SCAFFOLDING.
H13
Chaf. XXIII.
Fig. 2358.
posed of three frames or floors, supported upon braces and portions of St. Andrew’scrosses : it was so light that it could be moved with facility by means of copper wheels.
A suspended scaffold for a singleworkman is easily contrived by ropesand pulleys, and is frequently usedby painters: where the cornice hassufficient projections, as at St. Peter’s atPome, a pole may be suspended, intowhich a brace may be pinned withits end lodged on the architrave below ;such a pendant scaffold requires to bewell secured at top, either by loading itor pinning it down.
In all suspended scaffolds, we mustregard the timber employed as pulled inthe direction of its length, and the quan-tity of space through which it extends is directly proportionalto the number of parts extended, and to the weight.
Emerson says that a piece of oak 1 inch square, and 1 footlong, supported at both ends, bears 315 pounds before itbreaks, and the same will bear, when drawn in the directionof its length, 2835 pounds, or 1^ tons. We have not sufficientdata to enable us yet to decide upon the strength of thetimbers in the situation we are now considering, when drawnin the direction of their length; but the extension to whichthey are subjected is inversely proportional to the area ofthe section, or inversely as the breadth and thickness.
Suppose L the length in feet, W the weight in pounds, B the breadth in inches, and T the
thickness in inches, then "^-3 is the extension; or, when the extension is given,B * r . , . , , . , BxT
L x W : BxT. Where a is a quantity derived from experiment, then a = ^ x ^y*
SUSPENDED SCArrOLD.
Fig. 2359.
4x3