Chap. XII.
MACHINES, TOOLS, ETC.
997
Machines tor raising weights, or removing them, are principally levers, capstans,wheels, pulleys, inclined planes, screws, and their various combinations in the form ofcranes and derricks. Wheelwork is now employed in a variety of instances, its power inall cases being derived from the same principles as the lever, each wheel and pinion beingconsidered as composed of a series of bent levers, of which the axis is the common fulcrum,acting in succession on the teeth of the next wheel.
The Lever is the most simple and the most generally useful for raising weights to asmall height, but the extent of its action is still limited, although we are indebted toPerrault for an invention which has greatly increased its power; two pins are fixed in thelever at a short distance from each other, which slide in two pairs of vertical grooves pro-vided with ratchets, so that when the long arm of the lever is pulled by means of a rope,the nearer pin serves as a fulcrum, and the more distant one is elevated at the same timewith the weight, and detained in its place by the click ; when the rope is slackened, theweight sinks a little, and raises the pin, which first served as a fulcrum, to a higher placein its groove.
A Winch with an axis is a bent lever, and often used as a power, to draw water from awell; a vertical axis, with two or more levers inserted in it, constitutes the capstan ; toestimate their force all that is necessary is to add to their radius half the thickness of therope, and compare it with the arm of the lever.
The Gin for raising coals and other weights is an erect axis or drum, turned by theforce of horses working in a circle. The buckets are attached to the opposite ends of arope, passing round a drum, to which it is drawn by its property of adhesion: onebucket descends empty, as the other rises full, and when the motion of the buckets isrequired to be changed, the horses arc turned, or the wheels are made to impel the axis ina contrary direction, when any other moving power is employed.
The Capstan is not supposed to have been much employed in Britain previous to thesixteenth century : at its first introduction it consisted of an upright barrel or drum,turning on a vertical axle ; at the upper part was the head, in which were holes or sockets,for the insertion of the eight levers or bars, which were from ten to twelve feet in length.The action of the men employed to turn it round was necessarily constant, for if left aninstant, the weight would immediately run back or recoil.
An improvement in the method of pulling the capstan was first made in France byLalande, who contrived the palls, now attached to the lower parts of the barrel, whichturning on a pivot raise thecapstan, and press it forward overany obstacle, when they fall bytheir own weight into iron stepsmade to receive them, which incase of any recoil also check thedescent of the w r eight; when themotion is to be reversed, the pallsare turned over on their pivots,and they act in the other di-rection.
There being some difficultyin surging the rope on the plaincylindrical barrel, it was fur-nished with additional piecessecurely attached, and calledwhelps; to which, as well asto the barrel, the capstan headis bolted down ; they arc slightlyconical, for the purpose of allow-ing the rope to be received and delivered without rising or falling from its properplace. The power of such capstans depends upon the length of the lever or bar, or thedistance of the centre of pressure on the bar, and on the radius of the barrel, including thewhelps, and half the thickness of the rope.
The power with which a man pushes when working a bar, multiplied by the distancefrom the axis, expresses the effect of this power; when there are several, the power must
3 s 3
Fig. 1493.