'What was it? What happened?'

'A shark,' he replied sagely. 'Really big one, travelling fast. I saw it clearly. It must have clipped the end of your boat, bitten the rudder off. Never seen a beast like that in the lagoon before.'

The crew was delighted; this was much better than rotten wood or some ordinary accident. They would dine out on this for weeks. Bartoli, after expressing surprise that they hadn't noticed the fin sticking out of the water, offered assistance, which made Macintyre fretful. He wanted to go and get his torpedo back; he had no real idea what its range was, and it could be anywhere by now. It was his most treasured possession, and he did not want it to fall into the hands of some spy or rival, for he was convinced that all the governments and companies of the world were desperately trying to steal his secrets.

He need not have worried; Bartoli was too skilled for that. He knew quite well that no Venetian sailor would submit to being towed ignominiously into harbour by a bunch of foreigners. They were duly grateful, but turned the offer down. Then they rigged up a makeshift rudder from an oar, poking over the back rather as on a gondola, and after half an hour of enjoyable conversation, they set off again.

We all – and Macintyre in particular – breathed a sigh of relief when the felucca disappeared into the early morning mist; then we turned to the business of recovering his invention. I thought that the time had come to apologise.

'I think I had better find some way of compensating those sailors as well,' I ended. 'I imagine repairing that rudder will cost something.'

But no apologies were really necessary; Macintyre was transformed. From the anxiety-ridden fusspot of an hour or so ago, he was like a man who had just been told he had inherited a fortune. He positively beamed at me, his eyes sparkling with excitement.

'Did you see it?' he exclaimed. 'Did you see it? Straight as an arrow. It works, Stone! It works! Exactly as I said. If there'd only been some explosives in the nose I could have blown that boat to kingdom come. I could have sunk a battleship.'

'It would have been difficult to blame that on a shark,' I pointed out. But Macintyre waved my objections aside and ran up to the prow of the boat with a pair of field glasses.

We searched for about an hour for, although Macintyre was convinced it had gone as straight as an arrow, in fact it had a tendency to veer to the left a little. Not by much, but over several hundred yards, this made quite a difference. Also it had settled low in the water, only just visible on the surface, and that also made the search more difficult.

But we tracked it down eventually, embedded in a mudbank in water too shallow for us to approach in the boat.

'Now what do we do?' I asked as we gazed at it, some twenty yards away from us off our starboard bow, not daring to go any closer lest our boat also got wedged in the mud.

We spent half an hour throwing a hook tied to a rope towards it, hoping to hook the thing and then drag it towards us, but with no luck whatsoever. There was no point waiting for the tide to change, as there was none.

'Can anyone swim?' I asked.

A general shaking of heads, which I found extraordinary. It didn't surprise me that Macintyre couldn't, but I was amazed that none of his employees – brought up surrounded by water as they were – could either. I wondered how many Venetians drowned every year if this was normal amongst them.

'Why?'

'Well,' I said, now suddenly reluctant, 'I thought – just an idea, you know – that one of us could try to swim over to it. The water might be deep enough.'

'If you got stuck in the mud you'd never get out again,' Bartoli said. I didn't like that 'you'.

'Good point,' I said.

But Macintyre thought my untimely death would be a worthwhile price to pay. 'Take two ropes,' he said. 'One for the torpedo and another for you. Then we could pull both out. You can swim, can't you?'

'Me?' I said, wondering whether my father would have considered lying justifiable in these particular circumstances. On the whole he disapproved strongly of the practice. 'Well, a little.'

'Excellent,' Macintyre said, his worries all over. 'And I am deeply grateful to you, my dear sir. Deeply grateful. Although as it's your fault that the torpedo is there in the first place . . .'

Point taken. Very reluctantly I began to take off my clothes and peered over the side. I would have to let myself into the water very slowly, for fear of sinking down and becoming embedded in mud before I even started. It was cold and the water looked even colder.

Bartoli tied two ropes around my waist and grinned at me. 'Don't worry,' he said. 'We will not leave you there.'

And then I lowered myself gently into the water. It was even colder than I had feared, and I began shivering immediately. But, nothing to be done now; using a gentle breast stroke, I set off for the torpedo, trying to keep my legs as high in the water as possible.

The only danger came when I got close to the torpedo and had to stop. Then the water was only about three feet deep and my feet had slid across the mud several times; as I manoeuvred into position, I had to push down and I felt them slip into the mud properly. When I tried to hang onto the torpedo and drag them out, I realised they were stuck hard.

'I can't move,' I shouted to the boat.

'Tie the rope on to the torpedo! Stop pushing it further into the mud,' Macintyre shouted back.

'What about me?'

'We'll pull you out afterwards.'

Well, thank you, I thought bitterly. Still, he was right. That was what I was there for. I was so cold now that I could barely untie the knot, let alone push the rope through the propeller casing and tie it securely. My teeth were chattering uncontrollably by the time I was finally done.

'Excellent,' Macintyre shouted. 'Pull away.'

It took some effort by the people on the boat, but eventually the torpedo began to move, and once the suction was broken it slipped rapidly past me and into deeper water. Macintyre was all but dancing up and down in joy.

'Now get me out of here!' I shouted.

'Oh, very well,' came the reply, and I felt the rope tighten around my chest as they began to tug. Nothing happened. I moved a few inches, but the moment the pressure was relaxed, I sank back down even deeper than before. I was now getting frightened.

'Don't stop!' I shouted. 'I'm going lower. Get me out of here!'

Nothing happened. I glanced round and saw Macintyre, staring at me and stroking his chin. Then he talked to Bartoli. For a fraction of a second I was convinced he was going to abandon me there.

But no. Although what he planned was nearly worse. As he explained afterwards, the suction from the mud was too strong for them. All they were doing was pulling the boat itself into danger. They needed more power.

I saw Bartoli pulling up the sails, and Macintyre pulling up the anchor, and one of his men manning the oar to turn the ship. I realised with horror what they had in mind. They were setting sail, and were going to use the full power of the boat and the wind to try and dislodge me.

'You'll pull me apart!' I yelled. 'Don't do that.'

But Macintyre just waved cheerfully. The boat began to move, and I felt the rope tighten once more, until it was as taut as a bowstring and the pressure on my chest, the rubbing of the rough cord, unbearably painful. It was all I could do not to scream. I certainly remember thinking that if I was still in one piece at the end of this experiment I would thump Macintyre on the nose.

It got more and more painful; I could feel my body stretching as the mud refused to let me go, and that seemed to go on forever. Then, with the most disgusting slurping sound, it gave me up; my legs and feet were belched out of the mud in a huge cloud of foul-smelling water, and I floated free, trailing behind the boat as it headed towards Venice.