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LECTURE XVI. 



ON THE UNION OF FLEXIBLE FIBRES. 



JLlIE strength of cordage, and of other substances which ai"e employed in 

 the communication of motion, where flexiblHty is required, as well as the 

 utility of other flexible materials which serve for furniture or for clothing, 

 depends principally upon the lateral adhesion produced by twisting, or by the 

 intermixture of fibres. The union of flexible fibres, therefore, being fre- 

 quently subservient to the communication of motion, and the machinery, usually 

 employed for producing it, belonging immediately to the subject of the mo- 

 dification of motion, we may with propriety consider at present, as far as 

 our plan will allow us, those important branches of the mechanical arts, of 

 which the object is to effect a union of this kind. 



When a chain is made of wire, each link is separately bent, and remains 

 united with the neighbouring links in virtue of its rigidity: but the fibres of 

 vegetable and of animal substances must be united by other means. For 

 this purpose we have recourse to the force of friction, or rather of lateral ad- 

 hesion, and the fibres are so disposed, that besides the mutual pressure 

 which their own elasticity causes them to exert, any additional force applied 

 in the direction of the length of the aggregate, tends to bring the parts into 

 closer contact, and to augment the adhesion, in the same manner as we have 

 already seen that a wedge and a screw may be retained in their situations. The 

 simple art of tying a knot, and the more complicated processes of spinning, 

 ropemaking, weaving, and felting, derive their utility from this principle. 



When a line is coiled round a cylinder, for ' instance, in letting down 

 a weight, by means of a rope which slides on a post, or on such a 

 grooved cylinder as is sometimes employed to enable a person to lower him- 

 self from a window in cases of fire, the pressure on the whole circumference 



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