Chap. XII.

MACHINES, TOOLS, ETC.

1001

raised by the axle of the lastwheel: in the second case thepower must be applied to thefirst pinion.

The Crab employed for raisingweights has a winch and barrelworked by a wheel and pinion;the whole is fixed in a strongframe either of timber or iron,and may be moved into anysituation where it is required.A double handle enables twomen to lift considerable loads;and the machine is furnishedwith a brake, so that it maybe stopped at any time, whetherin raising or lowering a weight.Two crabs worked by six meneach, at the London Docks,raised a weight of 723,000 lbs.to a height of 16 feet in 8hours, equalling 2012 lbs. raised1 foot high per minute by oneman.

The Gin usually accompaniesthe crab in raising large stones;it is composed of three long stoutlegs, 12 or 14 feet in length and6 inches in diameter, meetingtogether at the top, and so placedthat two can be fixed at a certaindistance from each other bymeans of two iron bars placedhorizontally, one being 12 inchesand the other 7 inches long: aroller upon pivots turns be-tween two cheeks attached totwo of the poles, 7 or 8 inchesin diameter and 6 feet in length,20 inches of which are left

Fig. 1504.

CRAB.

I

.1

Fig. 1505.

square at each end, and holes are made in eachhandspikes, by which the roller is turned; thecylindrical, to wind up the rope.

to receivemiddle isThe transverse iron bars arefixed at one end to one of the poles by a bolt, and at the otherto the other pole with a bolt and key, which enables the ginto be separated and removed. Each cheek is fastened to thepole by two iron bands and two iron bolts, between which thecylinder revolves : the poles are usually hooped round at eachend, and to the upper are straps through which an iron boltpasses to keep them together, and hold up the block throughwhich the rope or chain runs to the crab or windlass. Theblock contains two pulleys, and before the machine is mountedfor use, it is laid down flat on the ground, the lower end of thesingle pole being turned over, for the purpose of fasteningthe upper block, and when the rope is turned round boththe machine is raised.

The Jack or Hand-jack is a portable mechanical power forelevating the end of a block of stone or piece of timber, toallow of rollers or blocks being placed underneath ; or to lift upgirders, brestsuminers, or to hold up a wall that requires under-pinning, and is one of the most useful mechanical contrivancesin the hands of the builder. The power of the common hand-jack is obtained by a rack and pinion, placed in a block ofwood about 30 inches long, 10 inches broad, and 6 incheswide : the rack has a double claw at its upper end, and asmall pinion which engages in the teeth ; the axis of the pinionis supported in two iron plates bolted to each side of the block,