DETAILS OF (PADDLE-WHEEL) OSCILLATING ENGINES. 193

CHAPTER Y.

DETAILS OF OSCILLATING ENGINES FOR PADDLE-WHEEL PROPULSION.

The details of marine engines form a widesubject for discussion, inasmuch that, althoughone effect is sought after by the severalmakers, each'differs in design and proportion.In dealing with this matter, practical exampleswill be quoted and dimensions adverted to,thus forming a certain standard for productionsof greater or less power. The illustrations,relating to the engines under direct notice, arecomposed, for the most part, of details from theexamples represented by Plate 30, produced byMessrs. James YYatt and Co.; but where anydeviation is made as a comparison, attentionto the same will be directed, but unless thisoccurs, it will be remembered to whose enginesthe details belong.

CYLINDERS.

The cylinders for oscillating engines do notdiffer much in design, excepting the arrange-ment of the steam ports and the position of thetrunnions. Those latter portions are generallysituated at a given distance from the top orbottom end, to retain an equal strain on thepiston rod when the cylinder is at the requisiteangles. The centre of gravity can also be main-tained by proportionating the position of theslide valve chest, and allowing the trunnions tobe in the centre of the cylinder’s length. Thegreat depth required for the main stuffing boxon the cover of the cylinder renders the cover

heavy,and thus,if the trunnions and slide valveswere at the centre of the cylinder’s length, theupper portion would be heavier than the lower,instead of both equal, as required. In somecases it is preferred to make the centre of thetrunnions, and that of the exhaust ports, on thesame horizontal line, rather than as shown bythe sectional elevation, Fig. 39—page 194. Inthis example it is seen that the trunnions areraised, while the valve casings are lowered fromthe centre of the cylinder’s length; or, to bestill more explanatory, the top of the exhaustpassage, and that of the trunnion tubes, are onthe same level; but the belt surrounding thecylinder is equidistant, vertically on each sideof the trunnions’ centres. The cylinder coverand stuffing box are of the ordinary kind, with ashallow gland and deep bottom bush. The raisedcurved portion on the cover serves alike forornament and a repository for any waste lubri-cant. The bottom end is cast with the body, andthe boring bar can be passed through the holeprovided. The slide valve is double-ported, ofthe equilibrium type, packed at the back withindia-rubberand a face ring. The sectional planrenders a ready conception of the arrangementof the supply and exhaust belts, or steam pas-sages, and the angular position of the slidevalves and casings. The trunnions, stuffingboxes, glands, and tubes, are the usual mode,

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