Chap. XXIII

SCAFFOLDING.

1417

PLAN OP SCAFFOLDINO AT THE APERTURE AT THE TOP OF THE DOME.

To the scaffold builder a knowledge of the variety of knots is important, and as ropesare used in several machines, it is .ry

necessary that every mechanic should n

understand how they can be secured "*

and tied together.

An ordinary knot is simply madeby twisting a rope twice round, andpassing the ends in opposite directions.

Loop knots for joining pieces ofrope together are passed through aloop in such a manner that, whenpulled tight, they are firmly united :they may cross each other, or sim-ply pass through each rope whendoubled. Fig. 2370. Fig. 2371. Fig. 2372.

The wall knot is made with the lays of a rope, and cannotslip; the bow-line knot is so firm that when fastened it isperfectly secure.

The sheep-shank knot, which shortens a rope withoutcutting it, is so arranged that it can be readily loosenedagain ; most knots are for the purpose of fastening onerope to another, which is effected by means of a small cordattached to the neck of the knot, and firmly tied about bothropes.

The fibres of a rope seldom exceed in length 3^ feet;they must therefore be well twisted together before theyare applied: large ropes are either cable-laid or hawser-laid;the former are composed of nine strands, the latter of three,and each of a number of primitive yarns: a rope 8 inchesin circumference may have 414 yarns equally divided amongthree strands.

The greater the obliquity of the fibres that compose arope, the greater will be their adhesion, but the greater alsowill be their immediate tension, in consequence of the actionof a given force in the direction of the rope; but the relative

Fig. 2373.

Fig. 2374.