cession of iron arches over the furnace tops, to keep them in shape after the fashion practised in Stephenson's locomotives, and to stay the boiler top to these.

Figs. 120, 121. represent another boiler with upright tubes (having the water outside of them), the design of Messrs. Rennie. This boiler is intended to supply steam to an engine on land, and its exterior is therefore composed of a cylinder of brick-work, between which and the boiler shell the smoke is made to circulate on its way to the chimney. This brick cylinder is surmounted by a cast-iron dome, the edge of which fits into a groove containing sand, and access can readily be had to the ends of the tubes by raising up this dome by the tackle applied to the eye fixed in its centre. This kind of boiler is identical in all its important features with that repre

Fig. 120.

fire-grate is 20-96, and the proportion of fire to heating surface is 1 to 20 nearly. One of these boilers supplies the engine of the vessel with steam, the cylinder of which is 26 inches in diameter, and the length of stroke 38 inches, the number of strokes per minute, 35 light, 26 to 30 when towing. The consumption of fuel is 10 cwt. of coal per four hours, including getting up steam. The tubes are of iron, and show no symptoms of injury from the application of heat above the water level. In some of the boilers constructed on this plan the lower tube plate has cracked between the tubes, and we think the breadth of an inch of iron between the tubes is insufficient in most cases. It would be an improvement, we think, to

zigzag the tubes tubes thus:

This would leave more iron between the tubes, and, at the same time, would not prevent the introduction of an instrument to scrape them when necessary, at least diagonally. It would be very expedient, too, to defend the tube plate from the heat of the fire by a fire-brick dome perforated opposite to each tube. In that case, however, it would be necessary to give an additional tube surface, as more heat would then pass into the tubes than if such a dome were not employed.

One very obvious objection to this plan of boiler will present itself to our practical readers, which is, that a deposit either of silt or scale will be liable to take place on the top of the tube plate, which will, in consequence, very soon burn out. This evil is to a great extent met by our suggestion to interpose a fire-brick dome between the tube plate and the fire; but we have not been able to discover that any sensible inconvenience has arisen from such depositures in boilers of this construction, even without the expedient we have just described. The agitation of the water, caused by the ascent of the steam up through it, is a powerful antidote to any settlement where it is going on, and the particles of matter mingled with the water, and out of which scale is fabricated, find their way by degrees to the sides of the boiler, where the water is tranquil, and there they finally settle. It is expedient, however, in boilers of this kind, either to blow off very frequently, or to have brine pumps, which change the water in the boiler rather faster than usual. It would be a further improvement to place a collecting vessel within each boiler. Instruments of this kind have hitherto been very little used in steam vessels, but in land boilers they have long been employed with advantage, though in that case the advantage is of less account. We shall at a more advanced stage of our progress give some specimens of collecting vessels as applicable to marine boilers, but may here, for the sake of making our recommendation intelligible, explain the simplest form of collecting vessel as applied to waggon boilers.

Fig. 122.

LAND BOILER WITH UPRIGHT TUBES, BY MESSRS. RENNIE.
Transverse Section.

sented in figs. 110, 111, 112, and 113. That boiler, however, is intended for a steam vessel, and is, indeed, the boiler of one of the tug vessels working upon the Thames. As we happen to have a very high opinion of this variety of boiler, we must consider its merits in some detail, and therefore refer back our readers to figs. 110, 111, 112, and 113, as it is to that specimen our remarks chiefly apply.

