By dividing this quantity by the units of heat required to convert 1 lb. of water of 212° into steam of the same temperature (14,131÷966), we have 14.63 units of evaporation, or 14'63 lbs. evaporated from and at 212°.

Coke contains 86 carbon, but no hydrogen or oxygen, and yields (14.500 multiplied by 86)=12,470 units of heat.

Wood, when dry, contains 50 carbon, and the hydrogen and oxygen combine without yielding heat; and yields (14,500 multiplied by '50) = 7,250 units of heat per lb.

Peat contains about one-third more units of heat than wood. These are the maximum heating powers of the above combustibles, for which at least 10 per cent. must be deducted for imperfect combustion. In practice it is impossible to utilize all the available heat, and it is distributed as follows:

:

Heat lost by radiation-10 per cent.

Heat lost by ashes falling unburnt through the fire bars-10 per cent. Heat lost by gases escaping at a high temperature to the chimney-20 per cent.

Heat used in producing steam in internally fired boilers-60 per cent. In externally fired boilers the loss is 10 per cent. greater.

The average evaporative power, of different kinds of fuels, is as follows:

1 lb. good coal will evaporate 9 lbs. water which has been raised to 212°. lb. of petroleum-Ditto.

ditto.

ditto.

Liquid-Fuels comprise all classes of fluid hydrocarbons, such as, the mineral hydrocarbons of bitumen and asphalt; oils obtained by destructive distillation of coal, shale and schist; and animal and vegetable fats and oils. The heating-effect of liquid-fuel, when injected into the furnace in the form of spray, is about 3 times as great as that of coal.

The consumption of coal per indicated horse-power per hour, in firstclass triple-expansion surface-condensing engines, is about 1 lbs., in double-expansion condensing engines, from 1 to 2 lbs. ; in single cylinder condensing engines, 2 to 3 lbs. ; in locomotives, 2 lbs.; and in high pressure non-condensing simple engines, 3 to 4 lbs.

CYLINDRICAL STEAM-BOILERS.

Boiler-Shells.-The resistance of a boiler shell to internal pressure, varies inversely as the diameter. A shell 2 feet diameter, will bear double the internal pressure of one 4 feet diameter, the thickness being the same in

M

both cases. The resistance of the plates varies as their thickness

A shell of inch thickness, will bear double the pressure of one inch thickness, the diameter of the shell being the same in both cases. The thickness of the plates should be in proportion to the diameter of shell. A shell of 6 feet diameter, will require plates double the thickness of one 3 feet diameter, to resist the same pressure. The pressure of steam being equal in all directions, the pressure inside the shell of a boiler, acts uniformly all round its circumference, and tends to maintain its form perfectly circular, and also to restore any departure of its shape from a true circle. The shell cannot, however, be made perfectly circular, owing to the plates overlapping each other at the longitudinal seams, but the amount of deviation caused thereby, is so small that it need not be taken into consideration. The circumferential strain, being the greatest from the pressure inside the shell, the plates should be placed lengthways round the circumference, that is, the fibre of the iron should run round the circumference, because the plates are strongest in the direction in which they were rolled. The longitudinal seams should not be in line from end to end, but they should break joint, thereby considerably increasing the strength of the shell, and the longitudinal seams should be placed away from the centre line, along the top and bottom of the boiler. The transverse joints, requiring only half the strength of the longitudinal seams, only require to be single-riveted ; but the longitudinal seams should be double-riveted.

Longitudinal Strain on Boiler-Shells.-The strain inside a boilershell, tending to rupture it longitudinaily in lines parallel to its axis, is found by multiplying the diameter in inches by the length in inches, and then by the pressure of steam per square inch.

Transverse Strain on Boiler-Shells.-The strain inside a boilershell, tending to rupture it transversely in lines at right angles to its axis, is the amount of pressure against each end of the shell, and it is found by multiplying the area of the end of the shell in square inches, by the pressure per square inch.

Length of Boilers.-The strength of a boiler is not affected by its length as regards internal pressure, but the liability to strain increases with the length; short boilers do more work in proportion than long ones. The minimum length of Cornish and Lancashire boilers, for confined positions, should be 2 times the diameter, and the maximum, and best working length, should be 4 times the diameter.

