Another Company has also a noteworthy vessel running on the Estuary of the Thames—viz., the London Belle, plying from London Bridge to Clacton-on-Sea. She is a triple-expansion paddle boat, and the first river steamer fitted with three crank triple-expansion paddle engines. She was built by Denny of Dumbarton, and can develop a speed of 19½ knots—i.e., twenty-three statute miles per hour, and is worked with great economy of coal consumption.
An example of a quadruple-expansion engine steamer may be found in the Tantallon Castle, one of the newest vessels for voyaging to South Africa. She is 456 feet long, over 50 broad, with a gross tonnage of 5636. She is fitted with quadruple-expansion engines of 7500 horse-power, and the stoke holes are well ventilated by large fans speeding round with great swiftness.
Improvements in steamship building had gone steadily on; and it is safe to say that a pound of coal, after the compounding principle came fully into use, did four or five times the work it accomplished before high pressure engines were fully utilised.
Let us enter the engine-room of a big liner, and see for ourselves. It is a triumph of engineering. Still, at first, you cannot understand anything of the complicated mass of machinery. Then you notice three large cylinders—for these are triple-expansion engines—with pistons shooting in and out downwards, and attached by connecting rods to the cranks of the propeller shaft below. The cranks are bent at different angles so that they can never all be in the same position at once. There is a maze of machinery and shining rods, bewildering to the uninitiated eye. But you gradually notice how absolutely regular every part is in its action, and how beautifully one part fits with another.
Then go before the furnace; you find yourself in front of a huge structure, at the bottom of which is the long fire box; above rises the heat box communicating with tubes over the furnaces, with the water circulating between. The water, indeed, is beneath the furnace, about parts of the heat box, between and above the tubes. The object is, of course, to obtain as great heating surface as possible. The tubes communicate with the funnel at their other end. Boilers are made of a “mild” steel which has, it is said, a most remarkable tenacity of 28 tons to the square inch. Consequently they are able to bear great pressure of steam.
STOKE HOLE.
Hot distilled water is admitted to the boiler from the surface condenser. This is a “box,” riddled with tubes, through which cold sea water is pumped. The waste steam, having done its work in the cylinders, is passed into this “box,” is condensed by touching the chilly tubes of sea water, and can be run off or pumped to a hot cistern, whence it is used to feed the boiler and be turned once more to steam. About 4000 tons of water an hour pass through the surface condensers of a large liner when she is at full work.