Chap. XII.

CRANES , ETC.

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a number of arms, according to the strengthrequired : when the entire crane turned withthe load, it was originally called rat-tail, andwhen only a portion moved, it was termedthe gibbet crane.

Double Crane> used for the constructionof breakwaters, is mounted upon a frame,and by means of rollers, and a working gear,can be readily moved forwards or back-wards: when advanced to the edge of thework one gib can be employed to lay a stone,whilst the other lifts one from the carriage,which is moved along a railway laid downfor the purpose. The gib has a sliding car-riage for laying the stones either close tothe shaft, or as far out as it will reach : itis worked by a sheeve and rope passing overof the spindle.

gib, and down by the side

the point of the

ABERDEEN : DOUBLE CRANE.

A gib crane with a shaft 40 feet in height and 12 inches square, turning upon a gudgeonat the bottom, is frequently used in building lock walls, &c; At the top is a similar