Chap. XII-

CRANES, ETC.

1011

foot high per minute by one man. At another trial the six men raised a weight of911,680 pounds 8 feet high in 11 hours, which was equal to 1841 pounds raised a foothigh per minute by one man.

The gib of a crane should always revolve wholly round its axis, and be able to deliverthe load at any point of the circle described by the axis of the gib : in the crane employedby Perronet at the bridge of Orleans, the whole machinery was made to revolve, whichmethod answers admirably well, if the necessary space can be spared.

The gib apparently worked well in one particular, for as the rope by which the stoneswere raised passed over the upper gudgeon, and then between two small vertical rollers,when it was turned round, the rope always laid fair to the windlass, while in manyother cranes the contrary takes place; so that when the gib is turned on its axis, the rope isso bent that it forms an angle more or less acute, which causes a great increase of friction,and a continual effort to bring the arm of the gib into a parallel position with the innerpart of the rope: much inconvenience arises also from attempting to keep the gib inits true position by guy ropes, or other contrivances where the crab or windlass is in afixed position, and not so attached to the crane as to revolve with it.

Perronet balanced this crane so equally, that when poised upon the post upon which itturned, it worked freely in any direction.

The windlass or barrel on which the rope winds had two wheels fixed on the axis, at somedistance apart, which being united with boards formed a large hollow cylinder or drum,and was worked by men walking upwards on the inside. But to this method there aremany objections: awheel 16 or 18 feet diameter does not afford sufficient leverage formen so placed to turn it with much effect, and they are often overpowered by the load,when, if one makes a false step, or slips down, the wheel runs back with accelerated velocity,and throws them against the sides and each other with tremendous violence, often occa-sioning death.

It is indeed surprising that such cranes should have been used almost universally by theengineers of the last century.

A walking wheel is, however, an advantageous method of employing the strength of alabourer, but the requisite dimensions of such a machine render it sometimes inconvenient,and unless large, it is not sufficiently powerful; if, however, it be worked on the outsideinstead of the inside, this objection is obviated, but there should always be some meansprovided to prevent the too rapid descent of the weight of the labourer, should he by anyaccident be compelled to quit his position, and a catch is usually introduced, which coun-teracts any retrograde motion, and in some instances a bar is suspended from the axis ofthe wheel, on which he can support himself with his hands.

Other wheels of this description are made to incline a little from the horizon, and thelabourer walks on its flat surface, but these arc not preferred, on account of the great lossof power whenever the machine is stopped: when horses or other animals are used totread a wheel, the path within it should present as little elevation as possible, but theirpower is always more advantageously employed in a circular mill-walk.

In the construction of piers it is often convenient to have a revolving crane that willpermit the stone not only to be lifted out of the lighter, but also passed round to the sideor position in which it is intended to be laid; and this can be easily effected by making

SEA BANK OF WESTON' CANAL.

the gib or arms sufficiently long, and bracing them equally on each side; when thusbalanced properly the whole may be turned on its pivot, with the load suspended at oneextremity; and for the construction of sea-walls, such a crane has been found highlyuseful.

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