Rope-yarns, for large rigging, are from a quarter of an inch to somewhat more than the third of an inch in circumference; or of such a size, that 160 fathoms of white yarn weigh from 34 to 4 pounds. The number of yarns in a strand of cordage varies from sixteen to twenty-five. The yards are made into cords of any length, by laying them ; and that we may have a rope of any degree of strength, many yarns are united into one strand, for the same reason that many fibres are united into one yarn.
The process for laying or closing large cordage, is as follows :—At the upper end of the walk is fixed a tackle-board. This consists of a strong oaken plank, called a breast-board, having several holes in it, fitted with brass or iron plates. Into these are put iron cranks called heavers, which have forelocks and keys, on the ends of their spindles. This breast-board is fixed to the top of strong posts, and well secured by struts or braces. At the lower end of the rope-walk is a similar breast-board fixed to a movable sledge, which may be loaded with weights when necessary. A Tor, which is a truncated cone, having scores in its sides for the strands, a long staff', and supported on a sledge or carriage, is placed between the strands, and, when necessary, gently forced into the angle formed by their separation. A piece of soft rope, called a strap, is attached to the handle of the top, by the middle, and its two ends are brought back, wrapped several times tight round the rope, and bound down. The yarns are formed into strands, each of which is knotted apart at both ends. The knots at their upper ends are made fast to the hooks of the cranks in the tackle-board ; and those at the lower end, to the cranks on the sledge. The sledge itself is kept in its place by a tackle, and t proper weight laid on it till the strands are stretched in their places. The tackle is now cast off, the cranks turned at both ends, and as the strands contract by the operation, the sledge is dragged up th • walk. When the strands are sufficiently hardened, they are taken of th • cranks, the cranks taken out, and a very strong crank put in the centre hole of the tackle-board. To this all the strands are now attached ; the top is placed between the strands, as before described, and the heavers at the tackle-board and sledge continue to turn as before. By the motion of the sledge-crank, the top is forced away from the knot, and the rope begins to close. As this advances, the rope shortens, and the sledge is dragged up the walk. The top mores faster, and at last reaches the upper end of the walk, the rope being now laid.
Such is the general and essential process of rope-making; and in the course of this process, it is in our power to give the rope a solidity and hardness which makes it less penetrable by water. Some of these purposes are inconsistent with others; and the skill of a rope-maker lies in making the best compensation, so that the rope may, on the whole, be the best in point of strength, pliancy and duration, that the of hemp in it can produce.
The following rule for judging of the weight which a rope will bear, is not Air from the truth. Multiply the circumference in inches by itself, and the fifth of the product will express the number of tons which the rope will carry. Thus, if the rope have 6 inches circumference, 6 X 6 = 36, the fifth of which is 7/ tons ; apply this to the rope of 31 inches, on which Sir Charles Knowles made his experiments, 3i x 3i = 10.25 ; one-fifth of which is 2.05 tons, or 4592 pounds. It broke with 4550. This may suffice for a general account of the mechanical part of the manufacture : but we have taken no notice of the operation of tarring, because it would be no easy task to enumerate all the various methods employed in different rope-works. It is evidently proper to tar in the state of twine oryarn this being the only way in which the hemp can be uniformly penetrated. The yarn is made to wind off one reel, and having passed through a vessel containing hot tar, it is wound upon another reel, and the superfluous tar is taken off, by passing through a hole surrounded with spongy oakum • or it is tarred in or hauls, which are drawn by a capstern, through the tar kettle.
Tarred cordage, when new, is weaker than white, and the difference increases by keeping. The following experiments were made by M. Du Hamel, at i Rochefort, in 1743, on cordage of three inches (French) in circumference, made of the best Rigs hemp.
M. Du Hamel says, that it is decided by experience, 1st. That white cordage in continual service is one-third more durable than tarred. 2d. That it retains its force much longer while kept in store. 3d. That it resists the ordinary Injuries of the weather one-fourth longer. Why, then, should cordage be tarred t The answer is, That tarring preserves cables and ground tackle, which are greatly exposed to the alternate action of water and air ; for white cordage, exposed to be alternately very wet and dry, is found to be weaker than tarred cordage; and that cordage which is superficially tarred is always stronger than what is tarred throughout, and resists better the alternatives of wet and dry.