'I'm almost disappointed,' I said. 'But isn't a gun more reliable? Less chance of something going wrong, and less chance of the other ship getting out of the way? And cheaper?'

'Possibly so, but to send a shell of equivalent power on its way you need a gun weighing some sixty tons. And for that you need a very large ship. Which has to be armour-plated, and carry a large crew. With a few of these, a corvette of three hundred tons and a crew of sixty will be a match for the largest battleship in the world.'

'The Royal Navy will thank you for that, I'm sure,' I said ironically.

Macintyre laughed. 'They won't. This will neutralise every navy in the world! No one will dare send their capital ships to sea, for fear of losing them. War will come to an end.'

I found his optimism touching, if misplaced. 'That would kill off demand for your invention, would it not? How many of these could you sell?'

'I have no idea.'

I did. If it worked, and he could persuade one navy to buy them, then he would sell them to every navy in the world. Admirals are as discerning as housewives in a department store. They must have what everyone else is having.

'Does it work?'

'Of course. At least, it will work, when one or two problems are ironed out.'

'Such as?'

'It has to go in a straight line, as I say. That is quite straightforward. But it also has to propel itself at a constant depth, not rising and falling. Through the water, not over the top of it.'

'Why?'

'Because ships are plated above the waterline, but not so heavily below it. Shells burst when they hit the water, so there is rarely direct damage under sea level, and so little need to protect the hulls so far down.'

'How much does it cost to make these?'

'I've no idea.'

'And how much will you try to sell them for?'

'I haven't thought about that.'

'Where would you manufacture them? You could hardly do it here.'

'I don't know.'

'How much have you spent on developing it so far?'

All of a sudden the boyish look of enthusiasm which had animated his face since he began talking about his machine faded. He looked his age and more so, careworn and anxious.

'Everything I have, or had. And more.'

'You are in debt?' He professed to like direct questions. Normally I do not, except where money is concerned. There I desire absolute and unambiguous precision.

He nodded.

'How much?'

'Three hundred pounds. I think.'

'At what rate of interest?'

'I don't know.'

I was appalled. However skilled Macintyre was as an engineer, he was no businessman. In that department he was a naïve as a newborn babe. And someone, I could tell, was taking advantage of that.

I do not object to such practices. Macintyre was an adult and far from stupid. He had entered into an agreement fully conscious of what he was doing. If he did so, that was his fault, not the fault of the person who was so exploiting his unworldly nature. It turned out, so he told me, that he had needed money, both to pay the wages of his men, and to buy the material necessary for his great machine, and had assumed he would be able to pay it off with a job he had taken on designing the metal work for a new bridge to be thrown across the Grand Canal. But that project had collapsed, so no payment was forthcoming, and the debts had mounted up.

'I arrived in Venice with enough money, so I thought, to live indefinitely. But this machine has been more difficult than I could ever have imagined. The problems to be solved! You cannot believe it. Building the case and ensuring it is watertight, designing the engine, the detonator, coming up with an entirely new device to regulate depth. It takes time and money. More money than I have.'

'So you are heavily in debt, with no assets to draw on, paying what I imagine is a high rate of interest. How long before you are unable to continue?'

'Not long. My creditors are pressing. They are insisting that the torpedo be tried out and quickly, otherwise they will call in their debts.'

'Can you do that?'

'I'm going to give a demonstration soon. If it works, I will be allowed to borrow more. But it is too early; much too early.'

He did not continue, and had no need to.

'I think you need a bookkeeper as much as you do a draughtsman or a machinist,' I said. 'Money is as important a component as steel.'

He shrugged, plainly uninterested. 'They're thieves,' he said. 'They'd steal my invention and leave me with nothing unless I was careful.'

'I hate to say it, but you are not being careful.'

'Oh, everything will be just fine next week. When the test is done.'

'Are you sure?'

He looked weary. 'Any sort of calculation in engineering I can do. But show me a contract, or a page of accounts . . .'

'With me, it is the precise opposite. Listen. If you wish, I could look through that side of things, see what the situation is precisely, and tell you – in words even an engineer could understand – how you stand at the moment. Only if you wish. I do not want to interfere in any way.'

I was very reluctant to make this offer, as it is generally unwise to give financial advice unbidden. But the look of hopelessness on his face as he talked of his debts was beguiling. And my mind was racing. An entirely new class of weapon could be formidably profitable, witness Mr Maxim's rapid-fire gun which, from small beginnings, rapidly became more or less obligatory equipment for every army in the world.

And the beauty of Macintyre's machine was that it was so wasteful. Unlike a cannon, which was (so to speak) a fixed investment, with the cost of employment quite low – only the amount needed to buy the shell and the gunpowder – the torpedo could be employed once only. Once sent on its way, the whole thing would have to be replaced. The potential for replacement orders was considerable and (if I knew my sailors) in a conflict they would fire them off like rockets on Guy Fawkes night.

Regular orders from an organisation with bottomless pockets. The prospect was enticing. Not least because I was fairly certain that Macintyre's aim, of eliminating war by making destruction certain, was as unlikely as it was noble. No weapon has ever made war less likely; they merely end wars more quickly by killing people at higher speeds. Until the mind of man invents something capable of killing everyone, that will not change.

But it seemed that the chances of Macintyre ever succeeding with his device were small to non-existent. He barely had the resources to finish one, so what chance had he of producing them in bulk? Who would provide the capital to fit out a factory, hire a workforce? Who would run it, ensure that the machines were properly made, sold and delivered? Macintyre had no idea of any of this, nor did he even know how to find those who did.

The whole situation was full of possibilities. If the machine worked.