Volume Twelve - Pre-Victorian to the Present Day - further aspects - 1898 - A Clever Piece of Engineering . . .

Next a gantry on two pairs of sheerlegs, as shown in plan in Fig. 14, were provided. There were adjusting screws on each sling or tackle, so as to keep the gates perfectly upright. This tackle was also required when putting the new gates in.

As a considerable amount of water leaked in, all above water was made quite tight. After this, to drive the water out and keep it from again entering, air was pumped in and a pressure maintained sufficient for the purpose, this being necessary not only when lifting the gates, but when carrying them along the dock to where they were landed.

The gates were not quite buoyant in the lower portions, about 40 tons additional buoyancy being required to keep them clear of the sills. It was for this purpose the pontoon was provided.

The first operation was to lift the gates so that the bottom would clear the sill. This was done by the tackle on the sheerlegs, as shown. The chain from the pontoon was then attached by a diver to claws placed under the gate, so as to afford easy attachment.

The top of the gate had to be pulled over by a crab on the opposite dock wall to get room to place the floating pontoon at the back, as shown in Fig. 13.

The pontoon chain being attached, the bottom of the gate was gradually raised by means of a lifting screw on the pontoon, as shown. There were, however, first inserted under the gate, when lifted by the tackles, blocks of timber which were loaded to keep them down.

The gate was lowered on to these blocks, and when it was pulled forward by the crab it tilted on the blocks and thus cleared the wall and the sill. The screw on the pontoon being tightened up as close as possible, the gate was floating by its own buoyancy in the upper part, and by the assisting pontoon in the lower portion, and was then towed to the end of the dock.

The next problem was to land the gate on the end of the slopes at the far end of the docks where the work was to be carried out.