Mr Simnel wiped his hands on his greasy rag, causing them to get even more greasy, and then proffered one to Lord Vetinari, who gently waved it away, saying, ‘I would prefer it if you dealt with Mister Lipwig, Mister Simnel. If I decide to allow you your fascinating … experiment, it will be to him that you answer, in the first part. Personally, I treasure my ignorance of how machinery works, although I am well aware that this is something of great interest to some people,’ he added, in a tone of voice that suggested he meant strange and secret people … busy people, excitable people, fiddling people, tinkering and volatile people. A kind, alas, who would say something as innocent as, let’s give it a try, it can’t hurt, surely? We can always hide under the coffee table.

‘My interest,’ continued Lord Vetinari, ‘lies in ways and means, opportunity, danger and consequences, do you see? I am given to believe that your remarkable engine is propelled by steam, heated until the boiler almost, but doesn’t quite burst. Is that not the case?’

Mr Simnel gave the Patrician a cheerful smile and said, ‘That’s about it, gaffer, and I’ve blown up one or three in testing, I don’t mind telling thee! But now, sir, we’ve got it right, sir. Safety valves! That’s the ticket! Safety valves made out of lead, bungs that melt if the fire box gets too hot so the water comes down and extinguishes the fire before the boiler blows.’

Simnel carried on, ‘Live steam is very dangerous, of course, to them that don’t have the knowing of it, but to me, well, gaffer, it’s as playful as a puppy. Sir Harry has allowed me to build a demonstration track, sir,’ and he gestured to the rails that led out of the shed and wound round the perimeter of the compound. ‘May I ask if you gentlemen would care to come for a little spin?’

Moist turned to Vetinari and said, with a flat face, ‘Yes, how about it … gaffer?’ And got a look like a stiletto. A look that said, we’ll have words about this later.

Vetinari turned to Simnel and said, ‘Thank you, Mister Simnel. I think on this occasion I will give that honour to Mister Lipwig. And I dare say Drumknott will be eager to accompany him.’

This was said brightly, but Drumknott looked anything but delighted at the opportunity, and frankly neither was Moist overjoyed, remembering too late that he had put on an expensive new jacket.

Moist asked, ‘Mister Simnel, why does your contraption need to run on rails, please?’

Dick Simnel smiled the expansive smile of a man who really, really wants to talk about his wonderful pet project and is now keen to illuminate every bystander to the point of boredom, and in the worst cases suicide. Moist recognized the type; they were invariably useful and in themselves amiable and quite without malice of any sort, but nevertheless they were implicitly dangerous.