The flue boiler consists of a box-shaped boiler with flat or slightly curved sides, within which is the furnace, having at its posterior extremity a fire bridge, beyond which the flue passes on nearly to the end of the boiler, when it rises and returns along the upper part of the boiler, entering the uptake of the chimney near the front of the same. The steam is partially dried by resting in contact with the casing of the uptake.

The sides of this kind of boiler being flat, necessarily require to be strongly braced by numerous ties, pitched from 14 to 18 inches apart, according to circumstances. The pressure at which such boilers are worked seldom exceeds about 33 lbs. per square inch, and is more generally about 20 lbs. per square inch.

The tubular marine boiler is shown in longitudinal section on Plate XVI. It is similar to the flue boiler in general form; but the large flue is replaced by numerous small tubes, whereby a larger amount of heating surface is obtained. The air required for combustion enters the ash-pit, passes through the fire into the chamber at the posterior end of the furnace, whence it finds its way to the uptake through the numerous small tubes described above.

Locomotive boilers consist usually of two parts, presenting in longitudinal section the aspect shown Plate XXV. The one part contains the furnace or fire-box, which is surrounded on all sides save the bottom by water; ties are used to strengthen the flat sides of the fire-box, and the crown of the same is furnished with ribs.

In designing boilers of any description, care should be taken so to form the various parts that there may be no impediment to the escape of the steam to the upper part of the boiler as rapidly as it is generated; and for this purpose the joints should be arranged so that the recesses formed may not detain the bubbles of steam as they rise; also the sides of flues should not be made vertical, but inclined, so that the water spaces may be somewhat wider at the top than at the bottom.

The manufacture of boilers is very simple. Where it is required to rivet various plates together, they are usually first punched, then placed in juxtaposition and the holes trued by broaching or rhyming them out.

The rivets are made of bar-iron, being formed with one head:

these rivets, when required for use, are raised to a cherry-red or white heat, inserted into their places, and there retained by holding a hammer against the head while the straight end is first hammered up into the form of a head, and then finished off in a conical or hemispherical form by means of swages which are called snaps. The riveting may be done either by hand or by machinery.

The stays are sometimes secured by screwing the ends and fitting nuts upon them; sometimes by riveting and sometimes by screwing and riveting. Riveting may occasionally be employed for securing metal when cold.

The tubes of multitubular boilers may be fixed either by riveting the ends over the tube plates, or by driving in ferules to spread the ends, the apertures in the tube plates being slightly conical; and, lastly, the tubes may be screwed.

Whenever plates intended to be riveted can at all conveniently be drilled, this method of perforation should be adopted, as by punching the metal is strained and the apertures thus produced are not cylindrical; also it is desirable that the plates shall not, when partly riveted together, be forced to fit by driving drifts through the opposite holes, as thereby a strain is thrown upon the shell to which it should not be subjected.

Land boilers are usually set in masonry, and marine boilers in

cement.

We have omitted to mention hitherto the appendages which are common to all boilers. These are safety valves, loaded according to the pressure under which the boiler is intended to work; the valve may be acted upon directly by a weight or through the medium of levers, as shown in the illustration, Plate XIII.; or they may be kept down by springs acting through levers, and this is the method commonly used in locomotives.

Steam-pressure gauges are also requisite; they were formerly made of a syphon-formed tube containing mercury, the difference of the heights of the mercury in the two legs of the syphon indicating the pressure of the steam. The most portable and convenient steam-gauge now manufactured is that of M. Bourdon, which consists of a curved tube into which the steam has free access, and the steam by its pressure tends to straighten the tube, this tendency being opposed by the elasticity of the tube. By

means of suitable connections the motion of the tube is communicated to an index placed upon a dial, graduated to show pounds pressure per square inch. These gauges are also made to show vacuums. Gifford's injector is now frequently appended to steamboilers, to act in place of a feed-pump. In this apparatus a jet of steam passes from the boiler through a mouth-piece, and is partly condensed, when it forces its way through another mouthpiece into the boiler again, carrying with it a quantity of feedwater Its action may appear paradoxical, but is in reality very simple, being as follows.

