This is done when it is intended to lower the rope or chain; but if a weight or goods be appended to it, the friction band m m is made to press against the periphery of the fly-wheel k by raising the lever n, which, having its fulcrum at o, draws the friction band tightly over the fly-wheel, and the goods are thus lowered with safety and expedition. But, for general purposes, the wheel and pinion turned by a winch is superior to all other modes of working cranes, and more particularly is it superior to any arrangement of reciprocating levers, as it produces a smooth continuous circular motion, avoids the jerks and concussions attending the latter, and saves a vast deal of friction, complexity, and expense; accordingly, we find the wheel and pinion now generally adopted almost to the exclusion of every other method. We have already stated that the barrel is sometimes attached to the jib so as to turn with it. The annexed engraving represents an excellent construction of a crane of this description, several of which are erected upon the wharfs of the Regent's Canal, a is an upright pillar of cast iron firmly fixed in a foundation of masonry; b a pin in the head of a, which supports the jib c, and forms the pivot round which it turns; d d two struts, supporting the extremities of the jib, and the lower ends resting on a collar e encircling the lower part of the pillar, which collar is suspended from the jib by the iron rodsff; g is one of the side frames supporting the barrel h; k a toothed wheel on the axis of the barrel, and turned by a pinion, on the axis of which is fixed the winch f.
The following figure represents a modification of the wheel and axle, known by the name of the " Chinese Crane," which, for simplicity of construction and immense power, far surpasses any other machine which is applied to purposes for which this is adapted; and however its modest and unassuming appearance may prevent its admission into the elegant companies of wheels and pinions, which we see associated together in this age of mechanical combination, there can be little doubt that it will eventually work its way into notice by its own merits, to the displacement of some of those complicated arrangements of wood, iron, and brass, which, in some cases, seem to be erected for no other purpose than for employing a horse to do the work of a man. The construction of this machine will be readily understood by reference to the figure, a b is a windlass, which is worked by a winch or handle c d. It will be seen that the windlass a b partakes of two diameters, that part from a to e being larger than the remaining part e b; the cord g is wound round the part ae of the windlass, and is passed under the movable pulley h i, and carried over the part e b of the windlass, on the opposite side to that from which it descended at a e; the weight to be raised is suspended from the pulley h i.
Now if a power be applied at d, and the windlass be caused to make one revolution, a portion of the cord g equal to the circumference of the part a e of the windlass, will have been wound on to the windlass; but the part e l of the windlass has also made one revolution, consequently a portion of the cord, equal to the circumference of e b, has descended, so that after one revo lution the cord will have been shortened a quantity equal to the different between the larger and smaller barrel of the windlass; but as this difference has been divided between the two parts of the cord g and k, it follows that the weight has been raised through a space equal to only half the difference of the circumferences a e and e 6. But as circles are to each other as their radii, the following simple rule may be deduced for calculating the power of these machines; as c d, the radius of the winch, is to half the difference of the radii of the parts of the windlass a e and e b, so is the weight w to the power whicli is necessary to produce an equilibrium.
For example, put c d=18, the radius a e=6, and the radius e 6=3; and suspend a weight of 108 lbs. from h i; we then have 6 - 3/2=11/2, and as 18: 11/2:: 108: 9; consequently a power of 9 applied at the point d would be equivalent to a weight of 108 lbs. acting upon the pulley h i. This subject may perhaps be better understood by referring to the annexed diagram, where a e represents the radius of the larger part of the windlass, and b the smaller radius, e d being the radius of the winch; and we may suppose d a to represent a lever, whose fulcrum is e; and as each of the ropes g k bear equal parts of the weight of 108 lbs. we represent the whole weight by two distinct weights g and k acting upon the points 6 and a of the lever da; and if we retain the proportions de=18, ae=16, and b e=3, we have a weight k on one side of the fulcrum, at the distance of b, whose quantity is 54; and we have a power of 54 acting on the opposite side of the fulcrum, at the distance of 3. Now it will easily be seen from the principles of the lever, that g will sustain a quantity equal to 1/2 k, consequently there will remain a weight of 27 acting at a, to be kept in equilibrio by a force applied at d; but ed is 3 times e a, therefore a power equal to 9 applied at d would balance a weight of 27 acting at a; which is precisely the same result as was obtained by the rule before laid down.