Battering CATAPULTA, and is represented on Plate LXXII. fig. 1. This catapulta is supposed to carry a stone, &c. of an hundred weight, and therefore a description of it will be sufficient to explain the doctrine of all the rest; for such as threw stones of twelve-hundred and upwards were constructed on the same principles.
The base is composed of two large beams 2, 3. The length of those beams is fifteen diameters of the bore of the capitals 9. At the two extremities of each beam, two double mortises are cut to receive the eight tenons of two cross beams, each of them four of the diameters in length. In the centre of each of the beams of the base, and near two thirds of their length, a hole, perfectly round, and 16 inches in diameter, should be bored: these holes must be exactly opposite to each other, and should increase gradually to the inside of the beams, so that each of them, being 16 inches on the outside towards the capitals 9, should be 17½ at the opening on the inside, and the edges carefully rounded off. The capitals 9 are, in a manner, the soul of the machine, and serve to twist and strain the cordage, which form its principle or power of motion.
The capitals are either of cast brass, or iron; each consisting of a wheel with teeth, C 10, of 2½ inches thick. The hollow or bore of these wheels should be 11½ inches in diameter, perfectly round, and the edges smoothed down. As the friction would be too great,
if the capitals rubbed against the beams by the extreme straining of the cordage, which draws them towards these beams, that inconvenience is remedied by the means of eight friction-wheels, or cylinders of brass, about the 13th of an inch in diameter, and an inch and one sixth in length, placed circularly, and turning upon axes, as represented at D 13, and B 12. One of these friction-wheels at large with its screw, by which it is fastened into the beam, is represented at A.
Upon this number of cylindrical wheels the capitals 9 must be placed in the beams 2, 3, so that the cylinders do not extend to the teeth of the wheels, which must receive a strong pinion 14. By the means of this pinion the wheel of the capital is made to turn for straining the cordage with the key 15. The capital wheel has a strong catch 16, and another of the same kind may be added to prevent any thing from giving way through the extreme and violent force of the strained cordage.
The capital-piece of the machine is a nut or cross-pin of iron, 17, seen at C, and hammered cold into its form. It divides the bore of the capitals exactly in two equal parts, and fixed in grooves about an inch deep. This piece, or nut, ought to be about two inches and one third thick at the top 18, as represented in the section at B; and rounded off and polished as much as possible, that the cords folded over it may not be hurt or cut by the roughness or edges of the iron. Its height ought to be eight inches, decreasing gradually in thickness to the bottom, where it ought to be only one inch. It must be very exactly inserted in the capitals.
After placing the two capitals in the holes of the two beams in a right line with each other, and fixing the two cross-diametrical nuts or pieces over which the cordage is to wind, one end of the cord is reeved through a hole in one of the capitals in the base, and made fast to a nail within-side of the beam. The other side of the cord is then carried through the hole in the opposite beam and capital, and so wound over the cross-pieces of iron in the centre of the two capitals, till they are full, the cordage forming a large skain. The tension or straining of the cordage ought to be exactly equal, that is, the several foldings of the cord over the capital-pieces should be equally strained, and so near each other as not to leave the least space between them. As soon as the first folding or skain of cord has filled up one whole space or breadth of the capital-pieces, another must be carried over it; and so on, always equally straining the end till no more will pass through the capitals, and the skain of cordage entirely fills them, observing to rub it from time to time with soap.
At three or four inches behind the cordage, thus wound over the capital pieces, two very strong upright beams 21 are raised: these are posts of oak 14 inches thick, crossed over at top by another of the same solidity. The height of the upright beams is 7½ diameters; each supported behind with very strong props 25, fixed at bottom in the extremities of the base 2, 3. The cross-beam 24 is supported in the same manner by a prop in the centre.
The tree, arm, or stylus 22, should be of sound ash. Its length is from 15 to 16 diameters of the bore
Catapulta of the capitals. The end at the bottom, or that fixed in the middle of the skain, is 10 inches thick, and 14 broad. To strengthen the arm or tree, it should be wrapped round with a cloth dipped in strong glue, like the tree of a saddle, and bound very hard with waxed thread of the sixth of an inch in diameter from the large end at bottom, almost to the top, as represented in the figure.
At the top of the arm, just under the iron-hand or receiver 27, a strong cord is fastened, with two loops twisted one within another, for the greater strength. Into these two loops the hook of a brass pulley 23 is put. The cord 29 is then reeved through the pulley, and fastened to the roll 30. The cock or trigger 31, which serves as a stay, is then brought to it, and made fast by its hook to the extremity of the hand 27, in which the body to be discharged is placed. The pulley at the neck of the arm is then unhooked; and when the trigger is to let it off, a stroke must be given upon it with an iron-bar or crow of about an inch in diameter; on which the arm flies up with a force almost equal to that of a modern mortar. The cushion or stomacher 23, placed exactly in the middle of the cross-beam 24, should be covered with tanned ox-hide, and stuffed with hair, the arm striking against it with inconceivable force. It is to be observed, that the tree or arm 22 describes an angle of 90 degrees, beginning at the cock, and ending at the stomacher or cushion.