extends round as far as the centre of the wing furnaces, and then sheathed with sheet-lead and bound with strong iron hoops.

Each boiler has two spring-loaded safety valves, 413 inches diameter, with easing gear led to engine-room platform, one steam stop valve 7 inches diameter, one valve for steam to winches and cranes 2 inches diameter, one valve for steam to whistle, one surface blow-off valve, one bottom blow-off valve, one main feed check valve, one donkey feed check valve, one salinometer cock, two sets of asbestos packed gauge cocks; also an efficient means of circulating the water in boilers while steam is being got up. The whole of the above valve chests are of cast brass, with the exception of the safety-valve and stop-valve chests.

The furnace fronts, doors, and centre bar bearer are made of wrought iron, and the dead plates and bars of cast iron. Furnace fronts below dead plate are fitted with damper doors, with a rack to keep them open to the desired amount. A wrought-iron door is fitted to lower part of bridge bearer in each furnace, so that ashes or coal thrown over the bridges may be removed.

3

The uptake is formed of inch and inch wrought-iron plates, with an air space of 2 inches between them. The smokebox doors have shield plates both outside and inside, and very strong hinges with brass pins riveted in. The funnel is formed of inch and inch plates, 43 feet high from fire bars, and 6 feet 6 inches diameter, with all necessary hoops and shackles for stays &c.

LECTURE XXVI. QUESTIONS.

1. Sketch a longitudinal section of a Cornish boiler; show clearly the method of connecting the ends to the shell and flue plates, and mark the water level. Where is the feed water admitted, and for what reason? (S. and A. Exam., 1887.)

2. Sketch a cross and a longitudinal section of a Cornish boiler, with a single-furnace tube and fire grate inside the tube. Describe your sketches by an index to the parts, and explain how the products of combustion pass from the furnace to the chimney.

3. Sketch and describe in section an ordinary cylindrical two-flued Lancashire land boiler with flat ends and internal flues. How are these flues fitted with cross water tubes. Enumerate the principal fittings and their

uses.

4. Sketch the front view of a Lancashire boiler, showing all the necessary fittings. State the uses of the principal parts in giving an index of them. 5. Describe Galloway's water tubes, usually fitted to cylindrical land boilers, and show how they are fitted into their position. What advantages are claimed by their adoption?

6. Describe the construction of a marine boiler with four furnaces of modern type for high-pressure steam. Sketch a cross and a longitudinal section, showing the water spaces, with a complete index of the various parts. How are the flat surfaces stayed?

7. Sketch front end view and a longitudinal section of a modern marine high pressure multitubular boiler, and calculate the heating surface of 320 tubes, each 7′ long and 2" 75 external diameter. Ans. 1612.7 sq. feet.

8. If the inside diameter of a Lancashire boiler shell is 5', and the outside diameter of each of the two internal flues is 25", what space is there inside the boiler for steam and water if the length is 21'4" ? Ans. 273'35 cubic feet.

9. The diameter of a cylindrica: boiler being 13' and the water level being one-fourth of the diameter from the top; if the boiler is 10' long, what is the volume of the steam space? Also, what is the pressure per square inch on the bottom of the boiler when the steam pressure is 100 lbs. by gauge? Ans. 295'5 cb. ft.; 104:25 lbs.

10. Why are the longitudinal joints in cylindrical boilers usually double rivetted, while the transverse joints are only single-rivetted? Sketch in longitudinal section a marine high pressure boiler. (S. and A. Exam., 1889.) See Lecture XXIX. of the Anthor's Advanced Text Book on "Steam and Steam Engines," for auswer to first part of this question.

II. Draw a vertical section through the fire-box and tubes of a lowpressure marine boiler. Describe the method adopted for strengthening the weak portions. Mark in your drawing the combustion chamber, and state the object which it fulfils. (S. and A. Exam. 1890.)

12. Give (1) a longitudinal, (2) a transverse section through a Lancashire double-flued boiler, showing the probable water-line. Where is the feed water admitted, and for what reason? What is done to strengthen the shell of the boiler at the parts where it is most liable to give way under pressure. How are the flues secured against collapse? (S. and A. Exam 1891.)

