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sectional diagram across the boiler; fig. 66 part longitudinal section, and fig. 68 part plan. The peculiarity consists in arranging a series of vertical conical tubes, aa, within the central flue, in a zig-zag form, connecting the upper and lower part of the boiler cc. The boiler is of the double-furnace kind, firing each furnace alternately.

The great efficiency of the locomotive boiler, in which the heated air passes through the inside of tubes which are surrounded externally with water, has directed the attention of engineers to the construction of boilers for stationary engines on this principle. Fig. 69 is a longitudinal section

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of Messrs. Gordon's multitubular boiler, and fig. 70 a transverse section. This form of boiler requires no brick-setting; but after being attached to the engine and to the chimney-flue, cán be set at work. They have double furnaces, which are fired alternately: d is the fire-door, ff the combustion

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chamber, ee the fire-bars, 9 the fire-bridge, aa the tubes, c the pipe from which to withdraw the sediment, b the man-hole door, mm the stays to strengthen the boiler, hh the standard for supporting the boiler.

The forms of boilers we have here given are such as are generally adopted for land or stationary engines; those for marine being arranged somewhat differently. It is impossible, under this division, to notice even a tithe of the arrangements introduced; we must content ourselves, therefore, with a few general notes.

The form of boiler originally introduced into steam-boats was merely

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fig. 71.

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à land-boiler, with large internal fues, like the Cornish boiler; the difference being, that in the latter the boiler was encased in brick-work, while in the former the external casing was of iron, leaving a space between, in which water was placed. The plan of the locomotive tubular boiler was next introduced, and with marked success. The next improvement was turning the tubes over the furnaces, as in fig. 71 : h the fire-door, b the fire-bars, ff the tubes passing through the boiler to the funnel-flue g. A form of boiler has been introduced, both for land and marine engines, which promises good results ; it is

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d known as the “vertical tubular," and is represented in the diagram, fig. 72, where a is the furnace door, b the lines representing the tubes; the heated air passes up through these, giving out the heat to the water with which they are surrounded. The tubes pass out at the top of the casing. As the water-level is below the top of the tubes, as at b, the steam is rendered thoroughly dry, or "anhydrous," as it is termed, and the inconveniences of priming are obviated.

The principle of boiler as patented by the Earl of Dundonald is considered good ; it has been adopted with marked success in

fig. 72. the line of Ocean American steamers of Collins. The diagram in fig. 73 shows the arrangement. The 6 boilers are provided with two rows of fire-places, and two tiers of tubes, one above the other, for the purpose of increasing the grate and boiler surface.” The improved form of this boiler, as secured by the last patent

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of the Earl of Dundonald, is given in fig. 74 : aa the fire-bars (there are four furnaces abreast), the heated air passes over the inverted bridge named the " economical heat trap;" along c and d, and down among the tubes, and finally up the chimney-flue e; by this arrangement, heated products of combustion are retained in contact with the most efficient part of the heating surface of the boiler, until the complete ab

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sorption of all the useful caloric is effected." The natural tendency of heated air is to ascend; and only to descend when deprived of its caloric or principle of levity. In the ordinary boiler, the heated air has a free and uninterrupted ascent to the funnel; but in the patent boiler it is conveyed through the flues to the summit of the tube-chamber, and there constrained to maintain that elevated position by its levity, until the water has effectually absorbed all the useful caloric held in combination, and the products of combustion, deprived of their superabundant principle of levity, descend to the interior aperture leading to the funnel, where they escape into the atmosphere at a low temperature. It will be noticed, that in this form of boiler the French principle of filling the tubes with water, making the flame pass outside, is carried out.

Having now illustrated the forms of boiler introduced both for land and marine engines, to the extent we deem necessary for the purposes of our work, we now proceed to illustrate the appliances of modern boilers.

In condensing, or low-pressure engines, the water is supplied to the boiler through a stand-pipe; this is of altitude to contain a column of water sufficient to counterbalance the pressure of steam in the boiler. A small cistern is placed on the top of this stand-pipe, which is supplied with water from the hot-well by the power of the engine ; a small valve is placed at the bottom of this cistern, which is opened by a lever; this lever is acted upon by the float in the inside of the boiler, the wire or rod connected with it passing through the top of the boiler, and fastened to the end of the lever: the arrangement has already been figured in Chapter II. In some instances, the wire or rod of the float passing through the boiler has stuck in its stuffing-box, and thus rendered the apparatus inoperative. A plan to obviate this has been recently introduced; this consists of a pipe, same height as the stand-pipe, containing a column of water; the rod of the float passing through this is moved without friction, and cannot stick. This apparatus of stand-pipe and float acts as a safety-valve in some measure, as before mentioned. The height of column of water in the standpipe is calculated so as to counterbalance the pressure of steam in the boiler ; but should the pressure of steam increase beyond a certain point, the water will be forced out of the pipe through the valve in the cistern, and thus give timely warning. The stand-pipe has another apparatus connected with it, by which the degree of combustion in the furnace is regulated. At the throat of the chimney a valve, consisting of a flat plate sliding in a frame, is placed; when this is forced fully into its frame the aperture of the chimney is closed, thus preventing the smoke, &c. from the furnace gaining access to the chimney: by this means the intensity of combustion is lessened, and the fire finally goes out. In proportion, therefore, to the degree of opening of the mouth of the funnel, so wisl be the intensity of combustion in the grate. The sliding-valve has connected with it a chain, which, passing over two pulleys, is finally passed down the interior of the stand-pipe, and connected with a float which moves up and down in it. As the pressure of the steam increases in the boiler, the water is forced up

the stand-pipe; this raises the float, and releasing the chain, the damper or valve at the mouth of the chimney is forced into its frame; the throat of the chimney is thus proportionally closed, the draught is diminished, the fuel in the furnace burns less fiercely, the pressure of steam is diminished, the water in the stand-pipe falls, and along with it the float; this tightens the chain, and the damper is pulled up.

In high-pressure engines, the chain of the damper passes over two pulleys, and is connected with a counter-balance weight, hung near the furnace door; the raising and depressing of the damper is done by hand.

In high-pressure boilers the water is supplied to the boiler by a forcepump, worked by the engine; the water being forced into the boiler against the pressure of the steam. A very ingenious apparatus for supplying highpressure

boilers with water, has been introduced by Mr. Turner of Ipswich. An elevation of this is given in fig. 75.

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This invention effects its object of rendering steam-boiler explosions arising from a deficiency of water next to impossible, by causing the boiler not only to regulate its own supply of water, but to sound an alarm almost instantly should the source of that supply fail; and to continue the alarm until its wants are supplied.

In fig. 75, A is a closed vessel, into which water is forced by the engine feed-pump through the pipe a; B is a portion of the boiler; v'is a ball.valve for controlling the entrance of water from a; c is a ram of an area

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