The diameter of that boiler is 6 feet; height, 11 feet 6 inches; diameter of fire-box, 5 feet 2 inches; distance from top of bars to lower tube plate, 1 foot 6 inches; distance from top of bars to ashpit bottom, 1 foot 7 inches; length of tubes, 8 feet; diameter of tubes, 2 inches; distance from centre to centre of tubes, 3 inches; diameter of tube plate, 3 feet 10 inches. There is a clear space of 1 foot 2 inches all round, between the tubes and the boiler shell. The height to which the water rises along the tubes is 5 feet, leaving 3 feet of the tubes to traverse the steam space. There are 145 tubes in all, and, reckoning their effective length at 5 feet, the effective tube surface is 381-53 feet. The fire-box surface is 20.96 feet, making a total of 412-03 square feet of effective heating surface. The area of the

We must first, however, premise that the addition of particles of extraneous matter to boiling water facilitates the extrication of the steam, and, in many cases, lowers the temperature at which ebullition is carried on. The generation of steam is most active in those situations where those particles exist, so that, indeed, the steam appears as if it came out of them, and the particles are continually buoyed up by the steam in its ascent, and prevented from settling where the ebullition is active. If a vessel with a narrow mouth be immerged in a boiler under such circumstances as to preserve the water within it in a tranquil state, the vessel will speedily become filled with depositure resembling mortar, which, could it have settled, would have been indurated into scale. Of this cirN

CORNISH BOILER AT MR. THOMAS CUBITT'S FACTORY, BY MESSRS. RENNIE.

Longitudinal Section.

Fig. 124.

with a crucial passage between them, and the athwartship passage need only be of a width sufficient to enable the attendants to light and clean the fires. We cannot here enter into the details of the application, but may remark that in those cases in which it may be inconvenient to drop the coal upon the fire through a slit from the top of the boiler, it will probably be found the best way to combine the boilers, so that one revolving grate will suffice for two sets of tubes, and the coals may then be introduced Fig. 126.

000

CORNISH BOILER AT MR. THO HAS CUBITT'S FACTORY, BY MESSRS. RENNIE. Bird's Eye View.

tice. a a represents the boiler, which has been cut, and a part removed for the purpose of showing the interior. b is a box into which the depositure falls through the neck c, within which is another cylinder, or rather frustrum of a cone, which may be raised or lowered to suit the water level and other circumstances. These parts are all made of sheet iron. d is an agitator for stirring up the depositure within the box previously to blowing out, for which purpose a rod passes through a stuffing box on the boiler top furnished with a handle, which may be turned round. e represents the pipe through which the depositure is discharged, ƒ the blowoff cock, and g the bottom of the boiler. It would be easy in a boiler

Fig. 125.

Scale two tenths inch=1 foot. BOILER OF STEAMER ZEPHYR, BY MESSRS. HORTON AND SON, LIVERPOOL. Front View and Transverse Section

between the tube cylinders, and immediately over the fire. One material advantage of this arrangement is, that there will be no smoke-a virtue often claimed for the Cornish plan of boiler, but without any title thereto,

Fig. 127.

with upright tubes to convert one or more of the tubes into collecting mouths leading to a sediment vessel underneath.

The remarks which we have already made respecting this species of boiler are sufficient, we conceive, to show that the objections which might be brought against it are not of a very formidable character. Its recommendations, however, are not of a negative kind, and it possesses two of weight enough, we conceive, to entitle it to a preference over most kinds hitherto projected. The first is, that it yields very dry, or, indeed, surcharged steam, the use of which is attended with economy; and the second, that it facilitates the introduction of the revolving grate described in page 53, and which, we are confident, will come into general use for steam vessels. There is nothing in the revolving grate too complicated for such an application, and it would be a matter of great importance to have the

Scale two tenths inch=1 foot.

BOILER OF STEAMER ZEPHYR, BY MESSRS. HORTON AND SON, LIVERPOOL Longitudinal Section.

farther than what arises from slow combustion, and the use of Welsh coal, and both of these points may be attained in any boiler. This, we think, will appear clear enough by a reference to the representation of a Cornish boiler given in figs. 123, 124, and 125. This boiler produces no smoke, but there is no peculiarity of structure to bring about such a result, and the whole efficacy arises from a slow combustion and peculiar coal-advantages which are not exclusively Cornish. The diameter of this boiler is 6 feet 6 inches; length 33 feet 6 inches. Length of furnace 6 feet; breadth of furnace 3 feet 10 inches. A tube of about 19 inches in diameter, filled with water, runs through the main flue, which flue is a continuation of the furnace.

