The axis of the trunnions is at right angles to that of the gun, and is usually about half their diameter below the latter axis ; but Dr. Gregory, in his Lectures on Gunnery,' recommends that It should intersect the axis of the gun, in order that the recoil of the piece in firing may act less on the carriage. The vent is two-ninths of en inch in diameter, and it enters the bore at about eight-tenths of an inch froth the bottom of the latter.
The diameter of the bore is called the calibre of the gun ; this is rather greater than the diameter of the shot by which the nature of the gun is designated, and the difference between them is called the windage. [WiNnson.] A scale of quarter degrees is graduated on the base ring, the zero line being in a plane passing through the axis of the gun and cutting the exterior surface immediately above the trunnions, and the inter section of such plane with the side of the muzzle is marked by a notch on the latter. These quarter-sights, as they arc called, serve to give the gun an elevation not exceeding three degrees ; and this is accom plished by lowering the breech till the division expressing the intended elevation and the notch on the side of the muzzle are in a line with the object. The axis of the gun will then be inclined to a plane passing through the object and the axis of the trunnions, in an angle equal to that which is indicated by the division above mentioned.
A point-blank disposition of the gun is that in which the zero notch on the base ring and that on the side of the muzzle are made to coincide in direction with the object, whether this line of direction be or be not parallel to the horizon. But when a noteh on the top of the base ring and one at the top of the muzzle are made to coincide in direction with the object, the gun is said to have the line of metal elevation. [Cusxsny.) The angle which the axis of the gun, in this ease, makes with a plane passing through the trunnions, and the object, varies with the nature and construction of the gun, but is generally about one degree.
For elevations greater than three degrees a tangent scale is employed ; this is a brass rod which slides up and down in a groove formed in the direction of a diameter to the base-ring of the gun, and is divided into quarter degrees. In using this scale the latter is drawn up till the graduation expressing the intended elevation is at the surface of the base-ring; then the breech of the gun is lowered till a notch at the top of the scale and that at the top of the muzzle are in a line with the object. The axis of the gun (allowance being made for the difference
between the semidiameters of the base-ring and muzzle [Dtsranv) ) is then inclined to a plane passing through the trunnions and the object in an angle indicated by the said graduation.
It was considered advisable at one time to convert a number of heavy gnus into a description of shell gun by boring or reaming up, that is, re-boring and enlarging the bore of various guns in the ser vice to the next higher calibre ; thus, 24-pounders were bored tip into 32-pounders. The practice was first recommended by Colonel Paixhans, in France.
Tho application of locks to naval ordnance was introduced by Sir Charles Douglas (captain of the fleet to Admiral Rodney in the action of 1782), and their efficiency was soon fully recognised. At first the locks were made with one flint only, and in the event of this becoming unserviceable the loss of time attending the fixing a fresh flint was found to be so great, that instead of renewing the flint, recourse was generally then had to the lintstock or port-fire. But this defect was removed by the construction of a lock which carried two flints, so disposed, that on the failure of one, by simply turning the nut, the other might be brought into use. This improvement was made in 1318, by Major-General Sir Howard Douglas; it was immediately introduced in the British navy, and was afterwards adopted in the land-service artillery. This was followed soon afterwards by the introduction of the percussion tube into the navy and heavy artillery, and the friction tube for field artillery, and indeed. for heavy artillery also often when on shore. The percussion tube is fired by a hammer which is made to strike simply by a pull on a lanyard attached to it. It is so contrived that after striking it is withdrawn from the vent and therefore not thrown back by the explosion of the powder. The friction tube is fired by sharply palling out a portion of the head which projects above the vent, and which is made with an eye for the insertion of the hook of the lanyard.