of which we shall give all the best specimens. But a good number of boilers upon the common flue plan are still made, so that some specimens of them are indispensable; and, indeed, there are still far more flue boilers in use than there are of any other description. In most of these boilers it is a fault that the furnaces are made too long and narrow, and the consequence is, that it is impossible to fire them on a long sea-voyage, especially in stormy weather. It is much preferable to restrict the furnaces to a moderate length, and give the bars a considerable elevation, so that they always be well covered with coal at the after ends. When the furnaces are very long, a good deal of air generally escapes into the flues at the after end of the bars, the effect of which is materially to lessen the generation of

steam.

Scale three-tenth inch=2 feet.

BOILERS OF STEAMER EAGLE, BY MESSRS. CAIRD AND CO. Elevation and Horizontal Section.

BOILERS OF STEAMERS THAMES AND MEDWAY, BY MESSRS. MAUDSLAY AND FIELD. Horizontal Section through Furnaces.

BOILERS OF STEAMERS THAMES AND MEDWAY, BY MESSRS. MAUDSLAY AND FIELD. Longitudinal Section.

Figs. 71, 72, and 73 represent the boilers of the Thames and Medway, two vessels of large size, constructed by Messrs. Maudslay and Field for the Royal Mail Steam Packet Company. The boilers of these vessels have been very successful, and are among the best specimens of the flue boiler as applicable to marine engines hitherto produced. We do not know of any boiler of this kind that engineers may imitate with greater safety, as regards their power of generating steam, though there are many specimens distinguished by a greater durability. In some of the boilers recently introduced in the Royal Mail Steamers, the furnaces stand athwartships, and the plan is attended with the material advantage that the coals trim more easily; for the coal reserve in this arrangement situated behind the boilers, and another depôt standing between the boilers and the engines, communicate immediately with the stoke-holes, whereby an easy transfer of the coal becomes accomplishable.

Figs. 74. and 75. are also views of the boilers of the Thames and Medway. These views are perpendicular sections through the lines A B and CĎ, shown in the horizontal section through the furnaces, fig. 71., and the horizontal section through the flues, fig. 72. The dotted sweeps at the two upper corners represent the ascent of the flue into the funnel. The flue narrows in width and rises in height as it approaches the chimney, for the same area is not required for the transmission of the smoke after its volume has been contracted by the communication of heat to the water, and a less depth of water above the flue suffices after the heat of the smoke traversing it has been well nigh expended. The bridges B (fig. 74.) are water bridges, and their superior ridges do not run in a horizontal, but in an oblique direction, the design of which is to facilitate the extrication of the steam. There are four boilers in all, and the boilers are fired from both ends.

Fig. 76.

H

B

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Figs. 76, 77, 78, and 79., represent the boilers of the steamers Dee and Solway, constructed for the Royal Mail Steam Packet Company by Messrs. Scott, Sinclair, and Co., of Greenock. These boilers, it will be remarked, have a double tier of furnaces one above the other, instead of being fired from both ends as in the preceding example. The length of the aftermost firing space is saved to the vessel by this arrangement, but the plan has disadvantages enough to overweigh this benefit. Of these, the chief perhaps is, that it is very difficult to fire the two tiers of furnaces effectually. The coals, moreover, do not lead from the coal boxes so conveniently, or so well maintain the trim of the ship, as when the boilers are fired from both ends. It appears to us, moreover, that the steam rising from the lower tier of furnaces must be cooled and weakened by its impact against the ashpit bottoms of the upper tier, into which a stream of cold air is always passing to maintain the combustion of the fires. The ashpits, are necessarily inconveniently shallow, and, as will be remarked by referring to fig. 78., it becomes difficult with such shallow ashpits to obtain the necessary inclination for the furnace bars. These considerations indispose us to this description of boiler, and indeed it is fast falling into disuse.

M

Fig. 80.

