The strength of twisted cordage has been made the subject of numerous experiments. fiAsumur, early in the last century, found that a ell.matio small hempen cord broke in different placca with 58, 67, and 72 its mean breaking weight being 65 lbs. ; while the three strands of which it was comp oaed bore 294, 334, and 35 lbs. nopectively; so that the united absolute strength of the strands was to: lies, although the average real strength of the rope was only 65 lbs., slowing a leas of strength from twisting equal to 33 lbei. It. appears that the cord used by was of very unequal quality, as another portion of it broke with 72 lies., while lin strands bore separately 26, 2S, and 30 lba.; which shows the diminution of *strength from twisting to have been from 84 to 72 lbs., the 1088 being in this instance only 12 lbs. The later experiments of Sir Charles Knowles indicates a diminution of strength nearly equal in amount to the first mentioned of Ithunsur. He found a whito or **Marred rope of 31 Inches* in circumference break, pn an average of several trials, with lbs.; while the aggregate strength of its yarns, which were 72 in number,and bore on an average 90 lbs. each, was 6480 lbs.; the loss being equal to 1926 lba., or about 30 per cent. Duhamel endeavoured to ascertain what degree of ,twist would produce the most useful effect_ lie caused some ropes to be made, so that only one-fourth of the length of the yarns was absorbed in twisting, instead of the usual proportion of These ropes were tried in shipping, and found to be lighter, thinner, and more pliant than those of the ordinary make. The following statement shows the comparative strength of ropes formed of the same hemp, and the same weight per fathom, but twisted respectively to three-fourths, and fifths of the length of their component yarns :— Much of the cordage used on shipboard requires the process of so-ring before it is fitted for its work. This consists in binding a smaller rope very tightly round a larger one, to preserve it from rut ting after friction. It is done as shown ink/. 4, where a horizontally The result of these experiments led Duhamel 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 bility, the outer covering Boon wearing off, or opening at bendings, so as to admit water, and occasion the rope to rot But while such untwisted ekeins of rope-yarns, or salrayrs, are unfit for most of the purposes to which cordage is applied, they are used with advantage for the tackle of great guns and some other purposes for which the greatest strength and pliancy are required. The usual reduction of length by twisting is one-third ; this applies to shroud or hawser-laid ropes; those which are reds/el:cid are further shortened, so that 200 fathoms of yarn are required to make 120 fathoms of cable. Ropes formed in the common manner, with three strands, do not require a heart, or central strand ; became the angles formed by the union of the three cylindrical strands are so obtuse that the pressure of the operation of laying or closing the rope causes the etranda to fill them up completely ; but when the number of strands exceeds three, a heart is essential to keep them equidistant from the axis of the rope, and to fill up the vacuity that would otherwise be left by their not meeting in the centre. The heart can however add very little to the strength of the rope ; as its fibre, lie much straighter than those of the outer strands, and, not being able to extend with them when the rope is stretched, arc soon pulled asunder. The following simple rule for calculating the strength of ropes is given by Robison :—Multiply the circumference of the rope in inches by itself, and the fifth part of the product will express the number of tons the rope will carry. For example, if the rope be 6 inches in circumference, 6 x 6= 36, the fifth of which is 7i, the number of tons which such a rope will sustain. The following rules for calculating the weight of cordage may also prove useful find the weight of shroud or rope, multiply the circumference in inches by itself, then multiply the product by the length of the rope in fathoms, and divide by 420, the product will be the weight in cwt.*. Example : to find the weight of a hawser-laid rope, 120 fathom long, 6 x 6=36 x 120= 4320, which, divided by 420 gives the weight of the rope, 10 cwt. 1 qr. 4 lbs. Again : to find the weight of cable-laid cordage, multiply its circumference in inches by itself, and divide by 4. The product will be the weight, in cwts., of a cable 120 fathoms long; from which the weight of any other length may be readily deduced. Example : required the weight of a cable, 120 fathoms long ; 12 x 12=144, divide by 4, and the product, 313, is the weight In mist Mr. Chapman, master ropemaker at Deptford Dockyard, in a treatise recently published on this subject, gives the following names, lengths, and weight* of certain kinds of rope or line as usually made in England :— stretched rope is being " or covered with spun yarns. A mallet, having a concave groove on the side furthest from the handle, is laid on the rope ; two or three tun's of spun yarn are passed tightly round the rope and round the body of the mallet. A boy
passes a ball of yarn continually round the rope ; while a man winds it on by means of the mallet, the handle of which serves as a lever to enable him to strain every turn as tightly as possible. The yarn thus appears like a screw whose threads pass almost transversely round the rope.
