STEAM ENGINE features, be so familiar to everyone as to require but short explanation. In the case of paddle vessels, a shaft is carried across the vessel, being continued on either side beyond the sides; to the extremities of this shaft, on the outside of the vessel, are attached a pair of wheels, constructed like under-shot water wheels, having a number of flat boards called paddle boards fixed upon their rims. As the wheels revolve, these paddle boards strike the water, driving it in a direction contrary to that in which it is intended that the vessel shall be propelled. The moving force imparted to the water thus driven backwards, by reaction on the vessel propels it. On the paddle shaft are fixed two cranks, placed at right angles one to the other, so that whenever one of them is thrown into the highest or lowest position the other is horizontal. These cranks are worked by strong iron rods called connecting rods, which are themselves either driven directly by the pistons of two steam engines, or are worked by beams moved by those pistons. Thus the medium of working becomes similar to that used in the ordinary land engines. The two cranks being placed at right angles, it follows that when one piston is at top or bottom of its stroke, and the crank is driven by it into the highest or lowest position, the other will be at the middle of its stroke, and the crank driven by it will be in the horizontal position. One of the pistons is therefore always in a position to produce the most advantageous effect on the crank at the moment that the other piston loses all power over the crank driven by it; and in the same manner it may be seen that while the power of one piston is augmented from zero to its greatest effect, the power of the other is decreasing from its greatest effect to zero. Thus the combined action of the two pistons is nearly uniform in its efficiency. If one engine only were used the motion of the wheels would be unequal, being most rapid when the piston is at the middle of the stroke, and slowest at the extremities. Nevertheless, single engines are sometimes used in paddle vessels with good effect. In the case of screw vessels, the action of the engines is similar, but the propelling shaft runs fore and aft above the keel, and protrudes at the stern, where a great screw propeller is hung upon it. [SCREW PROPELLER.] therefore, subject to the vicissitudes of tempestuous weather, the machinery must be protected by being placed below the deck. The space allotted to it being thus limited, great compactness is necessary. In paddle vessels, the paddle shaft being very little below the deck, the working beam and connecting rod could not be placed so conveniently above it. If a beam engine, therefore, be employed, two beams are so placed that one comes on each side of the cylinder, and is driven by the piston by means of a cross head attached to the piston rod, from the ends of which rods, called side rods, are carried downwards to the ends of the beams. From the other ends of the beams the connecting rod is presented upwards towards the crank. In screw vessels the engines are mostly made with the cylinders horizontal, or if vertical they are usually inverted, and work down to cranks on the screw shaft. [SCREW PROPELLER.] Side-Lever Engines.-One form of marine engine formerly in the most extensive use, and still employed in many paddle vessels, is the side-lever engine. But the oscillating engine has nearly superseded it. The general arrangement of the parts of a side-lever engine will be easily understood by reference to fig. 2, in which is represented in section a side-lever marine engine with a flue boiler, as placed in a steam vessel. The sleepers supporting the engine are represented at X, and the base of the engine is secured to these by bolts passing through them. S is the steam pipe leading from the steam chest in the boiler to the slides c, by which it is admitted to the top and bottom of the cylinder. The condenser is represented at B, and the air pump at E. The hot well is seen at F, from which the feed is taken from the boiler. L is the piston rod connected with the beam H, working on a centre K, near the base of the engine. The other end of the beam H drives the connecting rod M, which extends upwards to the crank, which it works upon the paddle shaft O; a a are the rods of the parallel motion. The framing by which the engine is supported is represented at Q R. The beam exhibited in the figure is shown in dotted lines, as being on the farther side of the engine. A similar beam, similarly placed, and moving on the same centre, must be understood to be at this side, connected with The steam engines used for navigation may the cross head of the piston in like manner by be either condensing engines or non-condensing a parallel motion, and with a cross tail attached engines. If the latter are employed, steam to the lower end of the connecting rod and to must be used having a pressure above the the opposite beam. The eccentric which works atmosphere of from 50 to 80 lbs. per square the slides is placed upon the paddle shaft 0; inch. Boilers in which steam is produced under and the connecting arm which drives the slides this pressure are considered in Europe so un- may be easily detached when the engine requires safe, that non-condensing engines with low- to be stopped. The section of the boiler grate pressure boilers are almost universally used and flues is represented at W U. The safety for navigation. In America, however, high-valve y is enclosed beneath a pipe carried up pressure boilers with non-condensing engines are extensively