182 LECTURE XVI. 



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 hy 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 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 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 in 

 undergoing 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 v^ry easily determined by calculation ; but it is 

 found by experience 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 ex- 

 ample 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 



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