1410

THEORY AXI) PRACTICE OF ENGINEERING.

Book 11.

values which suit them, according to a formula given by Boistard for this centre, andwhich makes a=191*09°, C = 75*37°, Y=115°, we shall find p = 128,730 kilogrammes,;/= 124,730 kilogrammes. In the same design the length of the piece be is 3*41 metres,that of the piece bf is 4*23 metres, and their square is 35 centimetres ; making then in theequation

a 6*

Q = (203368 45 + 21017476 be) —,

a= Z» = 0*35 and c = 3*41, we shall find for the weight which would begin to make thepiece be bend, Q = 1 6*7480 kilogrammes : and making in the same equation C = 4*28, weshall find for the weight which would begin to make the piece bf bend, Q,=. 121,400kilogrammes; thus the first of these two pieces is a little stronger than is necessary, andthe second would be too weak, if, as we lia.c supposed, the pressure transmitted by thebinding-piece ab were entirely carried on be and b f.

In seeking in the same manner what portion of the load transmitted by the binding-piece gh is supported by the pieces cd and di, we shall find that the pressure exercisedaccording to di is equal to 4149 kilogrammes, and that the pressure which exerts itselfaccording to cd is equal to 44,020 kilogrammes : thus the load of di is scarcely the third ofthat of bf, and as the length of the two pieces is the same, the square of di might be lessthan 35 centimetres ; cd is also much less loaded than be , but its length, which is 5*85 metres,is more considerable. In making in the preceding equation a = 5 = 0*35, and c — 5*85, we shallfind Q = 79,380 kilogrammes: notwithstanding, then, its greater length the piece cd isstronger than is necessary : thus we may conclude that of all the inclined timbers, regardbeing taken to the pressure compared with the length, bf is that which supports the greatesteffort ; that 35 centimetres square is rather small for this piece, but a little too large forall the others.

Scaffolding requires as much care in the selection of the timber employed, as for thatwhich is to constitute a part of the edifice : generally a scaffold is composed of uprightpoles, and ledgers laid horizontally and secured by strong cords; on the latteris laid the ends of shorter poles or putlocks, which have a bearing on the wall of thebuilding in holes left in the masonry purposely to receive them: for churches and publicbuildings a double scaffold is employed, which is braced in several directions, and, whereround poles are not obtained of sufficient strength, squared timber is made use of.The example given was erected by M. Dubrin, master carpenter, before the facade of SaintGervais at Paris , and was found to answer its purpose admirably : where the walls are to becarried up to a considerable height, it is requisite to spread the base of the scaffold, and tointroduce additional braces.

Scaffolding used in the erection of the Monument , by Sir Christopher Wren , occupied asquare, the side of which was 60 feet, and as there was a double row of poles, it left aninterior square, the side of which was 35 feet, in the middle of which the Monument wasconstructed. The entire width of the scaffold was therefore 12 feet 6 inches all around it,and at each angle of the inner and outer square was placed two poles, pitched upon thediagonals of the square: between these, in the middle of the length of the outside, were threepoles, and between them and the angle two others; each side of the inner square was dividedinto two, by two upright poles, so that there were 36 standard poles on the outside, and 16on the inside, in all 52, which were continued up, with a slight inclination, to the height of120 feet, after which single poles were found sufficient to construct the remainder of thescatfolding to the top : 28 ranges of ledgers were fastened to these upright standards, atdistances of 8 feet; those continued round the interior and exterior, forming as many stories,and supported the putlocks or cross timbers, on which the planks were laid; diagonal braces,each of which crossed four stories, were secured firmly on all sides, to the ledgers, from thefoundation to the summit, to which a well-contrived wooden stairs enabled the workmento mount and descend.

The Scaffolding for the Nelson Column in Trafalgar Square was designed and executedunder the direction of Mr. Allen. The whole was composed of unsawn timber, and thetotal height was 180 feet; 154 loads of timber, or 7700 cubic feet, were made use of, andthe cost of its erection was 240/. The upright timbers were slightly tenoned into thehorizontal, and where the joint was made, iron dogs were driven into the timbers in adiagonal direction across the joints; these afterwards were easily withdrawn, and the timberbut little injured : the base of the scaffold wqs 96 feet square, without measuring in theraking braces; there were in all seven stories, which diminished in height from 48

feet to 21 : the whole was strengthened by flying wind-braces, which were supported by

cross transoms, which projected outwards about 6 feet from the perpendicular of the scaffoldat each stage : at the summit worked a travelling machine, by which one mason could set

as much work in a dav as was formerly done in three.

At the London and Birmingham Railway Station , Euston Square , Messrs. Cubitt contrived