1006

THEORY AND PRACTICE OF ENGINEERING.

Book 11.

Fig. 1517.

DOUBLE CRANE.

cumferences are in the same proportion as their diameters; therefore the velocity of thepower will bear to the velocity of the weight the same proportion as the diameter of thewheel bears to the diameter of the axle, or as the weight is to the power.

When the upright post is secured by chains or ropes, the triangular timber frame is dis-pensed with, which is sometimes necessary in building a quay wall. By means of blocks thewhole of such an arrangementcan be lightened at pleasure,and made perfectly secure.

Double Crane , fixed on awharf or pier-head, is soframed and braced that it ba-lances exactly when turningon a pivot at the bottom.

Such a crane was used at theconstructions on the RochdaleCanal: the distance betweenthe ends of the gibs was 30feet; they were held togetherby a bar of iron, 1} inchessquare: their scantling was 8inches by 7J inches; thelength of each of the crossbraces was 27 feet 6 inches,and their scantling 7 inchesby 6. The upright post wasa foot in diameter, and thesegmental piece which swung round upon its head, 12 inches by 9. The wheels were 3 feet4 inches in diameter, the pinion 6 inches, and the rollers 12 inches, the latter being 2 feet9 inches in length.

Iron Pit Crane composed of acast-iron frame shaft and gib, hasusually three different powers, eachpower having a double purchase.

The height of the shaft is 20 feet,

11 of which is below the surfaceof the ground; it rests on a gudgeonabout 6 inches in diameter, andturns in a brass box. At the sur-face of the ground two plates arefixed to the shaft, 4J inches apart,between which are five rollers;when the crane is turned, thesemove round the inside of a cast-iron circle. The gib and pro-portions of a crane to carry eighttons is 21 feet in length, and thediameter of the winch is S feet;that of the pinion taken at thepitch line 5J inches with ten leaves:the wheel in which it works is3 feet 4} inches in diameter, witheighty-four teeth: the secondpinion is 7^ inches in diameter,with fifteen leaves, and the wheel3 feet 1^ nches in diameter, witheighty teeth : the third pinion is

1 foot 3^ inches in diameter, withthirty-two leaves, and the wheels

2 feet 5^ inches in diameter, withsixty-two teeth. The pinion com-mon to all the powers is 5| inchesin diameter with eleven leaves,and the wheel 4 feet and | of aninch in diameter, with 100 teeth,tion rollers for the chain 9 inches.

The Corvus or Raven of the ancients, according to Perrault, corresponded to our crane ;each comprised a strong arbor, or upright timber, firmly fixed in the ground, and sustained by

Fig. 1518.

WHARF CRANE.

The chain barrel is 16 inches in diameter, and the fric-