Chap. XXI.

LOCOMOTIVE ENGINES.

1273

The extreme width of the fire-box in sucli a case does not exceed 47J inches, and the ironplates with which it is surrounded arc in thickness about of an inch : the covering or

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top, however, where greater strength is required, has four wrought-iron rods rivetedto the upper surface, and the sides are held more firmly together by means of copper bolts,which are tapped and riveted. The aperture to admit the coals is usually formed by a ringof copper, and the iron plates of the casing are gathered in, and firmly riveted to it withcopper rivets.

The boilers are of a cylindrical form, about 8 feet in length, and 3 feet or more in diameter;the ends are closed by circular iron plates, the lower half drilled with holes, in which from70 to 140 longitudinal tubes of tough rolled brass are inserted by means of hoops of steel;the thickness of the brass out of which they are formed is that called No. 15 wire gauge;they are made truly cylindrical by drawing them through a mould : one end of these