140 LECTURE XVI. 



A single thread or yarn, consisting of fibres twisted together, has a ten- 

 dency to untwist itself ; the external parts are the most strained in the 

 operation and at first shorten the thread, until the internal parts have no 

 longer room for spreading out laterally, as they must necessarily do when 

 their length is diminished ; the elasticity of all the parts, therefore, resists, 

 and tends to restore the thread to its natural state. But if two such 

 threads are retained in contact at a given point of the circumference of 

 each, this point is rendered stationary by the opposition of the equal forces 

 acting in contrary directions, and becomes the centre, round which both 

 threads are carried by the remaining forces, so that they continue to twist 

 round each other till the new combination causes a tension capable of 

 counterbalancing the remaining tension of the original threads. Three, 

 four, or more threads may be united nearly in the same manner : a strand 

 consists of a considerable number of yarns thus twisted together, generally 

 from sixteen to twenty five, a hawser of three strands, a shroud of four, 

 and a cable of three hawsers or shrouds. Shroud laid cordage has the dis- 

 advantage of being hollow in the centre, or of requiring a greater change of 

 form in the strands to fill up the vacuity, and in undergoing this change, 

 the cordage stretches, and is unequally strained. The relative position and 

 the comparative tension of all the fibres in these complicated combinations 

 are not very easily determined by calculation ; but it is found by expe- 

 rience to be most advantageous to the strength of the ropes to twist the 

 strands, when they are to be compounded, in such a direction as to untwist 

 the yarns of which they are formed ; that is, to increase the twist of the 

 strands themselves : and probably the greatest strength is obtained when 

 the ultimate obliquity of the constituent fibres is the least, and the most 

 equable. This advantage is obtained in a considerable degree by Mr. Hud- 

 dart's* method of adjusting the length of the strand to its position in the 

 rope, and his registered cordage appears to derive a decided superiority 

 from this arrangement of the strands. A very strong rope may also be 

 made by twisting five or six strands round a seventh as an axis ; the central 

 strand, or heart, is found after much use to be chafed to oakum ; it should 

 be more twisted 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 to be frequently bent. 



Ropes are most commonly made of hemp, but various other vegetables 

 are occasionally employed ; the Chinese even use woody fibres, and the 

 barks of trees furnish cordage to other nations ; we have indeed in this 

 country an example of the use of the bark of the lime tree, which is 

 employed for garden matting. The finest hemp is imported from Riga and 

 St. Petersburg. The male and female flowers of hemp are on different 

 plants ; the male plants are soonest ripe, and require to be first pulled. 

 They are prepared for dressing by being exposed to the air, and the fibrous 

 part is separated from the dry pulp by beating and hackling. In spinning 

 the yarn, the hemp is fastened round the waist ; the wheel is turned by an 

 assistant, and the spinner, walking backwards, draws out the fibres with 



* Huddart's Patent registered Cordage, Rep. of Arts, xii. 80. Remarks on do. 

 4to. 1800. 



