So we’re ambling along the street towards Twickenham on a lazy Saturday morning, and the rugby crowds are beginning to bag every available parking space in preparation for a home game, and the Celt, who knows me well, asks: “How far do you feel like walking today?”

I say: I could walk a long way for the right destination.

We turn off the route to the Duke of Northumberland’s river, so called because it was not always there. It was cut for a number of specific purposes: a man made channel (possibly first dug in 1415 when Syon Abbey was created for the glory of God). The channel draws water from the River Crane in Whitton all the way to the magnificent and manicured Syon Park, the seat of the aforementioned and most privileged gentleman.

But there was another purpose for channelling the water in this fashion.

We walk along the path, and the sun is shining, and though we have been in the midst of conglomeration of London buildings and busy traffic just a few moments before, it all goes away, and dog walkers pass us on every side and the trees close in and the diamond clear water flows past beehives and ancient old trees and things long overgrown and mossy.

We are headed, my Celt tells me, for a mill.

And I happily build pictures in the air of pastoral mills where corn and wheat were ground for the locals by nuns and suchlike.

Which is foolish, because there are more kinds of mills in Heaven and Earth.

And my first clue that this was no ordinary mill is the signpost which indicates that I am walking towards a Shot Tower.

A fairytale-looking thing: fat at the bottom and thin at the top, old and mossy and like something Rapunzel once occupied, there in the middle of a dense wood.

Go on: guess. What do you suppose a shot tower is for?

In conversation with my Celt I stride into the ball park effortlessly: “Was it used to make musket shot?” I surmise, thinking myself awfully clever.

Yes, it was, he says, but how did they make the shot?

I hum and I haw. Musket shot would have been made of lead, I think aloud, and so did they put it into moulds? No, he says, think of the height of the tower….how could that be useful?

And I flounder. Throw it from a great height? But lead was bendy….

And the answer is simple and elegant and ingenious and aerodynamic and simply seductive.

This is how people made the shot.

They went to the top of the tower.

Before they left, they made sure a tank of cold water was waiting at the bottom.

And they sprayed molten lead in globules out onto the air through a copper sieve, showering gleaming grey droplets downwards from a great height: and Mother Nature, who does not ask questions about the health and safety of lead fumes, simply seized her part in the manufacture of shot and with the deftness of air itself shaped the lead into perfect spheres as it hurtled from the top of the tower to the bottom.

And what was waiting at the bottom?

Why, that water we mentioned,  to cool the lead shot and make it firm.

The History Bit: the first shot tower was patented in 1782 by William Watts of Bristol. He built it at his house in Redcliffe. The mill which has been spoken of was no flour mill but a Gunpowder Mills, an ammunition factory, hence the need for water passing by. For to stop a volatile substance like gunpowder from jolting and exploding one needed the stability of a boat on water to transport it about the place and finally away.

The Twickenham Shot Tower is well signposted locally. There are four shot towers listed for England, and this is a fifth; but there are plenty in the States, three in Australia and even ones in places as diverse as Finland and Latvia. Wikipedia lists a total of 27 worldwide.

I wonder if there is one near you?