used on some of the rivers. The arrangement of the parts of marine engines is different in several respets from land engines. Steam vessels being generally employed to navigate the open seas, and being, beside the chimney, and is inaccessible to the engine-man. The cocks for blowing out the salt water from the boiler (a process explained under the head STEAM BOILER) are represented at h, and the feed pipe at I. The nature and operation of the several parts STEAM ENGINE just mentioned will be understood by reference to the explanation of the structure and operation of the double-acting land engine, for, in fact, the marine engine, as here represented, is nothing more than a double-acting condensing steam engine, adapted in its form to the circumstances in which it is used in navigation. Since, however, the double-acting land engine, which has been described, is not provided with a slide valve worked by an eccentric, it will be proper to explain the form and mode of action of those important parts of the machine. An eccentric is a disc of metal perforated with a hole for the reception of the crank shaft; but this hole is not in the middle of the disc. The edge of the disc is encircled by a ring of brass, from which a rod proceeds to the connections of the slide valve, and it will be obvious that by the revolution of the shaft this rod will be drawn backwards and forwards, in Fig. 2. the same manner as if it were connected with | for moving astern, and by moving the link endways the valve is made to partake of the motion of that eccentric which is contiguous to it. By this apparatus the engine may be started, stopped, or reversed merely by shifting the position of the link with reference to the valve rod. The species of valve usually employed in side-lever marine engines is the D valve, so called because it is half a cylinder, and its cross section therefore resembles the letter D. Of this species of valve there are two varieties, the long D and the short D. In the long D the waste steam escapes from one of Fig. 4. the ports of the cylinder through the centre of the valve. In the short D there is no central passage in the valve, but the valve consists of two short and closed portions connected together by one or more rods. The long D valve has been most generally adopted. A valve of this kind is represented The flat projecting in fig. 4. part of the valve moves upon similar flat parts on the front of the cylinder, and when the valvebile is in the middle of its stroke the flat projecting portions accurately close the ports or passages by which the steam enters and leaves the cylinder. The round part of the valve opposite to each cylinder port is made tight by the application of a hemp packing, placed in the STEAM casing by which the valve is surrounded; and between the upper and lower packing steam is admitted to circulate round the valve-the steam pipe from the boiler having its debouche in the valve casing. If now the valve be in its middle position, so as to cover accurately both of the cylinder ports, it will be clear that if it be drawn upwards steam will be allowed to enter from the valve casing above the piston; and at the same time any steam or vapour will be allowed to escape from beneath the piston into the central part of the valve, which communicates with the condenser by means of a pipe penetrating the valve casing, either above the upper packing or beneath the lower packing. If the valve be forced below its middle position, the contrary operation will be produced. By moving the valve up and down, therefore, which is done by the eccentric, the piston is pressed up and down in the cylinder in the manner required. ENGINE vessels, and also exhibit a needless complication of parts. The strain has to be transmitted not merely through the piston rod and crank shaft, but also through the cross head and cross tail, side rods and connecting rod, and the side levers or beams. This multiplication of the moving parts obviously increases the risk of fracture, and the side levers themselves are peculiarly susceptible of accident from this cause, since by the properties of a lever of this class, the stress or strain upon the beam at the main centre or pivot is twice as great as the strain upon the piston. These defects of the side lever engine have caused a new species of engine to be introduced, called the oscillating engine, because the cylinder vibrates or oscillates somewhat in the manner of a pendulum. In this engine the top of the piston rod is coupled immediately with the crank pin, and as the piston rod moves up and down in a line coincident with the axis of the cylinder, while the crank pin revolves in a circle, it is necessary that the cylinder should be able to vibrate Fig. 5. Oscillating Engine.-Side-lever engines occupy an inconvenient amount of space in steam laterally, to enable the motions of the piston rod and crank pin to be reconciled with one another. The cylinder is consequently provided on each side with a short hollow pivot, or trunnion, on which it swings: and through one of these trunnions the steam enters the cylinder from the boiler, while through the other the steam escapes from the cylinder to the condenser. The alternate introduction of the steam above and below the piston is governed by a slide valve attached to the cylinder, and swinging with it; or, in large engines, two valves may be employed for this purpose, and by their suitable attachment to the cylinder they will balance one another. In steam vessels in which oscillating engines are employed, the cylinders are set immediately beneath the cranks, and the