16]
30. What is the diameter of the vent?
0.2 of an inch in all pieces except the eprouvette, in which it is 0.1.
31. What is the position of the axis of the vent?
The axis of the vent, is in a plane passing through the axis of the bore, perpendicular to the axis of the trunnions. In guns, and in howitzers having cylindrical chambers, the vent is placed at an angle of 80° with the axis of the bore, and it enters the bore at a distance from the bottom equal to one-fourth the diameter of the bore. As this inclination renders it easy to pull the friction tube out of the vent, that of the new 12-pdr. field gun, and the new columbiads has been placed perpendicular to the axis.
32. What are the quarter-sights of a piece?
Divisions marked on the upper quarters of the base ring, commencing where it would be intersected by a plane parallel to the axis of the piece, and tangent to the upper surface of the trunnions.
Note.—Not used in our service.
33. To what use are the quarter-sights applied?
For giving elevations up to three degrees but especially for pointing a piece at a less elevation than the natural angle of sight.