Fig. 150.

Scale one-quarter inch=1 foot. BOILERS OF H. M. STEAM VESSEL RETRIBUTION, OF 800 H. P., BY MAUDSLAY, SONS, AND FIELD, 1845.

Side View.

Fig. 151.

Scale one-quarter inch=1 foot. BOILERS OF H. M. STEAM VESSEL RETRIBUTION, OF 800 H. P., BY MAUDSLAY, SONS, AND FIELD, 1845. Longitudinal Section.

Figs. 146, 147, 148, 149, 150, 151, 152, and 153, represent the boilers of the Retribution, a steamer of 800 horse power; the engines being on the double cylinder plan of Messrs. Maudslay. There is nothing very peculiar in these boilers except their size; in other respects they very much resemble the boilers of the Great Western, and of the Thames and Medway, also by Messrs. Maudslay and Field, of which we have already given delineations. Figs. 154, 155, 156, 157, 158, 159, and 160, are different views of a sediment collector, as made by Mr. Armstrong of Manchester for marine boilers. The manner in which this contrivance acts has already been explained, and all that we have here to do is to give such forms of the machine as have been found the most convenient in practice. The sediment vessels are generally made of thin sheet iron, and an agitator with a handle extending through a stuffing-box to the outside of the boiler, is usually added to facilitate the blowing out of the silt and other foreign matters caught by the collectors. Of Lamb's scale preventer for marine boilers, an account will be found at page 179. In the manufacturing districts the main purpose for which collecting vessels is employed, is the prevention of priming.

Priming arises from insufficient steam room, an inadequate area of water level, or the use of dirty water in the boiler; the last of these instigations may be remedied by the use of collecting vessels, but the other defects are only to be corrected either by a suitable enlargement of the boiler, or by increasing the pressure and working more expansively. Closing the throttle-valves of an engine partially will generally diminish the amount of priming, and opening the safety-valve suddenly will generally set it astir. A steam vessel coming from salt into fresh water is much more liable to prime than if she had remained in salt water. or never ventured out of fresh. This is to be accounted for by the higher heat at which salt water boils, so that casting fresh water among it is in some measure like casting water among molten metal, and the priming is in this case the effect of the rapid production of steam. One of the best palliatives of priming appears to be the interposition of a perforated plate between the steam space and the water. The water appears to be broken up by dashing against a plate of this description, and the steam is liberated from its embrace. In cases in which an addition is made to a boiler or steam chest, it will be the best way not to cut out a large hole in the boiler shell for establishing a communication with the new chamber, but to bore a number of small holes for this purpose, so as to form a kind of sieve, through which a rush of water cannot ascend. In locomotives the same end is attained by the use of a perforated steam pipe extending from end to end of the boiler. Such a contrivance draws the steam off equally from the surface, instead of taking it from any one part; and boilers provided with it are enabled to work with so small a steam space that the steam domes are now being taken away from locomotives altogether. This expedient has not yet been adopted in steam vessels, though it appears to be applicable to them also with advantage. In some boilers priming appears to be mainly caused by a malformation which prevents the water from circulating freely, and the steam has therefore to pass up through the water, occasioning a great agitation, instead of the water being enabled to circulate with the ascending steam. The evil may be mitigated in such cases by the addition of pipes to the exterior of the boiler, which will permit a descending current to be established, to replace the water carried upward by the steam.

We cannot afford to surrender any more space to these specimens of boilers, and must now proceed to despatch what we have still to say on the subject of boilers in as few words as possible. We have already stated that a cubic foot of water raised into steam is reckoned equivalent to a horse power, and that to generate the steam with sufficient rapidity, an allowance of one square foot of fire bars, and one square yard of effective heating surface, are very commonly made in practice, at least in land engines. These proportions, however, greatly vary in different cases; and in some of the best marine engine boilers, where the area of fire-grate is restricted by the breadth of the vessel, and the impossibility of firing long furnaces effectually at sea, half a square foot of fire-grate per horse power is a very common proportion. Ten cubic feet of water in the boiler per horse power, and ten cubic feet of steam room per horse power, have been assigned as the average proportion of these elements; but the fact is, no general rule can be formed upon the subject, for the proportions which would be suitable for a waggon boiler would be inapplicable to a tubular boiler, whether marine or locomotive; and good examples will in such cases be found a safer guide than rules which must often give a false result. A capacity of three cubic feet per horse power is a common enough proportion of furnace-room, and it is a good plan to make the furnaces of a considerable width, as they can then be fired more effectually, and do not produce so much smoke as if they are made narrow. regards the question of draft, there is a great difference of opinion among engineers upon the subject, some preferring a very slow draft and others a rapid one. It is obvious that the question of draft is virtually that of the area of fire-grate, or of the quantity of fuel consumed upon a given area of grate surface, and the weight of fuel burned on a foot of fire-grate per hour varies in different cases in practice from 3 to 80 lbs. Upon the quickness of the draft again hinges the question of the proper thickness of the stratum of incandescent fuel upon the grate; for if the draft be very strong, and the fire at the same time be thin, a great deal of uncombined oxygen will escape up through the fire, and a needless refrigeration of the contents of the flues will be thereby occasioned; whereas, if the fire be thick, and the draft be sluggish, much of the useful effect of the coal will be lost by the formation of carbonic oxide. The length of the circuit made by the smoke

As

varies in almost every boiler, and the same may be said of the area of the flue in its cross section, through which the smoke has to pass. As an average, about one-fifth of the area of fire-grate for the area of the flue behind the bridge, diminished to half that amount for the area of the chimney, has been given as a good proportion, but the examples which we have given, and the average flue area of the boilers which we shall describe, may be taken as a safer guide than any such loose statements. When the flue is too long, or its sectional area is insufficient, the draft becomes insufficient to furnish the requisite quantity of steam; whereas if the flue be too short or too large in its area, a large quantity of the heat escapes up the chimney, and a deposition of soot in the flues also takes place. This last fault is one of material consequence in the case of tubular boilers consuming bituminous coal, though indeed the evil might be remedied by blocking some of the tubes up. The area of water-level we have already stated, p. 48., as being usually about 5 feet per horse power in land boilers. In many cases, however, it is much less; but it is always desirable to make the area of the water-level as large as possible, as when it is contracted not only is the water-level subject to sudden and dangerous fluctuations, but water is almost sure to be carried into the cylinder with the steam, in consequence of the violent agitation of the water, caused by the ascent of a large volume of steam through a small superficies. It would be an improvement in boilers, we think, to place over each furnace an inverted vessel immerged in the water, which might catch the steam in its ascent, and deliver it quietly by a pipe rising above the water-level. The water-level would thus be preserved from any inconvenient agitation, and the weight of water within the boiler would be diminished at the same time that the original depth of water over the furnaces was preserved. It would also be an improvement to make the sides of the furnaces of marine boilers sloping, instead of vertical, as is the common practice, for the steam could then ascend freely at the instant of its formation instead of being entangled among the rivets and landings of the plates, and superinducing an overheating of the plates by preventing a free access of the water to the metal.

In the Transactions of the Institution of Civil Engineers several papers are given by Mr. Parkes and others descriptive of experiments made by them on steam boilers. We have, in Table 1., collected a few of the principal