Those large ropes which ere said to be cable-laid are formed by the combination of smaller ropes twisted round their common axis, just as shroud-laid ropes are composed of strands twisted round their common axis. As cable-laid ropes are harder and more compact than others, this mode of formation is adopted for ropes to be exposed to the action of water, even though their thickness may not be very great. Originally all the yarns (from to .6. of an inch in thickness) composing a strand were of the same length : but Captain Huddart showed that in such an arrangement the outer most yarns must be more strained than the inner : he accordingly, in 1793, patented his register cordage, in which the strain is equalised throughout. The same gentleman invented machinery for almost every department of rope making, Ropes formed by plaiting instead of twisting are made use of for some purposes in which pliability is especiallrneeded, they being more supple and less liable to entanglement than those of the ordinary make. Such ropes are preferred for sash-lines, clock-lines, and generally where the • rope has to pass over pulleys of small diameter. • Flat ropes, which are much used for mining purposes, are either formed of two or more small ropes placed side by side, and united by sewing, lapping, or Interlacing with thread or smaller ropes, or of x number of strands of simi 4arly united. In either case it is necessary that the component ropes or strands be alter nately of a right hand and left hand twist, that the rope may remain in a quiescent state.
Many experiments were made by Reaumur, Knowles, and others, to test the loss of strength by the ordinary twist given to ropes. Duhamel prepared the following statement, to show the comparative strength of ropes formed of the same hemp, and the same weight per fathom, but twisted respectively to two-thirds, three fourths, of the length of their component yarns Degree of twist. Weight borne in two experiments.
4008 lbs. 4250 lbs.
4850 6753 6205 7397 The' result of these experiments' led Du bamel to try the practicability of making ropes without any twist, the yarns being wrapped round to keep them together. These had great strength, but very little durability. In shroud or hawser-laid ropes the usual reduction of length by twisting is one-third ; but cable laid ropes are further shortened, so that 200 fathoms of yarn are required to make 120 fathoms of cable. A hawser-laid rope 6 inches in circumference by 120 fathoms k720 feet) long, weighs about 10 cwts. A cable-laid rope inches in circumference and 120 fathoms long, weighs 36 cwts. A hawser-laid rope 6 inches in circumference will bear a weight of 140 cwts. The tarring of ropes somewhat
impairs their strength, but renders them more durable. Oils, fats, and tallow of various kinds have been occasionally used ; but tar is found to be the best preservative.
Several other kinds of vegetable fibre besides hemp have 'been made use of in the manu facture of cordage, and some appear greatly to exceed hemp in strength. The fibres of the aloe, long wool, hemp mixed with threads of caoutchouc, thongs of ox-hide, and several other substances, have been employed; but none are found to combine so many advantages as hemp.
Ropes formed of iron wire have been, within the years, introduced to a considerable extent, and have been found to effect a great saving of expense from their durability and superior lightness. , Ropes of twisted iron wire were used in the silver mines of the Harz Mountains twenty years ago ; they were nearly equal in strength to solid iron bars of equal thickness, and equal to hempen ropes of four times their weight. Wire ropes are now made in England under many patents. They are formed in various ways, according to their intended use. For standing rigging straight untwisted wires are employed, bound round with cloth or small hempen cordage saturated with .a solution of caoutchouc, asphaltum, or other preservative from rust. Flat ropes may likewise be made of straight wires, interwoven or wrapped with hempen yarn, or sewed be tween canvas, &c. Other ropes are formed much in the same way as those of hemp, the wires taking the place of rope-yarns, and being twisted into strands, and combined into ropes, both hawser-laid and cable-laid. The twisting should not be so hard as in hempen cordage ; and all the wires must he protected by an anti-corrosive composition, or by coating with tin, zinc, &c. It is sometimes the practice to twist wires round a core, either of wire, hemp cord, spun yarn, or other material to form a strand, and to lay such strands round a similar core when there are more than three strands in a rope.
The annexed table, showing the comparative size and weight per fathom for equal strength, may serve to show the great superiority of wire ropes over those of hemp, which they sur pass even in flexibility:— 'temp Rope. Wire Rope.
Weight per Weight per Equal to Size. fathom. Size. fathom. a strain ofInches. lbs. oz. Inches. lbs. oz. cwts.
3 2 4 11 1 4 50 4 3 15 11 1 9 70 5 6 0 1i 1 14 135 6 9 0 2 2 2 160 7 12 3 21 2 9 171 8 14 3 2i 4 1 198 9 19 6 3 5 • 306 10 25 0 31 7 1 486 11 30 0 4 11 6 585 12 36 8 41 15 12 701 Suspension bridges two now occasionally made of wire rope of enormous dimensions and strength.