As the velocity of the gases arising from the combustion of the powder is extremely great, a large amount escapes through the vent, which contributes nothing to the velocity of the projectile. It therefore follows, that the effect produced by a given charge will diminish as the diameter of the vent increases. Besides, on account of the increase of power in the current that escapes from them, large vents are more rapidly injured than small ones. [Sidenote: 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. 34. What is a _breech-sight_? An instrument having a graduated scale of tangents, by means of which any elevation may be given to a piece. [Sidenote: 17]] 35. How are the divisions of the tangent scale found? By taking the length of the piece, from the rear of the base-ring to the swell of the muzzle, measured on a line parallel to the axis, and multiplying it by the natural tangent of as many degrees as may be required; and then deduct the dispart. Thus, for 5° elevation, and the gun supposed to be 5 feet, or 60 inches long, multiply .08748, which is the natural tangent of 5°, by 60; the product gives 5.2488 inches; supposing the dispart to be 1 inch, the graduating of the tangent scale will be 4.2488 inches. 36. With what pieces are breech-sights used? Guns and howitzers. 37. What is a _pendulum hausse_?