A.—­The interior of the fire box is 3 feet 7-1/4 inches wide by 3 feet 5-1/2 inches long, measuring in the direction of the rails.  The area of the fire grate is consequently 12.4 square feet.  The bars are somewhat lower on the side next the fire door than at the side next the tubes, and the mean height of the crown of the fire box above the bars is 3 feet 10 inches.  The top edge of the fire door is about 7 inches lower than the crown of the fire box.  The fire box is divided transversely by a corrugated feather or bridge of plate iron, containing water, about 3-1/2 inches wide, and of about one-third of the height of the fire box in the centre of the feather, and about two-thirds the height of the fire box at the sides where it joins the sides of the fire box.  The internal shell of the fire box tapers somewhat upwards to facilitate the disengagement of the steam.  It is about 2 inches narrower and shorter at the top than at the bottom; the water space between the external and internal shell of the fire box being 2 inches at the bottom and 3 inches at the top.

657. Q.—­Of what material is the fire box composed?

A.—­The external shell of the fire box is formed of iron plates 3/8ths of an inch thick, and the internal shell is formed of copper plates 1/4 inch thick, but the tube plate is 3/4 inch thick.  The fire grate is rectangular, and the internal and external shells are tied together by iron stay bolts 3/4 inch diameter, and pitched about 4 inches apart.  The roof of the fire box is stiffened by six strong bars extending from side to side of the fire box like beams, and the top of the fire box is secured to these bars, so that it cannot be forced down without breaking or bending them.

658. Q.—­What are the dimensions of the barrel of the boiler?

A.—­The barrel of the boiler is 3 feet 7-1/2 inches in diameter, and 10 feet long.  It is formed of iron plates 3/8ths of an inch thick, riveted together.  It is furnished with 181 brass tubes 1-7/8 inch diameter and 10 feet long, secured at the ends by ferules.  The tube plate at the smoke box end is 5/8ths of an inch thick, and the tube plates above the tubes are tied together by eight iron rods 7/8ths of an inch thick, extending from end to end of the boiler.  The metal of the tubes is somewhat thicker at the end next the fire, being 13 wire gauge at fire box end, and 14 wire gauge at smoke box end.  The rivets of the boiler are 3/4 inch diameter and 1-1/2 inch pitch.  The plating of the ash pan is 5/16ths of an inch thick, and the plating of the smoke box is 3/16ths of an inch thick.

659. Q.—­Will you describe the structure of the framework on which the boiler and its attachments rest, and in which the wheels are set?