Here’s the first thing to say about Boris Johnson’s decision to purchase three water cannon for use in riot policing: he has got us a terrific bargain.

A brand new water cannon would generally set you back around £870,000, which is quite a lot of money, even in London. But by buying them second hand from the German federal police force, Johnson’s deputy Stephen Greenhalgh has managed to get the Metropolitan Police a job lot of three, for the low, low price of just £218,000.

That’s more than 90 per cent off. I mean, you would, wouldn’t you? If you walked past a shop selling laptops at 8 per cent of their regular price, you’d have to stop yourself from going in. Can we really expect our leaders to show any greater restraint?

The Association of Chief Police Officers (ACPO) quite fancies some toys of its own, and last January published this briefing paper, helpfully outlined the advantages of using water cannon in riot policing. For one thing, they can be used from a distance: this not only keeps the boys in blue safe from harm, it reduces the chance of minor scuffles that can escalate into something more dangerous. A cannon’s mere presence can have a deterrent effect, too, the briefing claims: in Northern Ireland, whose police force has six of the things, they’re “often deployed without being employed”.

Best of all, water cannon “provide a graduated and flexible application of force, ranging from spray or diffused mode to  forceful water jets”. In other words, those who fire them have at least some control over the appropriate level of force to use.

That is not something that can be said of other approaches to crowd control at a distance. If you don’t have a water cannon, the main alternative is ‘Attenuated Energising Projectiles’ which, ACPO tells us, are more commonly known as baton rounds. In fact they’re more commonly still known as rubber bullets (words that don’t appear anywhere in ACPO’s briefing). These, despite being rubber, are pretty nasty things: better to disperse a crowd by giving them a light hosing than to jump straight to shooting at them.

Oh, and you can water flowers with them. That’s nice, too.

That’s the good news. Here’s the bad: water cannon are indiscriminate. They’re not used to target individuals, but to target entire crowds. If you’re at the wrong protest, if you’re standing on the wrong place, it doesn’t matter how well behaved you are: you’re going to get blasted.

And ‘blasted’ is the word. We probably all realise by now that these things aren’t Supersoakers, but what happened when they were used in Stuttgart in 2010 is really, genuinely shocking. Here’s how the BBC reported it after the event:

“Dietrich Wagner – a 69-year-old retired engineer – was hit in the face at a protest in Stuttgart four years ago. His eyelids were torn and some of the bones around his eyes fractured, causing his eyeballs to fall out of their sockets.”

Let’s say that again. His eyeballs fell out of their sockets. That is an extreme case: no such injuries have ever been recorded in Northern Ireland. Nonetheless, it’s a reminder of quite how nasty these things can be.

The Home Secretary has yet to grant police the power to use water cannon in England and Wales. Even when that happens, senior police officers have repeatedly reassured that the technology will only rarely be used. (That briefing document identifies three protests where they may have been useful, one of which, hilariously, was the Countryside Alliance’s 2004 march on Parliament Square.) As ACPO itself admits, “whilst water cannon can have a deterrent effect, it must also equally be understood that its presence alone can be inflammatory”.

But the Metropolitan Police are, shall we say, not averse to using their crowd control powers to the maximum of their abilities. Inflammatory behaviour by riot police is hardly unknown either. Now they’ve got their toys, it seems probable that they’ll want to use them.