Cornish and Lancashire Boilers are more used than any other form of boiler, and cannot be surpassed for accessibility, simplicity, durability and economy: they are steady and good steam producers, they will burn the commonest qualities of fuel, and with a good draught they will burn any kind of refuse fuel. They should always be made with Galloway tubes, which strengthen the flues, increase the heating surface and circulation, and tend to equalize the temperature throughout the boiler, and thus prevent unequal expansion and contraction.

Cornish Boilers.-Cornish or single flue-tube boilers, are made from to 5 feet in diameter. The flue-tube is generally made one-half the diameter of the shell, and is fixed so as to leave a depth of 6 inches, between the bottom of the flue-tube, and the bottom of the shell, which is ample space for the proper circulation of the water, and leaves sufficient depth of end plate, to allow it to yield to the expansion and contraction of the flue tube. When less depth of water-space than this is allowed, the bottom part of the end plate is liable to crack, for want of sufficient flexibility, to allow for its springing during unequal expansion, owing to the top portion of the flue-tube becoming much hotter, and expanding more than the bottom portion, which causes the end plates to be forced out at an angle; to provide for this unequal expansion, the end plates should be made as flexible as possible.

Lancashire Boilers.-Lancashire, or double flue-tube boilers, are generally made from 5 feet 6 inches to 7 feet 6 inches diameter; the space between the two furnace-tubes should not be less than 5 inches, and that between the furnace-tube and the side of the shell should not be less than 4 inches.

End Plates of Cornish and Lancashire Boilers.-The back end plate may be attached to the shell by an inside angle-hoop, but in order to increase the flexibility of the front end plate, it should be attached to the shell by an outside angle-hoop. The end plate should be made out of one piece of plate, and the openings for flues should be cut out in a lathe.

Gusset-Stays.—The end plates of Lancashire and Cornish boilers should be stayed to the shell by gusset-stays, of single plates and double angle-steel. The number of stays will depend upon the size of boiler; large boilers should have 5 at each end above the flue tubes : at the front end, and one at the back end below the flues; two of the gusset-stays should be secured to the second belt of plates of the shell, and the bottom of the gusset-stays should not go nearer to the flue than 10 inches from the bottom rivet in the stay to the rivets of the angle-hoop connecting the flue-tube to the shell, so as not to injure the flexibility of the end plate.

Longitudinal Stay-Bolts.-The end plate of Lancashire and Cornish boilers, should be stayed with two longitudinal stay-bolts, one on each side. of the centre gusset-stay, at a good height above the flue, so as not to injure the flexibility of the end plates. The screwed part of the stay-ends, should be larger in diameter, than the body of the stay, so that the diameter at the bottom of the thread, may not be less than the plain part of the stay. The stay should be secured to the end plates, by nuts and washers both inside and outside.

Internal Flue-Tube.-As the pressure acts all round the circumference of a flue-tube, in order to make the pressure uniform the flue should be a true circle; any deviation therefrom. seriously weakens it, and the external pressure tends to increase the amount of deviation from the true circle, and to collapse the flue. When the plates overlap each other in the longitudinal seams, the flue cannot be made perfectly circular, and the amount of devia

tion caused thereby, reduces its strength to resist external pressure, to the extent of 30 per cent.

Longitudinal Seams of Internal Flue-Tubes.-When the workmanship can be relied upon, the longitudinal seams should be welded, otherwise they should be made with butt joints double riveted, with the strip on the outside of the flue.

Diameter of Flue-Tube.-The resistance of internal flues to collapse, varies inversely as the diameter, a tube 12 inches diameter, being double the strength of one 24 inches diameter, and as wrought-iron will sustain double the force to tear it asunder, that it will to crush it, the diameter of the internal flue should never exceed one-half the diameter of the boiler.

Length of Flue-Tube.-The resistance of wrought-iron flues to collapse, varies inversely as the length, a tube 5 feet long being double the strength of one 10 feet long; but as flues are constructed with several belts of plates, the ring seams add considerable strength to the flues, and by strengthening the ring seams the length is practically reduced to the distance between each ring seam; the best mode of strengthening the ring seams is the Adamson flanged seam, or the Bowling expansion hoop.

