gussets attached to the roof sheets by long angle irons; sometimes by running braces forward to the waist; by running them clear forward to the flue sheet; and by riveting heavy angle irons crosswise to the roof sheet as near as possible to where the brace ends are attached.

Q. What is the disadvantage of attaching the braces to the front flue sheets?

A. The vibration tends to produce leaks.

Q. What is another objection to the Belpaire type? A. The difficulty of keeping the top turns of the thread tight where they join the shell.

Q. Should a crown-sheet be perfectly level?

A. No, it should have such inclination that when the engine is on a level the back sheet end will be lower than the front, to keep water on the back part after the front end may have got exposed.

Q. Why does the crown-sheet of a long furnace slope toward the back?

A. To keep it covered in running down a very steep grade.

Q. Does not this make it dangerous for the front end of the sheet in running up a steep grade?

A. No; as the front end is nearer the center of the length of the boiler, it is not so apt to be uncovered as the back end.

Q. What is the action of the crown-bars?

A. They serve as trusses to keep the top sheet from buckling in.

Q. How are the crown-bars fastened?

A. They have at each end feet resting on the sidesheet beam, and holding them slightly above the sheet; they are double, and between them and the sheet is a thimble through which, as well as through the sheet and the bar, goes a bolt; then the bars are slung from the boiler-shell, so that they support the crown-sheet, and the boiler-shell holds up the bars.

Q. What is the advantage of crown-bars for supporting crown-sheets?

A. Greater ease of repair than where direct stays are used.

Q. What are the disadvantages of the crown-bar system?

A. It affords good chances for scale and mud to collect on the crown-sheet, is heavy and expensive, and the bars take up considerable of the water room on the sheet; it does not afford good facilities for inspection or for wash

ing out mud and scale; it is not practical for large boilers having wide crown-sheets and carrying high pres

sures.

Q. What is the advantage of having the crown-bar bolts and the holes through which they pass, slightly tapering?

A. They are more readily taken out in case leaks occur. Q. What is the advantage of having crown-bar washers tapering toward the sheet?

A. It gives more surface of the sheet in contact with the water, and lessens the liability to overheating around the bolt.

Q. Why are direct-stayed boilers flat-topped over the fire-box?

A. If they were not flat-topped, and were direct

stayed, the crown-stay bolts would not go squarely through the outside sheet.

Q. What difference would that make?

A. A full thread could not be had for the bolt in the outside sheet.

Q. Why is the dome on direct and radial stay boilers, ahead of the fire-box?

A. If over the fire-box, that portion of the crown-sheet directly under the dome could not be properly stayed. Q. What is a radial-stay boiler?

A. One upon which that portion over the fire-box is round-topped and the dome is ahead of the box.

Q. Why is that staying called radial?

A. Because the crown sheet is somewhat round-topped and the crown stay-bolts are run straight through it, radiating from the smaller circle (crown-sheet) to points proportionately spaced on the larger circle (outside sheet).

Q. How is that portion of the boiler-head above the crown-sheet supported?

A. By braces, the ends of which are attached to crowfeet on the shell and boiler-head itself.

Q. What is the disadvantage of sling stays?

A. The difficulty of giving all equal strain.

Q. What is the disadvantage of radial stays?

A. That some of them are at such an angle that they have too little hold in the thread.

Q. What precaution is it well to take with them?

A. To give the center six or more rows down, the length of the fire-box, button heads under the crownsheet.

CHAPTER X

THE BRICK ARCH

Q. How is the brick arch placed, and what are its functions?

A. It is built across the front of the fire-box, from side to side; and extends forward and upward, forming above the grate a diagonally-placed baffle-plate, preventing the flames and combustion gases from the front of the grate going directly into the lower tubes, and compelling them first to flow backward and upward; thus not only giving them time to get more thoroughly aflame, but causing more intimate mixture. Besides this, its fire-bricks get white hot and tend to assist the combustion when new coal is put on, especially with bituminous coal. It lessens black smoke by highly heating the unconsumed combustion products; also shields the fluesheet and the flues from sudden influx of air when the furnace door is opened.

Q. How are the bricks of the brick arch held up?

A. By bent tubes secured into the crown-sheet and the tube-sheet, thus making water communication between the water-leg and the water on the crown-sheet; or by tubes between the front and the back leg. (See Figs. 60 to 63 inclusive.)

Q. Have any experiments been made as to the exact value of the brick arch?

A. Yes. Mr. J. N. Lauder, of the O. C. R. R., took two engines of the same dimensions and in about the same condition, and put them to run alternately on the same trains, one having the Pennsylvania Railroad style of brick arch supported by water-tubes, the other a plain fire-box. They ran "opposite each other" for two months, and care was taken to see that no extra work was done by either that would lessen the value of the

performance report. For one month the engine with the plain fire-box ran 50.87 miles per ton of coal; that with the brick arch, 58.22. For the preceding month the advantage was about the same. The train-weight was 160

Figs. 60 and 61. Brick Arch on Water-tubes.

Figs. 62 and 63. Brick Arch on Water-tubes.

tons besides the engine; the run, 36 miles, made in 52 minutes, with eight or ten "slows" and several "knownothing" stops. The coal consumption was 34.3 pounds of coal per train mile with the brick arch and 39.3 with the plain fire-box, showing about four per cent saving.