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Description of the Direct Acting Screw Engines of various Makers.

and cast in a piece with the piston, there is a large pipe or trunk, to the centre of which one end of the connecting rod is attached. As the piston is moved backwards and forwards by the steam, the end of the connecting rod necessarily partakes of the same motion, and consequently turns the crank, the trunk being of sufficient diameter to enable the connecting rod to follow the motion of the crank without coming into contact with the sides of the trunk. The air pump, which is double acting and therefore made with a solid piston, is situated within the condenser, and lies in a horizontal position. It is worked in a very simple manner by means of a rod which passes through the piston and cylinder cover opposite to the point where the air pump is placed. There are two air pumps, one to each cylinder; and as the air pumps are double acting, each is necessarily provided with two suction or foot valves, and two forcing or delivery valves. Each of these valves consists of a number of india-rubber discs set upon a brass plate, as is now the usual practice in direct acting screw engines. The feed and bilge pumps are worked in the same manner as the air pumps, and consequently the whole of these pumps must have the same stroke as the piston. The valves are worked by means of the link motion, the arrangements of which are shown in the Plate. The steam escapes from the cylinder to the condenser through a large eduction pipe, which proceeds from the upper part of the valve casing, and slopes down to the condenser so as to enable any water, which might otherwise gather in it from the partial condensation of the steam, to run into the condenser by gravity. The cranks are forged in a piece with the shaft, and both are supported on each side of the connecting rod. The condenser with the air pumps within it is not nearly so wide as the cylinders, and therefore the eduction pipes converge as they approach the condenser.

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The same objections which apply to engines with four cylinders, apply to a considerable extent to this engine also, for the trunks may be regarded as additional cylinders, so far that they present a considerable extent of cooling surface to the air. Upon the whole, therefore we consider that this engine would be improved if the trunk were discarded in favour of two or four piston rods arranged as in Mr. D. Napier's steeple engine. The cylinders could then be brought nearer to the cranks and the length of the stroke might be increased, while the same amount of simplicity would still be preserved. Engines made upon this plan will, we are satisfied, be more economical in fuel than engines made with a trunk. This is a paramount consideration to which others ought to yield, and upon the whole we feel assured that these trunk engines will not preserve any very protracted vitality.

Engines of the Conflict. In these engines, constructed by Messrs. Seaward for H. M. S. Conflict, there are four cylinders lying in a horizontal position with connecting rods interposed between the cylinders and cranks as in a locomotive engines, and two vertical air pumps of short stroke wrought by separate cranks in the screw shaft.

Engines of the Vulcan, Forth, Seahorse, and Megara. - These engines, constructed by Messrs. Rennie, have also four horizontal cylinders, and in their leading features they are similar to the engines of the Conflict; but the air pumps lie at an angle with one another and are worked by a single crank at the end of the screw shaft. The condensers are tall flat chambers at the sides of the cylinders, and they box up the engines, very much. The design, though evincing a good deal of ability in the adjustment of the details, is not one likely to find much imitation, especially as such engines would be expensive to construct.

Screw Engines by Messrs. Blyth. - These engines, which are of 450 horse power, and were intended for the propulsion of a screw frigate of 74 guns, exhibit a large measure of engineering ability. The cylinders, which are oscillating, are made with two piston rods, which is better than one in the case of high speeds; and the modification also enables the crank to come closer to the cylinder cover, in which indeed a recess may be made to permit its more near approach. The screw shaft, which lies at a lower level than the shafts of the engine, is turned by means of a triangular frame, and it will be obvious that in this combination there can be no dead point, since one piston is in the middle of its stroke when the other

is at the end. It would be desirable to dispense with the cranks m the engine shaft for working the air pumps, and to work the air pumps from the cranks at the ends of the shafts, with recurved or eccentric crank pins. The engine shafts might then be made shorter, and in order that the steadiness of the framing might not be impaired, it might be made in a diagonal form, and the length of the bearings might also be increased.