engines occupy but little more in the length of the vessel than the diameter of the cylinder. In the shaft which connects the engines together, and which is called the intermediate shaft, a crank is forged, and this crank in its revolution gives motion to the pistons of one or more air pumps. The general nature of the arrangements at present employed in vessels of the most improved class, will be readily understood by a reference to fig. 5, which is a transverse section of the steam yacht Peterhoff, constructed for the emperor of Russia by Messrs. Rennie; and to fig. 6, which is a side view of the engines of the same vessel. A A are the cylinders; BB are the piston rods which are connected immediately with the cranks C C; D is a crank in the intermediate shaft, for working the piston bucket of the air pump E; F F are the slide valves STEAM ENGINE by which the admission of the steam to the | C is opposite. If the link be raised only enough cylinders is regulated; G G are double eccen- to bring the block C to the middle of the link, tries on the intermediate shaft, whereby the valves F F are moved. The purpose of the double eccentrics is to enable the link motion to be employed. H is a handle by which the engines may be stopped, started, or reversed. II are the steam pipes leading to the steam trunnions K K, on which, and on other trunnions connected with the pipe M, the cylinders oscillate. N N are pumps, the pistons of which are attached to the trunnions, and are worked by the oscillation of the cylinders. O is the Fig. 6. C waste-water pipe, through which the water which has accomplished the function of condensing the steam is ejected overboard. The same letters refer to the same parts in the two figures, and the second figure shows the arrangement necessary for feathering the floats of the paddle-wheels. Link Motion. The apparatus known as the link motion is represented in fig. 7, where A is the engine shaft on which two eccentrics are fixed; B is a central pin by which the link D, which connects the ends of the two eccentric rods, is suspended; and C is a brass or steel block which may be moved freely from end to end of the slot in the link D. The centre of this block carries a pin which connects with Fig. 7. Link Motion. the valve rod of the engine, and by raising or depressing the link, which is done by means of a suitable rod attached at the point B, the end either of the upper or lower eccentric rod is brought opposite to C, and imparts to the valve the motion of the eccentric to which the block there will be no motion imparted to the valve, and the engine will therefore stop; whereas if C be brought to one end of the link the engine will move ahead, and if brought to the other end the engine will move astern. By working the engine with the block C at a short distance from the end of the link, the engine, if constructed with a little overlap on the valve, will be worked more expansively; for as the extent to which a given amount of lap on the valve influences the cutting off of the steam varies with the throw of the valve, to reduce the throw with a given lap is tantamount to an increase of the lap with the same throw. Gridiron and Equilibrium Valves.—The increasing size of steam engines, especially as applied to steam navigation, and the increased difficulty of moving valves of great size by hand to start the engines, together with the need of rapid motion, have pointed to the necessity of some modification of the older forms of slide valve by which these qualities would be realised. The D valve has consequently been so modified in its construction as to take the pressure of the steam off the back of it. This is accomplished by introducing a packing opposite to the upper part of the cylinder port, and another opposite to the lower part. The pressure on the back of the valve is thus made the same as the pressure on the front, and the resistance due to the friction of the packing is the only force that has to be overcome. The form of valve employed in locomotives, and hitherto much used in oscillating and other engines, is the threeported valve, in which the upper and lower ports or passages leading to the cylinder, and a central port-communicating with the condenser in condensing engines, and with the atmosphere in high-pressure engines-are covered by a box with its open side against the cylinder, on which it is capable of moving steam-tight. But the length of the box is not sufficient to cover the whole three ports, but only one of the end ports and the central one. The box is set in a casing, and is surrounded by steam leading from the boiler; and the effect of moving up and down the box within the casing is to place each end of the cylinder alternately in connection with the boiler and with the atmosphere or condenser. This valve answers admirably for small engines, but the travel of the valve, which should be equal to the depth of the port, is too great to be convenient in the case of large engines moving at a high speed; and, further, the pressure of the steam against the bottom of the box is so great when acting over a large surface as to create an amount of resistance from friction that is not easily overcome. To remedy these defects, the gridiron and equilibrium forms of valve have been introduced. In the gridiron valve each port is divided into a number of narrow ports, by which the travel of the valve may be correspondingly abridged. In the equilibrium valve a ring is applied to the back of the valve, which rubs steam-tight against the STEAM ENGINE back of the valve casing. The interior of this ring communicates with the