1412
THEORY AND PRACTICE OF ENGINEERING.
Book II.
K
Fig 2355. SCAFFOLDING OF EUSTON SQUARE STATION.
similar arrangement wasmade, and the whole wasraised to 90 feet; the ma-terials were all hoisted bytravelling trucks mountedon tram-ways laid downfor the purpose. Squaretimber is decidedly prefer-able to round poles for thepurposes of scaffolding, asthere is comparatively littlewaste incurred; and thetimber, being thoroughlyseasoned by exposure, isbenefited rather than in-jured by its application tothis purpose. The travel-ling winch on a frameis now universally adopted ;where 10 or 15 tons are tobe hoisted at a time therewould be great economyin the employment of steam-power.
When poles or squared timbersarc employed in an upright position,as they are in a scaffold, the forcethat would bend them when actingin that direction must be exactlycalculated, for if bent in the slightestdegree, any additional effort willcause fracture. The strain is alwaysdirectly as the weight or pressure,and inversely as the strength, whichis also inversely as the cube of thediameter : it is also directly as thedeflexion, which will be directly asthe quantity of angular motion, andas the number of parts strained;that is, directly as the square of thelength, and inversely as the diameter.
The stillest rectangular post isthat in which the greatest side is tothe less as 10 is to 6, but thiswill bend in two directions, andbraces and struts must be introducedin a posiiion to prevent any changeof form ; the ledgers of our ordinaryscaffolds being secured by cords tothe upright standards, at distances of5 feet or more apart, prevents anybending of the latter, the cord passinground both the horizontal and ver-tical pieces, so that it secures thetwo firmly together, supporting themagainst each other in those partswhere they would most probablyyield to the weight they have tosustain. The expert scaffold-buildermakes his knots so that the hori-zontal poles or ledgers form a scriesof struts or straining pieces, and prevent the standards from yielding under any weight.
The Scaffold made use of at a chapel at Turin in Piedmont , for cutting the caissoonsin the vault, which was cylindrical, was well adapted for its purpose, and by means ofwheels, the position of which are shown in the plan, it could be moved either forwards orbackwards.
A similar scaffold was contrived by M. Mandar, the architect, at Paris , for the constructionof a roof erected under his direction, on the principles of Philibert Delorme ; it was com-
Fig.2357.
PLAN OF SCAFPOLD,