Longitudinal Expansion of Flue-Tube.-The flue expands more longitudinally than the shell, and unless provision is made for this expansion, the tube in expanding will become arched, and likewise will cause the end plates to spring out. This can be prevented by making the ring seams of the flue with Adamson's flanged joint, shewn at Fig. 142, which will allow the flue to expand sufficiently, and the strain on the end plates will be reduced; by using these flanged joints, besides strengthening the flues, the edges of the plates, and the rivet heads, are placed out of reach of the fire.

Strengthening Flue-Tube over the Fire.-In order to assist the flue-tube to retain its shape, in case of over-heating, and also to increase its resistance against collapsing pressure, strengthening rings, 3 feet apart, are in some cases placed round the flue at the furnace-end of long or weak fluetubes. They are in the form of light angle-hoops, and are secured to the flue-tube by studs spaced not more than 6 inches from centre to centre. The water-space under the ring is generally 1 inches.

Man-hole. The man-hole of Cornish and Lancashire boilers should be guarded with a strong wrought-iron raised mouth-piece, welded into one piece, flanged at the bottom, and riveted to the boiler with a double row of rivets, the diameter inside should be 16 inches, the height 8 inches, the thickness of the body should be equal to double the thickness of the shell of the boiler, and the flanges should be one-fourth thicker than the body. Cast-iron should never be used for this purpose, because it elongates much less with the same stress than wrought-iron, and as they both must stretch together, the cast-iron will give way long before the breaking strain comes on the wrought-iron. When a raised mouth-piece is not used, a strengthen ing-ring equal in thickness to not less than 1 the thickness of the shell,

and in width to 12 times the thickness of the shell, should be riveted on with rivets, at centres equal to 4 times the diameter of the rivets.

Mud-holes.-The mud-hole at the front of the boiler, beneath the furnace-tubes of Lancashire boilers, should be guarded with a strong wrought-iron mouth-piece, and the small mud-holes of vertical and other boilers, should be guarded with a strong mouth-piece, raised sufficiently to form a flat face for the cover to bed against.

Boiler Fittings for Cornish and Lancashire Boilers.-Every boiler should have two safety-valves, and two water-gauges: the one acts as a check on the other. The water-gauges should be fixed, so that the lowest visible point of the glass, is 5 inches above the highest point of the internal flue; the average working height of water above flues is from 9 to 10 inches. Height of deadplate above floor 2 feet 8 inches. Inclination of boiler towards blow-off cock, inch in 10 feet. Inclination of fire-bars towards back of boiler, 1 inch in 12 inches. The height of the bridge at the back of the fire-grate, should be made such, as to leave a passage over it, equal to one-sixth of the area of fire-grate. The mouthpiece of the furnace should be made of two wrought-iron plates, with an air-space between, the door of which should have a sliding grid on the outside and a perforated box baffleplate on the inside, for admitting air above the fire. The size of the perforations should not exceed & inch in diameter, and the sum of their areas should not be less than 3 inches per square foot of fire-grate surface.

Boiler Setting.-Cornish and Lancashire boilers, should rest upon fire-brick blocks, set on side walls, the width of bearing surface for the boiler, on each side, should be of an inch, for each foot in diameter of the boiler, each side flue should be 6 inches wide, carried up to the level of the furnace crown, and down to the level of the bottom of the boiler, the width of the bottom flue under the boiler, should be equal to one-half the diameter of the boiler, and the depth of the flue should be about 2 feet. When thus set, the flame after leaving the furnace-tube, passes under the bottom of the boiler, and returns to the chimney along the side flues. The face of the brickwork, at the front of the boiler, should be set back 6 inches, so as to leave the angle-iron and its rivets open. Fire-clay, instead of lime, should be used throughout, in setting the boiler.

Staying Flat Surfaces.—In a flat surface, such as the side of a locomotive firebox, each stay sustains the pressure on the square area of plate which surrounds it, whose side is equal to the distance between the centres of the stays; and the strain on the flat surface between the stays, is found, by multiplying the area in square inches between the adjacent stay-rods by the pressure.

The diameter of staybolts, for flat surfaces, should not be less than twice the thickness of the plate, and should never exceed three times the thickness of plate.

The working steam pressure of staybolts, per square inch of