HYDRAULIC LIFTING-DOCK. 39 



4 feet in diameter above the ground level, and sunk 12 feet in 

 the ground. The clear space between the two rows is 60 feet, 

 and the columns are 20 feet apart from centre to centre, and 

 are placed on each side of the excavated lift-pit in about 27 

 feet of water. There are sixteen columns in each row, giving 

 a length of 310 feet to the dock, but as vessels may overhang 

 at each end, there is a practical working length of 350 feet. 

 The columns were sunk in the usual manner. When the 

 requisite depth was obtained, the base was filled with concrete, 

 and covered with a layer of 2-inch planks, to act as a cushion 

 for the cast-iron seat on which the press rests. The columns 

 support no weight, but act solely as guides for the cross-heads 

 of the presses, which move in slots reaching from the top of the 

 presses (just clear of high-water) to the top of the columns. 

 The column is covered by a cap, and each row is firmly con- 

 nected together at the top by a wrought-iron framed platform 

 running from end to end of the dock on each side. This plat- 

 form forms a convenient permanent scaffold for raising the 

 rams. A scale is printed on each column to register the motion 

 of the cross-heads while rising or falling. 



" The presses or girders are arranged as follows : Each 

 column encloses a hydraulic ram of 10 inches diameter, with a 

 length of stroke of 25 feet; the top of the press is just clear 

 of the highest water, and it is kept in place by a collar or 

 diaphragm in the column. The rams are solid, and each carries 

 a boiler plate cross-head. From the ends of the cross head are 

 suspended, by wrought-iron bars, two iron girders, each of which 

 extend entirely across the dock to the corresponding column 

 and press on the opposite side. There are thus sixteen pairs 

 of suspended girders lying at the bottom in 27 feet of water 

 when the presses are lowered, but rising above the surface when 

 the presses are raised. They form a large wrought-iron plat- 

 form which can be raised or lowered at pleasure with a vessel 

 upon it. The sectional area of each ram being 100 circular 

 inches, a pressure of 2 tons per circular inch gives 200 tons 

 as the lifting power of each press, or 6400 tons for the whole 

 lift; but to find the available lifting power, there must be 

 deducted 620 tons, which is the weight of the rams, cross-head, 

 chains, and girders, leaving 5780 tons for the pontoon and 

 vessel. The presses were tested at 2^ tons per circular inch. 

 The girders are designed for carrying the vessel as a load at 



