VO: Now just when you thought it was safe to return to the water, here's Richard and Jonny and they're about to find out what it's like to be under an awful lot of pressure.
JONNY PHILLIPS: Tipping water over somebody's head might give them a bit of a shock if they're not expecting it, but it's not gonna move them.
Deliver a little more water with a little more force though and you can be pretty sure they'll notice.
JONNY PHILLIPS: But if you put a lot of water under a lot of pressure it can be very, very powerful indeed.
RICHARD AMBROSE:And we're going to show you with the help of these guys from the Hampshire Fire and Rescue Service, two hoses and this car. Are you ready guys?
VO: The engine and breaks are off.
RICHARD AMBROSE: Go on boy's fire it up.
VO: And the gears are in neutral. The water is blasting out at a pressure of twelve bar, that's twelve times atmospheric pressure, and the molecules of water are travelling at such velocity they create enough force to move the car.
RICHARD AMBROSE: That's typical. We're half way through filming and these guys have got a shout, they're off.
VO: While they're off putting out fires it gives us a chance to explain another use of water under pressure. To clean. It's used by councils to remove fat from the walls of sewers. In central London most of this comes from hundreds of fast food outlets, it gets flushed down sinks and washed down drains but because fat doesn't mix with water as the water drains away this stuff builds up on the walls eventually causing huge blockages.
One of the best ways to get rid of it is using high powered water jets, but high powered water doesn't just stop at cleaning stuff, concentrate your nozzle and create enough pressure and you can even use water to cut through steel or rock.
JONNY PHILLIPS: And this is the machine that does it.
RICHARD AMBROSE: And it's simply fed by mains water at the back.
JONNY PHILLIPS: That goes into this incredibly powerful intensifier pump.
RICHARD AMBROSE: And the water is fed right through to this nozzle, the cutting point.
JONNY PHILLIPS: Inside there is a sapphire with a point 2 millimetre aperture.
RICHARD AMBROSE: And that's also creating a vacuum to draw in the abrasive sand that's also required.
RICHARD AMBROSE: And the speed it's coming out is a thousand metres per second.
JONNY PHILLIPS: That's about the speed of a flying bullet.
RICHARD AMBROSE: So if you could pick that machine up and aim it at a concrete wall you could blast a hole in it from four metres.