Page:Philosophical Transactions - Volume 054.pdf/52

 but if the lever be pulled a little farther down than what is sufficient to lift the catch out of the teeth of the wheel, it will rub against the edge of the wheel V, and thereby hinder the too quick descent of the weight; and will quite stop the weight if pulled hard. And if the man should happen inadvertently to let go the lever, the elastic bar will pull it suddenly up, and the catch will fall down into the wheel, and stop the machine.

W W are two upright rollers, above the axis or upper gudgeon of the gib E: their use is to let the rope bend upon them, as the gib is turned to either side, in order to bring the weight over the place to which it is intended to be let down.

N. B. The rollers ought to be so placed, that if the great rope were stretched close by their outermost sides, the half thickness of the rope may be perpendicularly over the center of the upper gudgeon of the gib. For then, and in no other position of the rollers, the length of the rope between the pulley in the gib and the axle of the great wheel will be always the same, in all positions of the gib; and the gib will remain in any position to which it is turned.

When either of the trundles is not ufed in working the crane, it may be drawn off from the wheel, after the pin near the axis of the trundle is drawn out, and the thick piece of wood is raised up a little, behind the outward supporter of the axis of the trundle. But this is not material: for, as the trundle has no friction on its axis but what is occasioned by its own weight, it will be turned by the wheel without any sensible resistance in working the crane. Rh