atmosphere or the condenser accordingly as the engine is a highpressure or a condensing one, and by this expedient an equilibrium of pressure is established between the back and front of the valve, which discharges the friction and enables the valve to be moved with facility. In engines of large size, both of these expedients are generally introduced, as will be seen by a reference to fig. 8, which is a longitudinal section of part of one of the cylinders and of Fig. 8. one of the valves of the screw engines of the steamer Great Eastern. Here A A are the cylinder ports, each of which is split into two narrow ports before reaching the valve face; B is the central port or passage leading to the condenser; C C C C are portions of the valve casing, which is filled with steam, and D is the valve which, as represented, is opening the upper ports to the steam, while the lower ports are open to the condenser; E is a ring fixed on the back of the valve, and moving with it, which rubs on the door which constitutes the back of the valve-casing. A Equilibrium Valve. small hole is bored through the back of the valve, so as to enable the space within the ring to communicate with the exhaust passage B, by which expedient the pressure of the steam is taken off the back, and the valve is rendered easily movable. Gridiron Starting Cylinders and Valves.-In some large marine engines, small cylinders have been introduced to move the link motion by which the movements of the valves of the main engines are controlled. In other cases, starting valves have been introduced. Both expedients were first applied by Mr. Bourne, the first in 1836, and the second in 1852. Feathering Paddle-wheels.-In one variety of paddle-wheel, the float-boards which act upon the water are fixed in a radial position, like the buckets of an under-shot water-wheel, as has already been explained; and this species of wheel is largely employed both in the case of river and sea going vessels. Latterly, however, paddle-wheels with movable floats, of the kind represented in the preceding engravings, have obtained a marked preference, and vessels fitted with such wheels are found to realise a considerably increased speed. It will be remarked, that each float is hung upon an axis, whereby the inclination at which it enters or leaves the water may be altered; while by means of levers which are attached to the floats, and acted upon by rods converging to a centre eccentric to the shaft, a feathering motion is imparted to the floats by the revolution of the wheel. In river vessels the use of feathering wheels enables a small diameter of wheel to be employed without loss from angular impact; and with wheels of a small diameter, the engines may be worked with a greater speed, and will thus exert more power. In the case of seagoing vessels the diameter of the paddle-wheels cannot be materially reduced, whatever species of wheel is employed; for if the wheels were made very small, they would be immerged to the centre, or be out of the water altogether, if the sea should become boisterous: nevertheless, feathering wheels are advantageous in the case of ocean vessels also, as they act more beneficially than common radial wheels when deeply immersed, and with variable immersions, therefore, they maintain a greater average efficiency. There are objections, however, incident to the use of feathering wheels, which go far to balance these advantages. They are expensive both to make and to maintain. The wear and friction in such a multitude of joints is very considerable; and if any of the arms get adrift, or break, they will be whirled round like a flail, and may perhaps cut through the paddle box or even through the vessel. If the injury be of such a nature that the wheels cannot be turned round (and this has sometimes happened), it will follow that the engines will be virtually disabled until the obstruction can be cleared away; and if the weather be very stormy, or the vessel be in a critical situation, she may be lost in consequence of her temporary derangement. Nevertheless, feathering paddle-wheels have been on the increase in paddle vessels, and to diminish the risk of accident they are made very strong and sometimes of steel. The bearings and pins are usually made of iron covered with brass, and the eyes of the joints are lined with brass or, better still, with lignum vitæ or African oak. Proportions of Marine Engines.-In oscillating engines the piston rod is usually made one-ninth of the diameter of the cylinder, and the crank pin is made about one-seventh of the diameter of the cylinder. The diameter of the paddle shaft must have reference not merely to the diameter of the cylinder, but also to the length of the stroke of the piston, or, what is the same thing, to the length of the crank. If the square of the diameter of the cylinder in inches be multiplied by the length of the crank in inches, and the cube root of the product be extracted, then that root multiplied by 242 will give the diameter proper for the shaft in inches at the smallest part. The diameter of the trunnions is regulated by the diameter of the steam and eduction pipes, and these are each usually about one-fifth of the diameter of the cylinder; but it is better to make the steam trunnion a little less, and the eduction trunnion a little more, than this proportion. The steam and eduction pipes, where they enter their respective trunnions, are kept tight by a packing of hemp, which is compressed by a suitable ring or gland, tightened by screws. In land engines, the air pump and condenser are each made about one-eighth of the capacity |