13. Which joints of a Lancashire boiler are single-riveted and which are double-riveted? Explain why all the joints are not made of the same strength, and give some reason for such construction. Show by a sketch a double-riveted butt joint with cover plates. (S. and A. Exam. 1892.)

14. What is a Galloway tube, and what is its object? Sketch the transverse and longitudinal sectional elevations of a boiler in which such tubes are used. How are the ends of the boiler stayed? (S. and A. Exam. 1893.)

15. Draw a vertical longitudinal section of a single-ended return tube marine boiler of circular section. Show the method of staying the tube plate and combustion chamber. Show also the fire bridge, and state what end it serves. (S. & A. Exam. 1892.)

16. Sketch and describe the construction and action of a non-return feed-water valve for either a land or a marine boiler. Where and at what

level is such a valve placed on the boiler? (S. & A. Exam. 1896.)

17. Make a longitudinal and also a transverse section of a Lancashire boiler with its brickwork settings. Indicate the course of the gases through the internal and external flues of the boiler to the chimney. Show also the construction of the fire-bridge and method of supporting the fire-bars. (S. & A. Exam. 1896.) (See also the Author's advanced "Text-Book on Steam and the Steam Engine," and Munro's "Steam Boilers," published by Chas. Griffin & Co.)

LECTURE XXVII.

CONTENTS.-Boiler Mountings and Fittings-Safety Valves-Lever Safety Valves, with Examples-Dead-weight Safety Valve-Marine Boiler Safety Valve-Glass-tube Water Gauge.

Boiler Mountings and Fittings.-The necessary boiler mountings and fittings for an ordinary land boiler were enumerated in the index to parts given under the outside view representing a Lancashire boiler. Of these, we have already described the Bourdon steam gauge in Lecture XI. and the stop valve in Lecture XXII. Of the rest of the apparatus, that which claims our chief attention at present is the safety valve.

Safety Valves.—A safety valve is simply an ordinary circular steam-tight valve, fitted into a cast-iron chest, situated on the top of the boiler, and kept down with a pressure equivalent to that of the maximum working pressure of the steam on the under side of the valve. Its object is, therefore, to prevent the steam pressure in the boiler exceeding the working pressure. The pressure on the outside of the valve may be produced by a simple lever and weight as in the first figure, or by dead weight as in the figure (p. 268), or by a spring as in the figures (p. 269).

Lever Safety Valves.-The lever safety valve is, no doubt, the most common method of producing the required pressure on the valve in small land boilers. When properly made and fitted, it serves the purpose admirably. If, however, the weight on the lever is free to be moved, there is a liability of its being accidentally or intentionally shifted, which would bring a greater or less pressure on the valve. To prevent this, lockfast safety valves, marked with the precise load per square inch to which the valve is kept down, are now frequently demanded, and the following illustration shows very clearly how it is accomplished by means of a pointed bolt, lock, and key. The pointed bolt fits a niche in the lever at 40, and is held fast by the cross pin passing through the top of the weight, through the end of which pin the padlock ring is passed.

The rule for determining the dimensions of the valve, lever, and weight forms a very interesting example of the principle of moments. Adopting the same lettering and index to parts as in

Munro's book on Steam Boilers, from which the two next figures are taken

Let L

F

=

=

G

=

the length of lever in inches from the fulcrum to the point at which the weight, W, is suspended.

the length of lever in inches from the fulcrum to the centre of valve.

the length of the lever in inches from the fulcrum to the centre of gravity of the lever.

A = the area of the valve in square inches = πr2 =

W =

I d2.

4

weight of the cast-iron ball or block on the end of lever in lbs.

W weight of the lever in lbs.

=

P = pressure of steam in the boiler in lbs. per square inch at which it is intended the valve should lift and "blow off."

The lengths, L, F, and G, should be ascertained with great accuracy by a rule, and the weights W, W1, and Wo separately by an ordinary balance. To find the position of the centre of gravity or the distance, G, the lever should be balanced on a knife edge. The diameter of the valve, d, can best be found by a callipers.

Then,

P

=

W x L+ (W2 x G) + (W, × F)

Ax F