The highest qualities of the various articles are made from the best materials, such as the long line of the hemps and flaxes, and sound cotton. Medium sorts are produced from jute, various coarse fibres, and the tows of hemp and flax. Inferior th scriptions are obtained from the wastes and broken-up materials just mentioned, Each of these leading divisions possesses numerous grades, accordingly as the materials employed will permit of being assorted into different qualities.
It is probable that, as the fibrous plants of different countries become better known, numerous additions will be made to the list, as it is certain that for these purposes, there are many quite as suitable as any now in use, almost unknovvn save to scientific inquirers. The qualities requisite are pliability, softness, smoothness, strength, and a length of staple of from 3 in. upwards.
The word " rope," properly used, iinplies an article exceeding 1 in. in circumference ; smaller desmiptions are named " cords," "lines," "twines," "threads," &c., and their constituents, " yarns." After the prepaxatory stages have been gone through, the product of the first operation, which is spinning, is " yarn." A given number of yarns—more or less, according to the thickness of the article required—are twisted together to form a " strand "; three of these combined by the same rneans compose a "rope" ; whilst a similar union of three ropes constitutes a " cable." In making cordage of all kinds, the object of twistinethe fibres is to obtain increased length.
Singular as it may appear, it has been affirmed, and probably with truth, that this twisting of the fibres does not increase the strength, but considerably diminishes it. If a given number of fibres of equal length be placed in parallel order, and their tensile strength be tested, it will be found considerably greater than that of the same fibres twisted into a compact cord. This is bccause, in tbe first case, the strain upon each fibre is equal to that upon every other, and the total strength is that of all of the filmes added together. When these fibres are twisted, those forming the external layer, having to wrap round those constituting the core, are strained both in this process and when their tensile power is tested, and are the first to break, thus leading to the fracture of each in detail. From experiments that have been made, it has been found that this loss of strength amounts to 30 per cent. of the strain-bearing power of the untwisted fibres. The object, therefore, being to get continuity of length, all twisting which exceeds that necessary to prevent the fibres slipping over each other when a strain is applied is to be avoided, as entailing a loss of strength.
Hemp (see Fibrous Substancee, p. 934), the materiel of which the best ropes are ordinarily composed, as compared with flax, is much coarser and stronger, but is cultivated and treated in a very similar manner, the processes of retting, breaking, and hackling being like those of flax.