MARINE ENGINES.

415

Fig. 298A shows the crank shaft of the screw steamer Arizona . It

is built up of ffve pieces. Of thesefour are made of hammered androlled scrap iron, the fifth, or crankpin, being made of Steel. The dia-meter is 22^ inches.

The immense power of engines insome of the recently launched oceansteamships for the Atlantic Servicenecessitates correspondingly heavyand strong machinery; thus thecrank shaft of the Servia, City ojRome , and Alaska are about 25 inchesin diameter; those of the Servia andAlaska are solid, whilst-that of theCity of Rome is hollow. This shaft isof Steel and made by Sir J. Whit-worth’s process, the Steel beingknown as fluid compressed, this me-thod being adopted to insure uni-formity in structure. The processconsists in first of all casting the steelin a mould having a core; thereafter,andwhilestill fluid,thecasting is sub-jected to an intense hydraulic pres-sure, which forces the air and gaseousmatters out of the fluid mass. Aftersolidifying the metal is reheated andforged down to a length suitable forthe purpose for which it is to be ap-plied. A stronger shaft for the sameweight of metal is thus obtained.

The main framing (Fig. 299) onwhich the pillow blocks are cast forsustaining the cranked shaft may beregarded as the backboneof horizon-tal marine engines. It is subjected totensional, compressive, and twistingstrain, and must therefore be madeof great strength. Some makers adopt the 1 section, others the 4-,

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