Thanks both. The same enquiry elsewhere has directed me (thanks to Rob Griffith) to this treatise, which explains the procedure and associated risks very clearly. See pp.132-134 of the artillery section (597-599 in the PDF's numbering): https://www.google.co.uk/books/edition/Treatise_on_Fortification_and_Artillery/aGXzAAAAMAAJ
Thanks both. The same enquiry elsewhere has directed me (thanks to Rob Griffith) to this treatise, which explains the procedure and associated risks very clearly. See pp.132-134 of the artillery section (597-599 in the PDF's numbering): https://www.google.co.uk/books/edition/Treatise_on_Fortification_and_Artillery/aGXzAAAAMAAJ