All the ropes hitherto described are round or cylindrical ; but fat ropes are also made, chiefly for mining purposes. They 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 a number of strands of shroud-laid rope similarly united. In either case it is necessary that the component ropes or strands be alternately of a right-hand and left-hand twist, that the rope may remain in a quies cent state. The latter method of making flat ropes was first patented by Mr. Chapman, in 1807; and he considered it to afford the strongest possible combination of his belts or flat ropes appearing to be even stronger than sal rayes (which arc skeins of rope-yarns without any twist) of the same number of yarns. This seeming inconsistency is occasioned by the imperfection of hand-spun yarns ; because if each yarn bears its own strain unaided, it will break at its weakest part ; whereas, if combined, the mean strength of each will be rendered avail able. Huddart's flat-rope machine, of more recent invention, is shown in part in fig. 5. Supposing four round ropes be needed to make one flat rope, four reels are so placed that the ropes can unwind from them with facility, and pass side by side through a steam-heated box, where the tar becomes a little softened, and the ropes more easily worked. They next peas through a groove or recess closed in tightly at top, bottom, and aides, except holes at the sides to admit large needliss. A piercer, or sharp-pointed rod of steel, is then forced entirely through the whole of the four ropes, by leverage produced by steam power ; and a man immediately passes a needle and thread through the hole. Two piercers are employed alternately, one on either edge, making holes as fast as two men can introduce needles and thread. The thread here spoken of is sometimes yarn as much as half an inch in thickness, requiring great force to draw it tightly.
Much attention has been devoted to the discovery of the best method of preserving ropes from decay, especially when exposed to the action of water. The operation of tarring, which has been almost universally practised for this purpose, effects it very imperfectly, and materially diminishes the strength of the cordage. Taking the mean of several experiments by Duhamel, it seems that untarred ropes bore a greater weight, by nearly 30 per cent, than those to which the tar ring process, bail been applied ; and he states that it is decided by experience that white cordage in coutinpal service is one-third more durable than tarred, that it retains its force much longer when kept in store, and that it resists the ordinary injuries of the weather one-fourth longer. Notwithstanding these facts, it is found that for cables and ground-tackle, which are much exposed to the alternate action of water and air, tarring is a valuable preservative; though cordage that is only superficially tarred is said to be stronger than such as is tarred throughout, and better able to bear the alternations of wet and dry. The removal of the defects and bad qualities of common tar was the object of a patent taken out by Mr. Chapman. Unsuccessful attempts had been made to substitute oils and various fat substances, which would be insoluble in water, for tar ; but they had been found to impede the operation of twisting. Chapman improved the ordinary tar, first, " by boiling the tar in water one or more times, each of which extracts a portion of its superabundant acid, and its mucilage, which contains a disengaged acid ;" and, secondly," by continuing these processes until the tar has thrown off a larger portion of its essential oil, and becomes more pitchy than usual ; end, finally, by restoring the requisite plasticity through the addition of substances less injurious and less volatile, and therefore more continuous : namely, by the addi tion of suet, tallow, animal oils, or suitable expressed oils." Of the advantages attending this process, an idea may be formed from the subjoined statement of the relative strength of the cordage without any tar, with common tar, and with Chapman 'a purified tar. The rope contained twelve yarns in each strand ; part was tried immediately, and the rest steeped in water for about three months, then removed to a foundry atove for three month*, and finally kept at the mpery nine months ; when another trial gave the following results Sir Joseph hanks had some ropes tarred with teak tar, by way of experiment, and found them to be one-third stronger than those done with common tar. Tanning has been tried for the preservation of ropes, but apparently without realising any decided advantage. The solution employed in kyaniaing, and a solution of caoutchoue, have also been tried as preservatives ; but common tar still continues to be the chief substance used for this purpose.