274. Q.--What power was generated by a cubic foot of water in the case of the Albion Mill engines when working without expansion?

A.--In the Albion Mill engines when working without expansion, it was found that 1 lb. of water in the shape of steam raised 28,489 lbs. 1 foot high. A cubic foot of water, therefore, or 62-1/2 lbs., if consumed in the hour, would raise 1780562.5 lbs. one foot high in the hour, or would raise 29,676 lbs. one foot high in a minute; and if to this we add one tenth for waste at the ends of the cylinder, a waste which hardly exists in modern engines, we have 32,643 lbs. raised one foot high in the minute, or a horse power very nearly. In some cases the approximation appears still nearer. Thus, in a 40 horse engine working without expansion, Watt found that .674 feet of water were evaporated from the boiler per minute, which is just a cubic foot per horse power per hour; but it is not certain in this case that the nominal and actual power were precisely identical. It will be quite safe, however, to reckon an actual horse power as producible by the evaporation of a cubic foot of water in the hour in the case of engines working without expansion; and for boiling off this quantity in flue or wagon boilers, about 8 lbs. of coal will be required and 9 square feet of flue surface.

MODERN MARINE AND LOCOMOTIVE BOILERS.

275. Q.--These proportions appear chiefly to refer to old boilers. I wish you to state what are the proportions of modern flue and tubular marine boilers.

A.--In modern marine boilers the area of fire grate is less than in Mr. Watt's original boilers, where it was one square foot to nine square feet of heating surface. The heat in the furnace is consequently more intense, and a somewhat less amount of surface suffices to evaporate a cubic foot of water. In Boulton and Watt's modern flue boilers they allow for the evaporation of a cubic foot of water 8 square feet of heating surface, 70 square inches of fire grate, 13 square inches sectional area of flues, 6 square inches sectional area of chimney, 14 square inches area over furnace bridges, ratio of area of flue to area of fire grate 1 to 5.4. To evaporate a cubic foot of water per hour in tubular boilers, the proportions are--heating surface 9 square feet, fire grate 70 square inches, sectional area of tubes 10 square inches, sectional area of back uptake 12 square inches, sectional area of front uptake 10 square inches, sectional area of chimney 7 square inches, ratio of diameter of tube to length of tube 1/28th to 1/30th, cubical content of boiler exclusive of steam chest 6.5 cubic feet, cubical content of steam chest 1.5 cubic feet.

276. Q.--These proportions do not apply to locomotive boilers?

A.--Not at all. In locomotive boilers the draught is maintained by the projection of the waste steam which escapes from the cylinders up the chimney, and the draught is much more powerful and the combustion much more rapid than in cases in which the combustion is maintained by the natural draught of a chimney, except indeed the chimney be of very unusual temperature and height. The proportions proper for locomotive boilers will be seen by the dimensions of a few locomotives of approved construction, which have been found to give satisfactory results in practice, and which are recorded in the following Table:

NAME OF ENGINE

Great Britain.

Pallas.

Snake.

Sphinx.

Diameter of cylinder

18 in.

15 in.

14-1/4 in.

18 in.

Length of stroke

24 in.

20 in.

21 in.

24 in.

Diameter of driving wheel

8 ft.

6 ft.

6-1/2 ft.

5 ft.

Inside diameter of fire box

53 in.

55 in.

41-1/3 in.

44 in.

Inside width of fire box

63 in.

42 in.

43-1/4 in.

39-1/2 in.

Height of fire box above bars

63 in.

52 in.

48-1/3 in.

55-1/2 in.

Number of fire bars

29

.....

32

16

Thickness of fire bars

3/4 in.

1-3/4 in.

5/8 in.

1 in.

Number of Tubes

305

134

181

142

Outside diameter of tubes

2 in.

2 in.

1-7/8 in.

2-1/8 in.

Length of tubes

11 ft 3 in

10 ft 6 in

10 ft 3-1/2 in.

14 ft 3-1/4 in.

Space between tubes

1/2 in.

3/4 in.

1/2 in.

Inside diameter of ferules

1-9/16 in.

1-1/2 in.

1-5/16 in.

1-5/8 in.

Diameter of chimney

17 in.

15 in.

13 in.

15-1/2 in.

Diameter of blast orifice

5-1/2 in.

4-5/8 in.

4-1/2 in.

4-3/4 in.

Area of grate

21 sq. ft.

16.04 sq. ft.

12.4 sq. ft.

10.56 sq. ft

Area of air space of grate

11.4 sq. ft.

4.08 sq. ft.

5.54 sq. ft.

5 sq. ft.

Area of tubes

5.46 sq. ft.

2.40 sq. ft.

2.8 sq. ft.

2.92 sq. ft.

Area though ferules

4 sq. ft.

1.64 sq. ft.

2 sq. ft.

2.04 sq. ft.

Area of chimney

1.77 sq. ft.

1.23 sq. ft.

921 sq. ft.

1.31 sq. ft.

Area of blast orifice

23.76 sq.in.

16.8 sq.in.

14.18 sq. in.

17.7 sq. in.

Heating surface of tubes

1627 sq.ft.

668.7 sq. ft.

823 sq. ft.

864 sq. ft.

THE BLAST IN LOCOMOTIVES.

277. Q.--What is the amount of draught produced in locomotive boilers in comparison with that existing in other boilers?