel calling, and when I looked round he had the door of the starboard washroom open and smoke was coming out. Not much, just a little. I called the captain, and he hurried aft to see what it was, but it was nothing serious, just a few papers burning – somebody had been careless with a cigarette, I suppose."

"And everybody rose out of their seats and crowded to have a look?" I asked grimly.

"Yes. Captain Johnson ordered them all back to their seats -they were upsetting the trim of the plane."

"And you didn't think this worth mentioning to me," I said heavily. "No importance at all?"

"I'm sorry. It – it did seem unimportant, unrelated to anything. That was hours before the crash, so – "

"It doesn't matter. Who could have gone into the galley then -anybody in the front seats, I suppose?"

"Yes. They all seemed to crowd down past the middle – "

"They? Who were 'They'?"

"I don't know. What – why do you ask?"

"Because by knowing who was there, we might find out who wasn't."

"I'm sorry," she repeated helplessly. "I was a little upset for a moment, then Captain Johnson was in front of me shooing everybody back to their seats and I couldn't see."

"All right." I changed my approach. "This was the men's washroom, I take it?"

"Yes. The powder room is on the port side."

"Can you remember who went in there, say, any time up to an hour beforehand?"

"An hour? But the cigarette end – "

"Do you believe now that the fire was caused deliberately?" I asked.

"Of course." She stared at me, wide-eyed.

"Right. And we're dealing, obviously, with hardened professional criminals. The whole success of their plan depended on causing this excitement. Do you for a moment believe that they were going to let the whole thing hinge on the mere off-chance of a smouldering butt-end setting some papers alight – especially setting them alight at the correct moment?"

"But how – "

"Easy. You can get a little plastic tube with a central composition shield dividing it into two compartments. In one compartment you have a free acid, in the other a different acid enclosed in a glass tube. All you have to do is to crush the tube, break the glass, drop the tube in your chosen spot, walk away and after a predetermined time the acid that was in the glass eats through the shield, meets the other acid and starts a fire. It's been used hundreds of times, especially in war-time sabotage. If you're an arsonist looking for a cast-iron alibi and want to be five miles away when the fire starts, it's the perfect answer."

"There way a funny smell – " she began slowly.

"You bet there was. Can you remember who went there?"

"It's no good." She shook her head. "I was in the galley most of the time, getting the meal ready."

"Who were in the front two seats – those nearest the galley?"

"Miss LeGarde and Mr Corazzini. And I'm afraid that's not much help. We know Marie LeGarde can't have had anything to do with it. And Mr Corazzini is the one person I'm sure didn't leave his seat before dinner. He had a gin soon after take-off, then switched off his reading light, draped a newspaper over his head and went to sleep."

"Are you sure?"

"Quite sure. I always peek through the cabin door from time to time, and he was always there."

"That seems to cut him out," I said thoughtfully. "And reduce the number of suspects – though, I suppose, he could still have got an accomplice to plant the acid tube." Then, suddenly, I had what was, for me, an inspiration. "Tell me, Miss Ross, did anyone ask you earlier in the evening when dinner would be?"

She looked at me for a long moment before answering, and even in the fading light of the aurora I could see the understanding coming into her eyes.

"Mrs Dansby-Gregg did, I'm sure."

"She would. Anyone else."

"Yes. I remember now." Her voice was suddenly very quiet. "Colonel Harrison – but he doesn't count any more – and Mr Zagero."

"Zagero?" In my excitement I bent forward until my face was almost touching hers. "Are you sure?"

"I'm sure. I remember when he asked me, I said, "Are you feeling peckish, sir?" and he grinned and said, "My dear air hostess, I always feel peckish."

"Well, well. This is most interesting."

"Do you think Mr Zagero – "

"I'm at the stage where I'm afraid to think anything. I've been wrong too often. But it's a straw in the wind all right – a straw about the size of a haystack.. . . Was he anywhere near you when the radio fell? Behind you, for instance, when you rose and brushed against the radio table?"

"No, he was by the hatch, I'm sure of that. Could he – "

"He couldn't. Joss and I worked it out. Somebody had pushed one of the table hinges right home and the other until it was at the critical point of balance. Then as you stood up he pushed the other in. From a distance. There was a long-handled brush lying there -but it had no significance for us at the time.. . . When you heard the crash you whirled round, didn't you?" She nodded without speaking.

"And what did you see?"

"Mr Corazzini – "

"We know he dived for it," I said impatiently. "But in the background, against the wall?"

"There was someone." Her voice was barely more than a whisper. "But no – no, it couldn't have been. He'd been sitting dozing on the floor, and he got the fright of his life when – "

"For heaven's sake!" I cut in harshly. "Who was it?"