Re: NNG: Retiring Engineer Given a 21-Torpedo Salute

For very special occasions (visits by Royalty, opening of new ventures, and the like) the slate quarrymen here used to fire salutes with rock cannon.

Drill five, seven, or more inch diameter holes in a suitable rock face, place black powder at the hole bottom, topped off with a detonator and fuse. Light the fuses in series and lo, a five, seven, or more gun salute, entirely produced by quarrying equipment and supplies. The last firing of a rock cannon seems to have been during a visit by the Duchess of Kent in 1951 to Blaenau Ffestiniog.

There's a good Wikipedia article, search on 'rock cannon Wales'