I've walked this footpath countless times and each time it seems special. There's always something new to note or fresh to feel. But today I've got a grid reference to pinpoint a blast from the past.

I'm in the Vale of Ffestiniog, alongside the track of the old railway. Opened in 1836, it was powered by gravity, ponies and a water wheel. The brakes were released and gravity took the slate trains all the way down to Porthmadog, then the ponies brought them all the way back to Blaenau. But just beyond the western end of Tanygrisiau reservoir the slate wagons went uphill, one at a time, driven by waterwheel.

From the day the railway opened this short uphill section was a bottleneck and work commenced on the first Moelwyn tunnel, the one that is now flooded by the reservoir. Six years later it was opened and thereafter the train truly ran downhill all the way by gravity.

Just a few yards from the old bit of uphill track there is a slab of rock embedded amongst the heather and wild grasses - a bit like any other slab of rock except that this one has 17 holes drilled into it. No question of a compressor-driven power tool, but good old hand drilling, five inches into solid granite.

This was not a training ground for apprentice quarrymen, but a rock cannon to be fired on special occasions. The holes were part filled with black powder and covered with stemming (crushed stones) through which a goose quill filled with powder acted as a detonator. Connecting the various holes was a line of goose fat embedded with more black powder. Light the touch fuse, stand well back and enjoy the show.

Too little stemming, and the explosions would be damp squibs. Too tightly packed with stemming, and the rock would be blasted to smithereens. There were many accidents. The ingredients needed to be well balanced and preferably fired on a day without rain.

This particular cannon is known to have been fired when the railway was first opened, and again in 1842 when the tunnel was completed. But it is just one of many. Throughout Gwynedd more than 200 have been recorded.

The design of later cannons was refined, with channels cut into the rock to link the holes together. Sometimes there are as many as 160 holes in a cannon. The greatest concentrations of cannons occur around quarries, especially those associated with the landed gentry - the reason being that they had a large number of VIP guests to impress, and what better way to do so than with a display of rock cannons.

Cannons would be fired to celebrate weddings, jubilees and declarations of peace, but probably the most extensive displays would be to welcome royal visitors.

The Daily Mail's account of the Prince of Wales's visit to Blaenau in 1923 gives an idea of how impressive they must have been: "Each mountain sprang into eruption. The Prince sat in his car and crushed his cap into his hand. And while the roar of the explosions rose above the cheering he banged a fist into the palm of his hand and said, 'I have never seen anything more wonderful - never'."

Wouldn't it be great if we could get these cannons into action again? But then again, I doubt whether my dogs would appreciate them! Nor the Health & Safety Executive.

If you would like to find out more about the history and location of rock cannons, Griff Jones's The Rock Cannon of Gwynedd is an excellent read. It's not available from Amazon but can be bought at the reception of Plas Tan y Bwlch.

Huw Jenkins