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.
If now the telescope be directed on the target and this level be brought to the centre of its run, the angle of sight can be read - if afterwards any range ordered is put on the sight and the gun truly layed, this bubble will be found in the centre of its run - so that if thereafter the target becomes obscured the gun can be relayed by elevating till the bubble is in the centre of its run, or at a completely concealed target the angle of sight can, if the range and difference of level are known or can be measured from somewhere near the gun, be put on by means of the micrometer screw, and the gun subsequently layed by putting the range in yards or degrees on the sight drum and elevating or depressing till the bubble is central.
It was ranged by varying the charge, and layed for line by means of a line and plumb bob Laying aligned on a picket.