Suppose the area of the orifice from the boiler to be one square inch, then the steam passing from this aperture with any given velocity, it may be partially condensed without losing this velocity, so that the same amount of energy will be concentrated upon a smaller area; hence, when so partially condensed, it can readily re-enter the boiler and carry other water with it. This apparatus will not act if the temperature of the water be much above 110° Fah.

In addition to the above appendages, man-holes, mud-holes, furnished with doors, and blow-through cocks, are requisite, to allow of the cleansing of the boilers; also guage-glasses to show the level of the water in the boilers.

CHAPTER XVI.

ON PROPELLERS.

THE three purposes which propellers are intended to fulfil are, the propulsion of ocean steamers, river steamers, and canal steamers, the latter consisting of tugs only; and the conditions to be satisfied are somewhat different for each class. Ocean steamers, besides requiring efficient power for the arduous work occasionally before them, demand that the machinery should be so placed as to be as safe as possible from enemies' shot; river steamers require to be compact; canal boats must be of light draught, compact, and must have their propelling apparatus of such form as may not cause injury to the banks. For these purposes only two propellers have hitherto been brought into general use, namely, the paddlewheel and the screw; and to these, and one form of the hydraulic propeller, we purpose now to devote a few brief remarks.

The paddle-wheel, being the longest established, first demands attention. It is manufactured in two forms: paddles with radial float-boards, and feathering paddles. In the first the float-boards are firmly fixed upon radial arms; and in the second they are formed so as to be movable upon an axis, their positions with regard to the horizon being regulated by means of rods, of which the outer extremities are attached, by pins, to arms upon the axes or gudgeons which carry the float-boards, their inner ends being similarly connected with the periphery of a ring fixed somewhat eccentrically to the paddle-shaft. The action of the floats of a paddle-wheel is as follows. Let us direct our attention to one floatboard, the engine being at rest and the vessel in still water. Then if the engine be started, a pressure will be exerted upon the water behind the float, which will pass through the water, a certain amount of motion being at the same time communicated to the vessel itself, the velocity attained being proportional to the pres

sure existing between the float-board and the water. Now, it is evident that while the float-board is at rest, no pressure is exerted⚫ upon the water; but when it begins to move, resistance becomes manifest, such resistance increasing as the square of its velocity; hence if there be any motion of the vessel, there must also be some yielding of the water in a direction opposite to that of the vessel, and if the yielding or part of the yielding of the water takes place in any other direction, there is a loss of power. With the common radial paddle-wheel the water yields in a variety of directions, corresponding with the positions of the various floats at any moment. And besides this, a portion of the water between the float-boards necessarily acquires some centrifugal force, which throws it out from the wheel radially, and thus some energy is wasted in useless work. With the feathering paddles the action is somewhat different from the above; but there exist to some extent the same disadvantages.

The screw-propeller has for some time enjoyed a reputation superior to that of the paddle-wheel, notwithstanding that its use is accompanied by serious disadvantages. In the first place the situation of the screw tends to remove from the stern of the vessel the back water, thereby leaving a deficiency of pressure at the stem of the vessel, which is equivalent to increased resistance at the bows; again, considerable centrifugal force is imparted through the water in contact with the screw, which is accordingly dispersed radially; a corresponding amount of energy being wasted, and at the same time the concussion of such water as passes upwards against the dead wood of the vessel produces vibration. The resistances which a screw has to overcome in a heavy sea are probably on the whole much more uniform than those which are opposed to the motion of paddles, whereby the alternate racing and stopping of the engines are much reduced, but nevertheless, this injurious action exists in a very considerable degree.

The hydraulic propeller has been brought forward at various times in a variety of forms, but hitherto it appears to have failed as a practical propeller. We may instance Ruthven's propeller, also the hirudine propeller. Ruthven's propeller consisted of a fan or rotatory pump, which expelled water through the extremities of channels towards the stern of the vessel. The motion of the vessel was due to the reaction of the issuing jets of water; the