It was a sailing barge, and there was a flurry of anxiety that there wasn't going to be enough wind, but eventually, at half-past four in the morning, the bargee declared that we could go, and we set off – very slowly indeed, the boat creeping along at such a pace that an hour later we were still just off the Salute. By six we were in the dead waters north of Murano, where the lagoon was shallow and few boats, only those with the most shallow of draughts, ever ventured. It was a magical experience in a way: to sit in the prow of the vessel smoking a cigar as the sun rose, and wild ducks flew low over the marshes, seeing Torcello in the distance with its great ruined tower, and far away the occasional sail – red or yellow – of one of the sailing ships that endlessly crisscrossed the lagoon.

Macintyre was not the best company, continually fussing over his invention, unscrewing panels and peering inside with an old oil lamp held over him by Bartoli so he could see what he was doing. Adding a little oil here, tightening a bolt there, tapping an instrument and grumbling under his breath.

'Nearly ready?' I asked when I had seen enough of birds and got up to walk back to the middle of the ship.

He grunted.

'I will take that to mean "No, it needs to be stripped down and rebuilt entirely,"' I said. 'Macintyre, the damned thing is either going to work, or not. Bung it in the water and see what happens.'

Macintyre glowered at me.

'But it's true,' I protested. 'I've been watching you. You aren't doing anything important. You're not making any real changes. It's as ready as it will ever be.'

Bartoli nodded behind him, and lifted his eyes to heaven in despair. Then Macintyre sagged as he accepted that, finally, he could do no more; that it was time to risk his machine in the water. More than that: to risk his life, for everything that made him what he was he had embedded into the metalwork of his torpedo. If it failed, he failed.

'How does it move, anyway?' I asked. 'I see no funnel or anything.'

There was nothing quite like a stupid remark to rouse him. Immediately, he straightened up and stared at me with withering contempt. 'Funnel?' he snarled. 'Funnel? You think I've put a boiler and a stack of coal in it? Or maybe you think I should have put a mast and a sail on as well?'

'I was only asking,' I said. 'It has a propeller. What makes it turn?'

'Air,' he replied. 'Compressed air. There's a reservoir with air at three hundred and seventy pounds per square inch pressure. Just here.' He tapped the middle of the torpedo. 'There are two eccentric cylinders with a sliding vane to divide the volume into two parts. In this fashion the air pressure causes direct rotation of the outer cylinder; this is coupled directly to the propeller, you see. That way, it can travel underwater, and can be ready for launch at all times, at a moment's notice.'

'If it works,' I added.

'Of course it will work,' he said scornfully. 'I've had it running dozens of times in the workshop. It will work without fail.'

'So? show me,' I said. 'Chuck it over the side and show me.'

Macintyre straightened up. 'Very well. Watch this.' He summoned Bartoli and the others, and they began to put ropes round the body of the torpedo, which was then rolled carefully to the side of the boat, and lowered gently into the water. The ropes were then removed, and the torpedo floated, three-quarters submerged, occasionally bumping softly against the side of the boat. Only a single, very thin, piece of rope held it close by, attached to a small pin at the rear. That, it seemed, was the firing mechanism.

Macintyre began rubbing his chin with anxiety. 'No,' he said. 'It's not right. I think I'd better take it out and check it over again. Just to make sure . . .'

Bartoli began to shake his head in frustration. 'Signor Macintyre, there is nothing left to check. Everything is just fine.'

'No. Just to be on the safe side. It will only take an hour or . . .'

Then I decided to intervene. 'If I may be of assistance . . .' I said.

Macintyre turned to look at me. I grabbed the thin piece of rope in his hand and gave a sharp tug.

'What the hell do you think you're doing?' he screamed in shock. But it was too late. With a quiet ping, the pin popped out of the torpedo, which immediately gave off a whirring, gurgling noise as the propeller began to spin at high speed.

'Whoops,' I said. 'Sorry. Oh, look, off it goes.'

True enough. The torpedo accelerated at an impressive speed in a straight line at a slight angle to the boat.

'Damned interfering fool,' Macintyre muttered as he pulled out his watch and started staring at the torpedo as it grew smaller and smaller in the water. 'My God, it works! It really works. Look at it go!'

It was true. Macintyre told me later (he spent much of the trip home poring over a piece of paper, working out his calculations) that his torpedo accelerated to a speed of about seven knots within a minute, that it travelled with only a 5 per cent deviation from a perfectly straight line, and that it was capable of going at least fourteen hundred yards before running out of power.

At least? Yes. I had been a little hasty in my desire to force Macintyre to get on with the business of testing. I should have made sure there was nothing in the way first of all.

'Oh, my God,' Bartoli said as he looked out appalled. The torpedo, still clearly visible, was now at maximum speed, all five hundred pounds of it, travelling a few inches underwater, heading straight for a felucca, one of the little vessels used often enough for fishing, or transporting food around the lagoon. The crew could be seen quite clearly, sitting in the stern by the rudder, or leaning on the side, admiring the view as the sail billowed in the light wind.

A peaceful scene; one that painters travelled many hundreds of miles to capture on canvas, to sell to romantically inclined northerners desperate for a bit of Venice on their walls.

'Look out!' Macintyre screamed in horror, and we all joined in, jumping up and down and waving. The sailors on the felucca looked up, grinned, and waved back. Crazy foreigners. Still, a pleasant morning, why not be friendly?

'How much gunpowder is in that thing?' I asked as I jumped up and down.

'None. I put fifty-four pounds of clay in the head instead. And it won't use gunpowder. It will use guncotton.'

'Yes, you told me.'

'Well, remember it. Anyway, I can't afford to waste it.'

'hat's lucky.'

The felucca kept going, the torpedo as well; it was going to be a close-run thing. Another quarter of a knot and the boat would pass over the torpedo's course entirely and it would miss. All would be well, if only the boat would go faster or the torpedo would slow down.

Neither obliged. It could have been worse, so I assured Macintyre later. Had the torpedo hit amidships, then something of that weight and that speed would undoubtedly have stove a hole right through the thin planking, and it would have been hard to pretend that a fourteen-foot steel tube wedged in their boat was nothing to do with us.

But we were lucky. The boat was almost out of the torpedo's path; almost but not quite. Macintyre's invention clipped the end of it; even at a distance of four hundred yards, we heard the cracking, breaking sound as the rudder gave way, and the boat lurched under the impact. The sails lost the wind and began flapping wildly, and the crew, a moment ago waving cheerfully and idling their time away, launched into stunned action, trying to bring their vessel back under control and work out what on earth had happened. The torpedo, meanwhile, went silently on its way, and it was clear no one on the felucca had seen it.

Bartoli was brilliant, I must say. Naturally, we steered towards the stricken boat, and he had a quick word with the crew. 'Never seen anything like that before,' Bartoli called in Venetian. 'Amazing.'