Stones are lifted and set into position in the work by means of ropes and pulleys supported by shear-legs (as Fig. 925), which are scaffold poles let into or bedded in the ground securely; or the pulleys are secured to the apex of two poles held in position by guy ropes, as Fig. 926; and when the work gets beyond the use of these methods, above the ground, derrick cranes are employed to hoist up all materials and to bed stones, though they are often used on the ground floor in place of shear legs or guy ropes and poles. They consist of two legs, from platforms or beds, fixed on the various floors sufficiently weighted to prevent any slipping out, inclining up to an apex to which an upright is raised from another platform immediately under the apex, on which also stands the apparatus for winding up the chain, which goes up to the apex and thence to the end of the derrick or gib, which is hinged at the bottom of the foot of the post beside the winding apparatus, as shown in Fig. 927. This derrick or gib is raised up and down as required, and, through a pulley at its top, the chain to lift the stones s worked from the winding apparatus.

Hoisting Tackle PracticalBuildingConstruction01 756

Fig. 326.

Hoisting Tackle PracticalBuildingConstruction01 757

Fig. 928.

When the stones are too heavy for ordinary derrick cranes travellers have to be constructed, consisting of strong platforms made of balk-timber raised on each side of the wall up to a height so that a trussed beam from one to the other can work or travel along above the required height of the wall. These trussed beams - for they are in pairs - run on wheels on rails fixed on the upper face of the top beams of the framing on each side; and they support the jenny or crab which runs across, from side to side, on wheels guided by rails on the trussed beams.

Hoisting Tackle PracticalBuildingConstruction01 758

Fig. 929.

Hoisting Tackle PracticalBuildingConstruction01 759

Fig. 930.

The jenny is used for hoisting the stones, and can be moved to any position along the traveller beams, which in themselves can be worked along the side framing, as the sketch will indicate (see Fig. 928).

The stones lifted by either of the above methods are grasped by nippers, as Fig. 929; and also with a round block, as Fig. 930, on one side, whenever the face might be injured by the nippers tightening when the weight hangs on them.

Another method is to cut a mortise in the top bed of the stone, and insett a Lewis, as tig. 931, which, it will be seen, is dovetailed in, and secured to the chain or rope.

Fig. 931.

Underpinning is the building of walls underneath an existing wall, or structure, such as cellar-walls below a building where there has been no cellar previously. These works require great care, so that settlements or the collapse of the building are guarded against; and with this view all underpinning should be done in cement-mortar, and in very short lengths, 2 or 3 lineal feet only of excavation being taken out at a time, and the work built up and wedged tightly and completely to the existing wall above before any more earth is taken out from under the wall further on.