Figs. 126. and 127. represent a small tubular boiler constructed by Messrs. Horton and Son for a coasting steamer called the Zephyr. This boiler has been found to perform well, and is, in every respect, satisfactory. The tubes are of iron, 3 inches in diameter, and 6 feet long. Length of furnace 6 feet, number of tubes 168. 2 engines. Consumption of coal per hour, about 6 cwt. The pressure of steam is about 5 lbs. on the square inch. Figs. 128, 129, and 130. represent another boiler, by Messrs. Miller, Ravenhill, and Co., of the tubular kind. It is identical in all its main features with the boilers of the Tagus already described, and the same remarks apply to it. It has been found expedient to introduce a jet of steam into the chimney of this vessel to quicken the draught. The tubes are of brass, 31 inches in diameter, and the tube plates are of iron. There is a galvanic action between the brass and the iron in consequence of this arrangement, which shows itself, not at the ends of the tubes, but at the ends of the athwartship stays which bind the sides of the boiler together; and the iron plate around these stays very soon acquires the appearance of having been scooped out by a knife. It is probable that in all boilers the action will not take place at the same spot; but, in several cases of brass-tubed boilers which have come under our observation, the action has been very remarkable in the situation we have mentioned. The evil would probably be obviated by the application of a washer of zinc. It would be a great benefit indeed to coat the shells of boilers with zinc both inside and outside; and should the plan of boilers with upright tubes become general, so as to make cylindrical boiler shells an article of manufacture, it would be very easy to accomplish this end by the electrotype process. One advantage of such boiler shells is, that they might easily be made by the riveting machine: though, indeed, we do not despair of seeing malleable iron pipes drawn of a sufficient

Scale three tenths inch-1 foot.

BOILER OF H. M. STEAMER INFERNAL, BY MESSRS. MILLER, RavenHILL, AND CD.

Back View one-half in Section.

diameter for boiler shells; and we shall then have malleable iron cylinders for steam engines, if cylinder engines then be made at all, of which we are by no means certain.

Figs. 131, 132, and 133. represent the boilers of the steam vessel Ocean. The tubes of these boilers are of iron 31 inches in diameter and 9 feet long; furnaces 7 feet long and 2' 1" wide. There are three boilers in all: the centre one with three furnaces, and the two wing ones with two furnaces each. Total breadth of boilers 19 feet; total length 14 feet; total number of tubes 378; two engines-diameter of cylinder 56 inches; length of stroke 5 feet; pressure of steam about 4 lbs; consumption of coal about 18 cwt. per hour. In ordinary coasting vessels we do not approve of the plan of firing from each end, as the length in the vessel occupied by an additional firing space is a manifest waste of room; and there will be no difficulty in short voyages, about maintaining the trim of the ship on account of the stowage of so evanescent a cargo as coal in the wake of the furnaces. The object should, we conceive, be to obtain the requisite number of furnaces in the breadth of the ship, and then to add a sufficient number of tubes above the furnaces to extract the heat given out in them. In cases where this cannot be altogether, and yet can be nearly done it is a less evil, we conceive, to put a few furnaces in a second tier into the midship boiler, rather than change the whole plan of the boiler by firing from both ends. In most cases we believe it will be possible to get sufficient room for the furnaces in the breadth of the ship if the engines be wrought expansively; and this may be done, without affecting the power, either by applying high pressure steam to the existing size of cylinders, or by introducing larger cylinders, and retaining the steam at a moderate pressure- that is, under 10 lbs. The latter plan in the case of new vessels is, in our judgment, greatly the preferable one; and we should recommend all new steam vessels to be made with the cylinders very large, while the boilers remain of the same dimensions as at present, except in the case of vessels impelled by the screw, where the engines may be worked at a very high speed - an increase in the speed being equivalent to an enlargement of the cylinder. A great force is thus available for the propulsion of the vessel when necessary, whether from adverse circumstances of wind and water, or otherwise, merely by diminishing the degree of expansion; while, in fair winds and smooth water, it becomes necessary to work with a very high measure of expansion to keep the engines supplied