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Fortress of Louisbourg National Historic Site of Canada
Recherche sur la Forteresse-de-Louisbourg Lieu historique national du Canada
Extracts of Matters of Historical Interest from "The Huissier, News For and About the Fortress of Louisbourg Heritage Presentation Staff" By The Fortress of Louisbourg Heritage Presentation Staff
(September 15, 2004)
Cannon Sizes at Louisbourg
By Sandy Balcom
Cannons were identified by the weight of the cast iron shot (cannonballs) they fired. Shot were made in specified diameters to achieve the standard weights. At Louisbourg in 1744, the standard weights of French shot and hence cannon sizes were 2, 3, 4, 6, 8, 12, 18, 24 and 36 livres. In addition, Louisbourg also had a number of English cannon in comparable sizes, but made to English measure. The French livre weighed approximately 1.09 English pounds resulting in slight variances in the diameters of shot and cannon bores.
Further differences came from variations in the windage of cannon bores used by the two nations. Windage is the amount that the diameter of a cannon's bore exceeds the shot it fires. Windage eases loading and prevents a too tightly-fitted shot from damaging the bore when fired. Writing in 1780, John Muller noted the windage of English pieces as being 1/20th of shot diameter and that of French guns as being 1/27. Guillaume Le Blond writing earlier in the century noted the windage of French guns as being at least two lignes (just over 3/16th of an inch).
Like weights, there were slight differences between French and English measures of distance, the French pouce being the equivalent of 1 1/16 inches. Taking the differing measures into account, Muller compared the shot diameters of the two nations. He found the French 24 livre ball compared closely to the English 27-pounder (5.808" to 5.679"), the French 32 to the English 37 (6.393" to 6.408") and the French 36 to the English 42 (6.648" to 6.684"). The last comparison confirms the appropriateness of the 1745 New England practice of describing captured French 36-livre guns as 42-pounders.
Muller noted some inconsistencies in making the conversions. In some shot sizes the French livre equated to 1.08 English pounds and in others to 1.145 pounds. The errors may have originated in the tables for French shot and bore sizes that he used. It should be noted that period authorities expected some discrepancies in calibrating bores due to wear, inconsistencies in manufacture and inaccuracies in measurement.
Editor's note: The two cannons fired most frequently during our military demonstrations are an 18-livre cannon (on the barbette in the King's Bastion) and an 8-livre cannon (on the waterfront or éperon). Examples of 12 and 24 livre cannons can be found in the Dauphin demi-bastion.