ing which the door is open. Closing the door, communication between drum and blower is opened by a second part of A between B and the jet blower; thus driving a blast into the fire-box. At first, when most air is required for the new fuel, the high-pressure steam in the drum drives in considerable; this diminishes gradually; and the length of time during which this subsidiary supply continues may be regulated for a given steam pressure and a given length of time during which the fire door is open. But always the higher the steam pressure and the longer the door is open, the more air forced in.

Every time the throttle is closed the regulating valve is half closed, as seen in Fig. 48, and steam passes to the drum and also to the blower. When the throttle is opened again the distributing valve is entirely closed and the blast kept up for a while to supply the place of the but slowly-occurring exhaust puffs.

At the lower end of the valve A is a smaller one connected by a small pipe L with the blower. When the throttle is shut and A half open (Fig. 49) this is opened to serve as a blower. When necessary to slice the fire, etc., with open fire-door, the air jet can be turned aside. The distributing valve is then as seen in Figs. 42 to 47.

Q. In what different ways may fire-box sheets fail?

A. Either by gradual failure, having a good many small cracks; or by hidden failure or rupture.

Q. In what direction are the small cracks, generally?
A. Vertical.

Q. Where are they most numerous?

A. Radiating from the stay-bolts, and usually from one stay-bolt to another in the same vertical row; seldom horizontally between stay bolts.

Q. On which side are they?

A. On the fire side, sometimes extending through the thickness of the sheet, first going through next to the stay bolts.

Q. What condition usually accompanies such cracks?

A. Corrugation.

Q. In what part of the sheets does one not find the most cracks and corrugation?

A. In the lower half.

Q. In case of sudden failure or rupture, how do the sheets fail?

A.. Usually by a single crack or rupture from a foot to several feet long; sometimes from the mud ring to the crown sheet; but ordinarily in the lower half of the sheet, upward from the mud ring or from a few inches above it, and always near the middle of the side sheet lengthwise.

STAY-BOLTS.

Q. What sort of strain is there on the fire-box?
A. One tending to crush it in.

Q. What resists the tendency to crush in the side sheets? A. To a very slight extent their own stiffness; to a very great extent the stay-bolts, extending from the inside to the outside sheets. (See Fig. 50.)

Q. What arrangement should be made with stay-bolts or tie-bolts of fire-boxes?

A. These should be tubular, or should have a small hole lengthwise in the outside end, extending beyond the plate, so that if the bolt breaks there will be a leak at the break, to give warning.

Q. What purpose is served by the small hole drilled in the outer end of stay-bolts?

A. If the bolt breaks or cracks in the water space, water rushes out at the hole and gives notice.

Q. How are these stay-bolts fastened?

A. In some engines they are riveted over; in others they are screwed in; in some, both screwed and riveted.

Q. How should stay-bolts be fastened into the side sheets? A. Their ends should be screwed in and then riveted over.

Q. Suppose that a fire-box has on it a pressure of 160 pounds per square inch, and that the stay-bolts are four inches between centers; what will be the strain on each bolt?.

A. There will be 16 square inches held by each bolt, making 2,560 pounds that the bolt will have to hold.

Q. What is the object of riveting over the stay-bolt ends? A. To "make assurance doubly sure"; because sometimes screw-threads strip, and again the bulging of the sheets from undue expansion will tend to open out the holes, leaving the entire strain on the bolt-heads. If there were no heads the bolts would then be useless.

Q. What kind of stay-bolts are used in England for firebox walls?

A. Copper.

Q. When a stay-bolt breaks, what takes the strain?

A. The eight bolts nearest thereto.

Q. How much is their strain thus increased?

A. One-eighth each.

Q. What is the principal cause of the strain on stay-bolts? A. The inner fire-box expands more than the outer.

Q. What is their usual distance apart?

A. About 4 inches between centers.

Q. What may be said of stay-bolts as a cause of explosion? A. After low water and hot crowns they are the most usual causes of explosions.

Q. Where do they usually break?

A. Close to the outer sheet.