.182

JLECTURE XVI.

length is diminished; the elasticity of all the parts, therefore, resists, and tendsto restore the thread to its natural.state. But if two such threads are retainedin contact at a given point of the circumference of each, this point is renderedstationary by the opposition of the equal forces acting in contrary directions,and becomes the centre, round which both threads are carried by the remain-ing forces, so that they continue to twist round each other till the new com-bination causes a tension, capable of counterbalancing the remaining tensionof the original threads. Three, four, or more threads may be united nearlyin the same manner: a strand consists of a considerable number of yarns thustwisted together, generally from sixteen to twenty five, a hawser of threestrands, a shroud of four, and a cable of three hawsers or shrouds. Shroudlaid cordage lias the disadvantage of being hollow in the centre, or of re-quiring a greater change of form in the strands to fill up the vacuity, and inundergoing this change, the cordage stretches, and is unequally strained.The relative position and the comparative tension of all the fibres in these com-plicated combinations are not very easily determined by calculation; but it isfound by experience to be most advantageous to the strength of the ropes totwist the strands, when they are to be compounded, in such a direction as tountwist the yarns of which they are formed; that is, to increase the twist ofthe strands themselves: and probably the greatest strength is obtained whenthe ultimate obliquity of the constituent fibres is the least, and the mostequable. This advantage is obtained in a considerable degree by Mr. Ilml-dart’s method of adjusting the length of the strand to its position in therope, and his registered cordage appears to derive a decided superiority fromthis arrangement of the strands. A very strong rope may also be made bytwisting five or six strands round a seventh as an axis: the central strand, orheart, is found after much use to he chafed to oakum; it should he moretwisted than the rest, in order to allow it to extend a little; such ropes are,however, unfit for running rigging, or for any use in which they are liable 'tobe frequently bent.

Ropes are most commonly made of liemp, hut various other vegetables arcoccasionally employed; the Chinese even use woody fibres, and the barks oftrees furnish cordage to other nations; we have indeed in this country an eX'ample of the use of the hark of the lime tree, which is employed for gardenmatting. The finest hemp is imported from Riga and St. Petersburg. Th«

2

9