Owing to the conical shape of the early muzzle-loading guns, if one trunnion were higher than the other, the " line of metal " would no longer be in the same vertical plane as the axis; in consequence of this, if a gun with, say, one wheel higher than the other were layed by this line, the axis would point off the target to the side of the lower wheel.
Guns without dispart sights cannot be layed at elevations below the clearance angle.
This was of wood; the third sight, also of wood, was for guns without a dispart patch, which consequently could not be layed at elevations below the dispart.
It was ranged by varying the charge, and layed for line by means of a line and plumb bob Laying aligned on a picket.
This enabled the gun to be layed from some little distance behind, so that the layer could be clear of recoil, and continuous laying was thus possible.