eight angle irons at the corners further strengthen the structure thus formed. The reason of this arrangement is that wrought iron plates from their flexibility offer but a small resistance to compression in the direction of their thickness, as they bend or buckle with a comparatively small force. The five vertical plates, however, which form the sides of the cells, are placed in the position in which they offer a maximum resistance to compression, namely with their width or depth in the direction of the strain; and the angle irons and the plates E and F serve to keep them in position and give great rigidity to the structure. The centre plate of the cells is

inch thick, and the two remaining plates each inch thick. The sectional area of the concave or front part of the crane subjected to compression is therefore

Attached to the back of the crane is a tail piece or box of wrought iron, containing cast iron weights acting as a counterpoise to the jib. The chain is attached to the crane. by a bolt and nut at the point of the jib, and passes round four pulleys, two moveable and two fixed, in the end of the jib; it is then conducted down in the interior of the jib over three rollers to the barrel, which is also in the tube near the ground. On each side of the crane a strong cast iron frame is fixed for receiving the axles of the spur wheels and pinions. Four men, each working a winch of 18 inches radius, act by two 6 inch pinions upon a wheel 5 feet 3 inches diameter; this in its turn moves the spur wheel, 6 feet 8 inches diameter, by means of an 8 inch pinion, and on the axle of the former the chain barrel, 2 feet in diameter, is fixed. Hence the advantage gained by the gearing will be

and as this result is quadrupled by the fixed and moveable pulleys, the power of the men applied to the handles is multiplied 632 times by the gearing and blocks. A break wheel, 5 feet 2 inches diameter, is fixed on the other end of the spindle of the spur wheel; and the power applied at its circumference is accordingly multiplied about 100 times by the gearing and blocks.

At the level of the ground the crane is firmly fixed in a strong cast iron frame, the outer edge of which is a circle of 11 feet 3 inches diameter; and on the edge of the well a similar ring is embedded in the masonry and secured by long holding-down bolts, leaving a space of 10 inches all round between it and the inner ring. In this space a number of strong cast iron rollers are placed, 10 inches in diameter, to prevent friction and facilitate the movement of the crane as it revolves round its axis. Upon the cast iron ring on the quay wall is fixed a circular rack, composed of cogged segments bolted together, into the teeth of which a pinion works, whereby the crane is made to revolve. This pinion is worked by a worm and wheel placed in the counterpoise box; and two men are sufficient to move round the crane with 60 tons suspended from the extreme point of the jib. In working the crane the men stand upon a cast iron platform attached to it a few inches above the level of the ground.

This crane, taking it altogether in regard to its strength, height, and the extent to which the weight raised can be swung round, is probably one of the first and most powerful in Europe. It can raise or lower boilers in and out of the holds of ships of the line; pick up the heaviest ordnance from any of the decks; and ship or unship masts

with the same or greater ease than is now done by the large shears used for that purpose in any of the dockyards. In fact a colossal crane of this description, 120 feet high, was submitted to the surveyor of the navy as a substitute for the large masting shears at Woolwich, which were worn out, but owing to some other arrangement the project was not carried into effect.

Fig. 67.

Fase by mi lequa in raising henry A me I esse as this has since been tream acari and is also an engine cirle madhe pa beth! the crane, so time wicked by seam ega mea inci cylinders with Ek The I start of a commucive, by which they er reed with the greatest ease.

Fig. 67 is a general view of a ten ton crane on the same. principle, with its engine and boiler attached. This crane sweeps a circle of 50 feet in diameter, and lifts to a height of 26 feet above the platform. The cylinders of the engine are 6 inches in diameter, to obtain a rapid elevation of the weight, and have link motions and reversing gear, as in

the larger cranes. If necessary they may be at once disconnected, and the cranes worked by hand.

The tubular principle has also been applied to railway travelling cranes, as shown in fig. 68, which is a sectional elevation of a 15 ton crane upon its truck. This crane revolves upon a steel pivot at p, and is balanced by a weight of about 8 or 10 tons attached behind the jib at W. In other respects it is precisely similar in construction to the stationary cranes previously described. It sweeps over a circle of 25 feet in diameter, and lifts to a height of 18 feet above the rails. The chain barrel is protected from the weather within the jib, as shown at a.

From the above description it will be seen that the great advantages of these constructions is their extreme lightness as compared with the weights they have to lift, and their powers of being extended to any amount of strength, height, or sweep that may be wanted. In conclusion, they are admirably adapted for the loading and discharge of cargoes from vessels in dock, and for the transfer of the load to any point within the limits of a circle described by the chain suspended from the jib.