1418
THEORY ANI) PRACTICE OF ENGINEERING.
Book II.
position and comparative tension of all the fibres employed can scarcely be estimated, andit must depend in an essential degree upon the quality of the hemp or material employed.
Fig. 2375.
Fig. 2378.
3 %
Fig. 2379.
Fig. 2380.
Fig. 2381.
Ropes passing through Rings may be variously secured, either by taking one end througha series of loops, or by a triple twisting through a single one. Emerson found that a goodhempen rope, of 1 inch in circum-ference, would bear 1000 poundsweight suspended to it, whilst a rodof iron of the same dimension wouldsupport 3 tons, and another of yellowfir, 2 inches in diameter, would carry7 tons.
The rigidity of ropes or cords, andthe difficulty of bending them intoany given curve, prevents the form-ation of a close knot, particularlywhere they are of large diameter: theresistance arising from this stiffness ^is as the weights which stretch thecords, multiplied by their thickness,and divided by the radii of curvatureof the surfaces over which they pass.
The Scaffold Builder must form hisknots or make fast his ropes in theway best adapted for his purpose ;when a St. Andrew’s cross is formedby two poles, to be tied tightly toge-ther, two or more ropes are lashedaround them, drawn tight and madefast; a single or double loop may bepassed around the head of a postaccording to the security required.
Mariners make use of a greatvariety of knots which have beenadopted by engineers, to secure theheads of piles or attach horizontaltimbers laid against them ; a doubleloop, or thrice twisting round, andafterwards passing the rope througha double loop, is an effectual fasten-ing, and prevents the knot from slip-ping in any direction.
Fig. 2383.
Fig.2384
Fig. 2385.