There are three boilers in each vessel, with six furnaces in each boiler, making eighteen furnaces in each vessel. The boilers represented are properly those of the Solway, for those of the Dee, instead of being each rounded on the top in the manner shown in the drawing, have the top carried in one sweep from the one side of the boiler to the other, so that the tops of the several boilers form conjointly part of an ellipse. The total heating surface of these boilers is 4156 square feet; total fire surface 468504 square inches; weight of each boiler 27 tons; the diameter of the cylinders of these vessels is 73 inches; length of stroke 7 feet; nominal horse power 400; heating surface per horse power 10-3 square feet; fire surface per horse power 117 square inches.

In furnaces with two lengths of furnace bars, it is a good plan to make the centre bearer double, so that the ends of the bars may have a space between them through which the ashes will be precipitated; the space thus left enables the bars to expand without injury on the application of heat, whereas, without some such provision, the bars are very liable to get burned out by their centres bending up into the furnace, or else the lugs which carry the bearer bars will be perpetually being carried away. A similar space should be left between the fore end of the bars and the dead plate at the furnace mouth, and care should be taken that not only the ends of the bars do not touch, but that the heels of the bars do not rest against the furnace bearers.

The bridges of these boilers are, it will be observed, of brick, and come tolerably close to the furnace top. In such cases it is expedient to make the upper part of the bridge consist of one or two fire brick blocks which may be lifted off when a person requires to enter the flues to sweep or repair them. The continual knocking down and building up of bridges becomes otherwise very expensive. In boilers of this construction it is difficult to light one tier of fires after the others have been thoroughly kindled, as the fires first lighted keep the lead in the draught. It might be anticipated that by firing the tiers of fires alternately, the smoke would be burned, but such is not found to be the effect.

Scale one-eighth inch=1 foot.

ORIGINAL BOILERS OF GREAT WESTERN STEAMER, BY MESSRS. MAUDSLAY, SONS, AND FIELD.
Section through Furnaces F.
Section through Flues f.

Figs. 80, 81, 82, 83, 84, 85. represent the original boilers of the Great Western steamer as made by Messrs. Maudslay, Sons, and Field, and figs. 86. 88, 89, and 90. the present boilers of the same vessel, which are of the tubular description. These tubular boilers are rather short of steam, the fault being that the tubes extract the heat from the smoke too effectually, and leave but little waste heat to maintain the draught: but this evil may easily be remedied, either by putting a steam blast pipe into the chimney, as in the case of locomotives, or by cutting a hole through the water-space that separates the inferior and superior smoke boxes at the after end of the boiler, so that some of the smoke will ascend the chimney without passing through the tubes at all. These perforations might be fitted with dampers, so as just to allow enough hot air to escape through them to maintain the draught, and no more.

Fig. 85.

f

Fig. 90.

88888000

Provision has been made in these boilers for the introduction of a fan blast, which would at once cure the evil of a defective draft. The furnaces would, in this plan, be made close, and the blast would be introduced into a chamber at the back of the ashpit A., fig. 90., from whence it would pass into the ashpits by necks ▲ A A, fig. 86. One inconvenience, however, of a fan blast thus applied is, that the smoke comes out of the furnace doors very much when they are opened.

12 feet.

Scale nine sixty fourth inch=1 foot.

PRESENT BOILERS OF THE GREAT WESTERN STEAMER, BY GREAT WESTERN STEAM CO., 1844. Longitudinal Section.

The original boilers of the Great Western, as represented in figs. 80. to 85., contained of flue surface 2950 square feet, and of furnace surface 890 square feet, making 3840 square feet of heating surface; area of fire grate 202 square feet: capacity of steam room 1150 cubic feet; weight of boilers and steam pipes 202 tons; weight of water 80 tons; average consumption of coal 1000 tons per voyage out and home of 27 or 28 days. In the tubular boilers, figs. 86. to 90., the tube surface is 5900 square feet, smoke box surface 830 square feet; furnaces 420 square feet; making 7150 square feet of heating surface; area of fire grate 145 square feet; weight of boilers 56 tons; weight of water 52 tons; capacity of steam room 1320 cubic feet; average consumption of coal per voyage out and home of 29 days 696 tons. The speed of the vessel, it will be observed, has somewhat declined with the new boilers, but there is a greater economy in fuel upon the same distance. The horse power of the Great Western is about 400; the particulars of the two boilers will therefore stand as follows: