The hull is of "mild steel," 20 per cent. stronger than iron. The wheels are of steel, and are 35 feet in diameter outside the buckets. The buckets are 14 feet long and 5 feet wide, each bucket of steel seven eighths inch thick, and weighing 2,800 pounds without rocking arms and brackets attached. The total weight of each wheel is 100 tons. The wheels are "feathering," and turn at the rate of 24 revolutions a minute.

The boat has a compound, vertical, beam, surface-condensing engine of 7,500 horse-power. The high-pressure cylinder is 75 inches in diameter, and 9 feet stroke of piston. The low-pressure cylinder is 110 inches in diameter, and 14 feet stroke of piston. The surface-condenser has 15,000 square feet of cooling surface and weighs 53 tons. Of condenser-tubes of brass there are 141 miles. Her working-beam is 34 feet in length from centre to centre, 17 feet wide, and weighs 42 tons. The section of beam-strap measures 9 × 11 inches. The main centre of the beam is 19 inches in diameter in its bearings. The shafts are 27 inches in diameter in main bearings, and 30 inches in gunwale bearings; they weigh 40 tons each. The cranks weigh 9 tons each. The crank-pin is 19 inches in diameter and 22 inches long.

The boilers contain 850 square feet of grate-surface and 26,000 square feet of heating surface. The products of combustion passed through two super-heaters, 8 feet 10 inches inside diameter and 12 feet 4 inches outside diameter, by 12 feet high; thence into two smoke-stacks, the top of each being 101 feet 1 inch from the keel.

The dining-saloon is 108 feet 4 inches in length by 30 feet in width, and 12 feet in height. There are 12 miles of electric-lighting wire, and, including annunciators, firealarm, etc., there are 20 miles of wire and 12,000 feet of steam-pipe. There are capacious gangways and staircases, lofty cornices, and ceilings supported by tasteful pilasters, the tapering columns of which, in relief, flank exquisitely

tinted panelling throughout the length of her saloons. Every convenience known to civilization, and which can contribute to the ease and comfort of the traveller on land or when afloat, is included in the internal arrangements of this floating caravansary.

FIG. 155.-Tandem Compound Engine. (Scale)

The latest types of marine engine are those compounded engines in which the number of engines in series is three, or even more, usually driving three equidistant cranks, and those which are designed to drive two, or even three, screws independently. In the extension of the principle of compounding in multiple-cylinder engines, it is

probably desirable to restrict the number of cranks to three, even with a pair of low-pressure cylinders, or in the quadruple-expansion engine; both as a matter of economy and

to secure smooth working with minimum friction. The balance is usually practically perfect and the full advantage of compounding is attained.

In these cases the construction of all the engines which constitute an element of the compounded machine is com

monly substantially the same in general, the differences being principally in the proportions of the steam-cylinder

FIG. 157.-Engines of "Paris" and "New York."

and its accessories. The triple-expansion engine thus usually consists, as a whole, of three similar simple engines, side by side, so arranged, as to size of cylinder and disposition of pipes and valves, that they work as a series in taking and exhausting steam. There are, however, a number of successful arrangements of three- and of four-cylinder engines driving but two cranks and in which the "tandem" disposition of cylinders is adopted with good

results.

The engraving represents one set of the triple-expansion engines of the twin-screw sister-ships, the City of Paris and the City of New York. Their general arrangement is well shown. Each set drives one screw. The magnitude of these great engines is indicated by the altitude of the working platforms and the reversing wheel. This may be taken to represent a standard and very satisfactory disposition of parts and general proportion of engines.

A good sample set of figures for the proportions and performance of these engines are:

Steam-cylinders, diameter, inches..

Stroke of pistons, feet.

Ratios of volumes.... 1; 2.489; 6.304 or 0.402; 1; 2.53

Steam-pressure, per gauge, lbs.

Revolutions per minute...

Vacuum, inches..

Mean pressures, lbs.

45, 71, 113

...

148

87

26

64; 32; 14