to the under side of the upper door and sliding out and in with it, thus bringing the point of suspension immediately into the centre of the shaft.
AIR-LOCK AND HOISTING GEAR FOR CAISSONS.
_Figs._41_to_44,_Page_25.png)
The mode of working these looks was as follows: A 3-ton steam crane with fixed jib (see Figs. 36 and 51), but provided with quick-moving slewing gear, was set up near the lock so as to centre the air-shaft with its heaving chain; the top door of the lock was then drawn back and the skip or bucket, about 3 ft. in diameter and 4 ft. high, holding about 1 cubic yard, was lowered down into the lock and the shackle of the inside winding chain attached to it. The upper door was now closed, and a slight turn of the engines tightened the chain and lifted the bucket just clear of the lower door. A cock, or valve, which communicated with the compressed air in the working chamber, was now opened and pressure thus admitted to the lock. The sliding doors were provided with india-rubber joints, and small thumbs or tappets, actuated by a lever, were used to force the door hard up against the india-rubber, in addition to which the gradually increasing air pressure helped to close the joint. When the upper door was thus secured, a turn of the interlocking wheels set the lower door free; this was now drawn back, and the bucket was lowered right down to the bottom of the working chamber, and there filled with the spoil of the excavation, or else the chain was taken off and attached to one already
