The form preferred for cannon may be defined that of a right cone, obtruncated at the apex, and from which a small cy linder has been subtracted, to form the barrel. The greatest force of the ignited powder being exerted at the breech, a cannon is of course made thickest at that part : its thickness diminishes but little for about a quarter of its length, when it is suddenly reduced in the breadth of its mouldings in that part ; it is again reduc ed abruptly at about half its length, and then continues diminishing very gradually to near the muzzle, where it is again en larged; it having been found that the shot in departing was apt to exert a greater force against the gun in that place. The two parts of the gun, where the thickness is increased, are called the reinforces. Some guns have been made in foreign countries, which have no reinforces or in crease of thickness at the muzzle ; a very beautiful one of this description of brass, of a large size, now lies in St. James' Park, as a trophy of war, brought front Egypt by the victorious troops command ed by General Lord Hutchinson.
In heavy and medium brass guns, the first and second reinforces are similar frustrums of right cones; and conse quently, when produced, their outlines will be parallel to each other, and are dis tant one-sixteenth part of the calibre. The exterior diameter of the piece is also diminished by that quantity, and its out line is drawn to the muzzle astragal.
In light guns, the length of the piece must be divided into eighteen equal parts, of which 5 parts are taken for the breech and first reinforce ; 4 parts for the second reinforce ; 9 . . . for the chase ; 2i . . for the length of the muzzle ; part for the diameter of the neck.
The axis of the trunnions are 8 parts from the breech, and half a calibre below the axis of the piece. The position of the trunnions of heavy and medium brass guns is as three-sevenths of the length of the piece, from the extremity of the breech, and half a calibre below the axis of the piece, reckoning to their centres. The diameter of the trunnions are each one calibre, and their length the same, allowing for the projection of the second reinforce ring ; their faces are parallel to the axis of the piece. The trunnions of medium and light brass guns have shoulders, which are a tenth of the dia meter of the trunnion in breadth, and of sufficient depth to clear the projection of the second reinforce rings.
The vent fields are one-seventeenth of the breech and first reinforce.
The chase girdles are one-fourteenth part of the chase.
The length of the muzzle is equal to the diameter of the second reinforce ring in heavy gulls ; and in medium guns, one seventh of the length of the piece. The
diameter of the swell of the muzzle is equal to the diameters of the second rein force rings.
The bottom of the bores of heavy brass guns is a plain surface, meeting the sides hi a small arc described with the radius of 1-24th of a calibre ; in medium and light guns they are hemispherical, and their vents form an angle of 75 degrees with the axis of the piece ; making in light guns one-third of the calibre, and in medium one-fourth of the calibre, from the extremity of the bore.
The vents of heavy guns are a fifth of an inch in diameter.
In medium and light guns there is a portion of metal beneath the neck of the cascable, for receiving the loop of the elevating screw. The lower part of it is the arc of a circle, described with a radi us equal to the semidiameter of the neck; of the position of the centre is one-fourth part of the distance from the extremity of the breech moulding to that of the but ton, and is one-fourteenth of the diame ter of the neck below it.
Medium and heavy guns are cast with dolphins, by which they are occasionally suspended, and they consequently should. be placed over the centre of gravity of them, or rather so that the breech may preponderate in a small degree.
From the following dimensions of the wheels and axles of an heavy 12 pound er and of a light six pounder, some idea may he formed of the proportion of other parts of their and also of those of pieces of artillery of the other rates.
Heavy Light 6 12 pounder. pounder in. ft. in.
Diameter of the wheel 4 : 9,506 Height of the axletree 0 : 8,250 0: 6, Thickness of ditto . . 0 : 6,625 0: 5,250 Length of ditto . ... 6 : 8, 5:3, The bed of the 12 pounder is 3 feet 3 inches in length.
The most usual mode by which cannon are discharged is, by applying a kindled match to the touch-hole. Locks, on a si milar principle to musket locks,have heen tried in sea service, and have been found to perform very well, but their use is by no meaus general.
A very great improvement has been made in matches by 31. Leroy, who has found that small rods of lime-tree, and some other soft woods, prepared with in fusion of nitrate of lead, or nitrate of cop per, form matches much superior to the common sort. For the method of making them, see the articles MATCH and PORT FIRE.
For the construction of iron guns for battering pieces, and garrison and ship guns, mortars, howitzers, and for other particulars relative to artillery in general, see the articles CANNON, MORTARS, How ITZERS, GUNNERY, and PROJECTILES.