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pressure. The balls ƒ balance the valve and levers when out of action. An index is fitted on the wheel a, which shows the height of the water in the boiler. This apparatus is very elegant in its general appearance, and has the merit of being self-acting.

In addition to these and other kindred contrivances, to notice all introduced for the last thirty years would take a goodly sized volume. The actual level of the water in the boiler is made visible by the contrivance known as the "water-guage;" this will be found described in the chapter on Locomotives. We pass on, therefore, to notice other appliances of the boiler. The pressure of the steam on the water in the boiler has been taken advantage of by Mr. Wilson, of Low Moor Iron Works, in his “safety apparatus for boilers," shown in fig. 79, which is part of a longitudinal section of a boiler with internal furnaces: a is a vertical pipe, like an ordinary feed water-pipe for low-pressure boilers. It is open at its lower

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fig. 79.

end b, which is set about one inch above the top of the furnace-plates of the flue C, and passes up through the top of the boiler to a height sufficient to obtain a column of water rather more than equal to the greatest steam pressure at which the boiler is intended to be worked. Near the upper-end of this pipe, a branch d is formed upon it, forming a junction with the second vertical pipe e, which passes downwards outside the boiler-end; and its extremity is bent to terminate in the front of the furnace at f; or if two or more furnaces are fitted in each boiler, then a communication of the same kind must be formed with each of them. The height of the dischargebranch d is such, that at the ordinary working steam-pressure of the boiler, the water forced up the pump by this pressure shall stand at some point beneath the branch d. Should, however, the steam-pressure be increased by any means beyond the due working pressure, the water will rise higher up the pipe a, until, if the pressure increase, it reaches the branch d; when it will pass through the latter, descend the pipe e, and pour into the fur

nace.

This discharge of water will at once damp the fire, and therefore reduce

the steam pressure; whilst, at the same time, it will draw the attention of the engine-attendant to the fact, that all is not right with the boiler. The open end b of the pipe a in the boiler is not exposed directly to the ebullition of the water, which would tend to derange the action of the apparatus; but is protected from this influence by insertion into a short length of wider pipe g, open at top and bottom, the water standing at the same level within it as in the boiler. In this way the water contained in the guard-pipe g is kept in a comparatively undisturbed state, the stilling effect of the pipe being further assisted by its expanded or funnel-shaped upper-end. In attaching this apparatus to a boiler, the branch d must be set at such a height as will allow of the obtainment of a column of water in the pipe a, of a head pressure a little more than the pressure to which the safety-valve h is loaded, so that the latter may have a short range to permit it to work freely when in good order, without the protector or water-discharging apparatus being brought into play. Thus, when the slight difference between the head-pressure in the water-pipe a, and that to which the safety-valve is loaded, is exceeded, owing to the sticking of the latter, or from any other cause, then the protector acts at once upon the fire. When the water-level in the boiler falls below the open end b of the water-pipe, which would endanger the overheating of the boiler-fiues, a portion of the water contained in the pipe at the time will drop out of the end b into the boiler, whilst the remainder will be forced up the pipe by the steam pressure. As the upperend of the pipe is closed, with only a small aperture to prevent its acting as a syphon when water passes down its branch d, the steam and water blown out will pass down the descending pipe e, and damp or extinguish the fire. This discharge of steam being so contrived that it shall take place before the parts of the boiler liable to overheating are quite bare, the damping of the fire will always remove all danger of explosion or injury to the boiler; which may be again set to work, after the supply to it of a proper quantity of water, which may be supplied at once without fear of any injurious effects. The advantages of this apparatus are, that it renders the dangerous accumulation of steam impossible, being entirely free from valves or connections, which are liable to derangement; whilst it can never fail to call the attention of the attendant to any objectionable increase of pressure, and it will remove the cause of the over-pressure unassisted.”

Where the steam in boilers

reaches a degree of pressure likely to be dangerous to its safety, what are termed "safety valves" are used. Mr. Fairbairn calls them the only "legitimate outlets" in circumstances such as the above. Two should always be provided to every boiler, of sufficient capacity to carry off the quantity of steam generated by the boiler. To prevent additions being made to the weight, Mr. Fairbairn recommends a 66 lock-up valve," as represented in fig. 80. a is the valve; b is a shell of thin brass,

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opening on a hinge, and secured by a padlock; it is of such a diameter as to allow the waste steam to escape in the direction of the arrows. c is the weight, which may be fixed at any part of the lever to give the desired amount of pressure, but which cannot be fixed or altered unless the boiler is opened to allow a man to get inside. d is a handle, having a long slot, by which the valve may be relieved or tried at any time, to obviate the liability of its corroding or being jammed; but the engineer cannot put any additional weight upon the valve by this handle. The sticking of the valve in its seat is always a fruitful source of danger.

As a valuable aid towards obviating such disasters, we here illustrate and describe, in fig. 81, the very neat valve registered by Mr. James Nasmyth, of the Bridgewater Works, at Patricroft, which has met with very general and high approval, as uniting in a most simple combination all the qualities which can tend to the formation of a true and perfect safety-valve.

