from The Century Dictionary and Cyclopedia
- n. A front sight placed on the rimbase of a cannon. A lug is usually left on the curved surface to form a base for the sight.
Sorry, no etymologies found.
The trunnion-sight is designed to be used only when the required elevation passes the limits of the other sights.
Officer of the piece, who sets the trunnion-sight at the proper degree of elevation and clamps it there.
The 2d Captain, having adjusted the trunnion-sight  to a given angle with the axis of the bore (usually 45°), orders "Raise" or
All the crew, except Nos. 1 and 2, will bowse on train-tackles, until the mortar is in the desired direction, when the order "Well!" will be given by No. 2, who attends spirit-level and trunnion-sight.