Chap. XI.

MECHANICS.

935

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Some pulleys are made to revolve on rollers;the pivot that passes through their centre is madeto turn upon two other rollers at each bearing;there are many varieties of friction rollers.

The most efficacious method perhaps of diminish-ing friction is by the application of friction-wheels :for any friction arising from one body beingdragged over another may be changed into thatwhich arises from one body rolling upon another :when the moving force is not exerted in a per-pendicular direction, but obliquely, as in thegudgeon of a wheel, it will press with greaterforce on one part of the socket than on anyother.

Some are so mounted that thepulley contains four others that aremovable, and which revolve rounda fixed cylinder in the middle ; thefour wheels are retained in theirposition by two cords which passover their thickness, and hold themtogether.

The force of friction, which re-sists the sliding of different bodieson each other, is closely connectedwith lateral adhesions ; the frictionof all solid bodies may be regardedas a uniform retarding force, neitherincreasing nor diminishing as thevelocity of their bodies is changed ;when bodies are in contact with each

other and at rest, their adhesion is greater than their friction, and this is required to befirst overcome.

Sometimes eight small wheelsare arranged in a manner to form aregular polygon, being held togetherby irons which are riveted overtheir respective centres, the rivetforming the pin upon which theyrevolve around the fixed centre.

"When wheels are fixed on thesame pivots, and turn with the samevelocity, there is considerable frictionand consequently wear, for the ropepassing over the wheels with differentdegrees of velocity, it cannot butslide variously in the differentgrooves, with the exception of oneonly. The object then in any ar-rangement of pulleys ought to be the making them all revolve on their axles at one time,so as to avoid unequal wear, and at the same time their grooves on their exterior shouldhave different velocities, equal to those of the rope which passes over them.

Chains of a variety offorms have been appliedinstead of ropes topulleys; that which iscommonly used was in-vented by De Vaucan-son, a celebrated me-chanic ; each chaincarries a tooth, whichpasses into a notch madeon the edge of thepulley.

Other pulleys have hadthe same principle ap-plied, but in a manner

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