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The total absence of all spindles or other guiding agencies, which have hitherto proved so fertile a source of mischief and uncertainty in the action

and permanent perfect condition of this vital part of a boiler apparatus, will be seen at a glance to characterise the design of this absolute safety-valve.

The chief feature of novelty, however, which distinguishes this improved safety-valve from all others hitherto proposed, consists in the peculiar and simple manner in which the motion of the water in the boiler is employed, as the agent by which the valve is prevented from ever getting set fast in its seat. The swaying to-and-fro sort of motion which at all times accompanies the ebullition of water in boilers, is made to act upon a sheet-iron appendage to the weight directly attached to the valve; and as the rod which connects this sheet-iron appendage and weight to the valve is inflexible, it will be easily seen how any slight pendulous motion given to the sheet-iron appendage is directly transferred to the valve; and as that portion of the valve which rests in the seat is spherical, the valve not only admits of, but receives, a continual slight motion in its seat in all directions, as the result of the universal pendulous motion of the appended weight, as acted upon by the incessant swaying motion of the water during ebullition. It will be seen that, as the spherical portions of the valve and seat are of equal width, the edges of their respective surfaces pass and repass each other continually, and so maintain and continually tend to improve the perfect spherical fit and agreement between the valve and its seat.

It may be proper to observe, that when the steam is nearly up to the desired pressure, the valve rests on its seat with a pressure next to no pressure at all, and is then, as it were, floating on steam. This action is common to all good valves; but the observation may tend to show how a slight movement of the water affects the valve in its seat. A pipe may lead the steam escaping from this valve to the manager's-office, and there give audible notice of its escape by acting in a steam-whistle there placed.

Still further to obviate the risk of boilers exploding from a dangerous pressure, "fusible plates" are introduced sometimes on the top of the boiler. These are made of a composition calculated to melt at a comparatively low temperature. Fusible plugs are also introduced into apertures on the boiler below water-line; these melting when the water gets too low, allow the steam to pass off. Little reliance, however, is to be placed

on these contrivances.

All boilers are supplied with gauge-cocks, the nature of which has been already described; by these the level of the water in the boiler is ascertained.

Mercurial pressure-gauges are frequently used to indicate the exact degree of pressure in the boiler. A glass tube is placed in a small cistern filled with mercury; one end is closed, the other opens to the interior of boiler in the steam space: the steam passing down the tube presses on the surface of the mercury; by this means the air in the upper part of the tube is compressed, and in proportion to the compression so is the pressure in the boiler. In improved gauges, a scale is attached which shows the temperature of the steam as well as its pressure in the boiler. In fig. 82 an improved pressure-gauge is shown: aa is the pipe introducing steam from the boiler; b the pipe for leading off the water of condensation; cc the snugs for fixing the gauge in any convenient position. In mercurial gauges adapted for low-pressure boilers, the mercury is placed in a syphon tube, one end of which opens into the boiler, and the other is open to the atmosphere; the pressure of the steam raises the mercury in the short leg of the

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syphon, and acts on a small plug floating on it; to this is attached a pointer, which shows on an index the degree of pressure. When the steam gets of very high pressure, the mercury is blown out of the tube; in this way acting as a safety-valve.

The consumption of smoke arising from the fuel used to raise the steam in the furnaces of boilers, has long been a favourite project among inventors; and the adoption of a good plan to effect this desideratum, always of importance, has more recently attracted earnest attention, from the fact of many Corporations making it imperative on those who employ steam-engines to consume their own smoke. We cannot pretend to notice all the plans introduced for this purpose, but must content ourselves with noticing only one; that, however, the most recently invented, and which has already taken a high place in the rank of effective plans. This, the invention of Mr. John Lee Stevens of London, is shown in the diagram in fig. 83. "The invention," says Mr. Lee, in his prospectus, from which our diagrams are taken, "consists in the combination of two sets of fixed fire-bars, the first of which is fed by the scoria and cinders voided from the second or upper set of fire-bars, with a calorific plate, as shown in the diagram; by which arrangement, the current of air entering at the lower part of the furnace passes through two strata of fire, and thence between the calorific plate and the bridge; and is thus so intensely heated, as continuously to produce the entire combustion of the gaseous products of the fuel, without the formation of smoke." In fig. 83, b the first and a the second set of bars; c the calorific plate, faced with fire-bricks; d the bridge, e the furnace-flue, h the furnacedoor; i shows the direction of the current of air: this diagram shows the application of the principle to the Cornish boiler for land-engines. In fig. 71 the application is shown to marine boilers; the same letters of reference apply to this diagram as to fig. 83.

fig. 82.

Still further to make the steam-engine automaton or self-acting in its arrangements, mechanism has been introduced by which the labour of the fireman or stoker is suspended, and the furnace supplied with fuel in proportion as required. Numerous contrivances have been introduced for this purpose; we shall only notice one, and that patented by Mr. Dean of Stockport, and in which neighbourhood it has been introduced with success. "It consists, first, of a double self-acting feed-apparatus, one side of which is caused to supply the furnace with fuel, whilst the other is at rest, and vice versa, alternate; and, secondly, in placing a partition-wall in the fur

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