ON THE UNION OF FLEXIBLE FIBRES. 139 



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 

 adhesion, 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 pro- 

 cesses 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 himself from 

 a window in cases of fire, the pressure on the whole circumference is to the 

 weight, as twice the circumference to the diameter ; supposing, for ex- 

 ample, that the friction of rope on metal were one tenth of the pressure, 

 then a single coil of rope round a cylinder of metal would support about 

 two thirds of the weight ; or if the weights acting on the different ends are 

 different, the adhesion may be a little greater or less than in this proportion, 

 according to the manner in which the rope is applied. If such a rope made 

 two or three coils, it would be impossible to apply a force sufficient to cause 

 it to slide in the grooves. 



From considering the effect of a force which is counteracted by other 

 forces acting obliquely, we may understand both the effect of twisting, in 

 binding the parts of a rope together, and its inconvenience, in causing the 

 strength of the fibres to act with a mechanical disadvantage. The greater 

 the obliquity of the fibres, the greater will be their adhesion, but the greater 

 also will be their immediate tension, in consequence of the action of a given 

 force in the direction of the rope : so that after employing as much ob- 

 liquity and as much tension, as is sufficient to connect the fibres firmly in 

 all cases of relaxation and of flexure, and to prevent in some measure the 

 penetration of moisture, all that is superfluously added tends to overpower 

 the primitive cohesion of the fibres in the direction of their length.* 



The mechanism of simple spinning is easily understood ; care is taken, 

 where the hand is employed, to intermix the fibres sufficiently, and to 

 engage their extremities as much as possible in the centre ; for it is obvious 

 that if any fibre were wholly external to the rest, it could not be retained 

 in the yarn ; in general, however, the materials are previously in such a 

 state of intermixture as to render this precaution unnecessary. Where 

 we have a number of single continuous fibres, as- in reeled silk, they are 

 sufficiently connected by twisting, and we have no need of spinning. In 

 bc4h cases such machinery has been invented for performing the necessary 

 operations, as is both honourable and lucrative to the British nation. 

 * See Hooke's Experiments on Cordage Birch., ii. 393. 