Screw Engines by Messrs. Stothert and Slaughter.— The main peculiarity of these engines is, that the air pumps are worked at a lower speed than the engines, or make fewer strokes than the pistons. This object is accomplished by the interposition of gearing, which brings down the speed of the air pump motion by the reverse of the arrangement used in geared engines for bringing up the speed of the screw. The object of this arrangement is to enable the air pumps to work without shock, although the form of valves usual in show engines are employed. There are two cylinders placed in a diagonal, position in a vertical plane at right angles with the keel. As both the cylinders stand at an angle of about 45° with a vertical line, and, therefore, at an angle of 90° with one another, the effect will be the same when the piston rods are connected with one crank, as when the piston rods of two parallel cylinders are connected with separate cranks standing at right angles with one another, and one crank for the two engines will therefore suffice. Instead of a crank, however, a disc of metal is here used with a projecting pin, and the shaft on which this disc of metal is fixed, gives motion to a pinion, gearing into a larger wheel, on the side of which there is a pin which gives motion to horizontal rods in connection with bell crank levers, which work the air pumps. The air pumps stand vertically is cisterns situated upon each side of the metal disc with which the pistons of the engines are connected. This engine has been contrived to overcome a difficulty which no longer exists, and the whole arrangement of the air pump machinery, therefore, involves superfluous complication. Before the introduction of indiarubber valves for the air pumps, it might have been serviceable, but any such device as this is now too late.

Engines of the Frankfort. These engines, constructed for the Frankfort by Messrs. J. and G. Thompson, are among the best examples of direct acting screw engines that we yet possess. They are simple, compact, and substantial, and upon the whole are a very eligible class of engines for merchant vessels, but for war vessels they, of course, would not be suitable, as they would come above the water-line. The general outline of the plan resembles that employed by Mr. Nasmyth for working his forge hammer, but in the forge hammer the engine is, of course, single acting, and has no crank or connecting rod. It will be seen that there are two cylinders and two air pumps, with one hot well standing between them. The condenser is a large square vessel standing between the cylinders and hot well, and the eduction pipes from the cylinders enter it at the top, and an injection cock delivers water through the sides of the condenser opposite each eduction pipe. The whole of the other details are so obvious, from an inspection of the drawing, as not to require further description. The engine is regulated from the deck, as will be apparent from the position of the starting handles. These engines are of 40 inches diameter of cylinder, and 2 feet 9 inches stroke, and the pressure of the steam in the boiler is 16 lbs. per square inch. A good many engines upon a plan nearly resembling this, have been made by Messrs. Caird and Co., Messrs. Miller, Ravenhill, and Co., and other makers.

Engines of Swedish steamer. These engines, designed by Mr. Carlsund for a Swedish steamer are the same, as regards the position of the cylinders, as the engines of Messrs. Stothert and Slaughter already described; but the air pumps are worked by arms extending from the piston rods, and have, therefore, the same length of stroke as the pistons themselves. The cylinders are fixed between two iron bulkheads extending across the vessel, whereby their weight and pressure are very effectually distributed over the hull. Mr. Carlsund's object in this arrangement of the cylinders was to enable the vessel to be made with a triangular cross section, and at the same time, to get the screw shaft kept as deep as possible in the water. This object the arrangement very effectually attains, while it is not exceptionable in other respects. The air pumps are situated within

Description of the Direct Acting Screw Engines of various Makers.

the condensers which are placed below the cylinders, and the con-
necting rods are forked, the piston rods being guided by their pro-
iecting ends, which pass through suitable eyes. The pistons are
made cupped, and the cylinder covers are cupped also, so as to fit
into the pistons, and to afford room for the stuffing boxes of the
piston rods. The piston packings are formed in a similar way to that
now usual in this country for the pistons of locomotives. Each
piston is first made like a solid plug that will exactly fit the cylinder,
and two grooves are then turned in it into which grove rings are
fitted of a larger diameter than the cylinder itself, but with pieces
cut out to enable them to enter the cylinder as is usually done in
common packing rings, and the piston, with these rings upon it, is
then forced into the cylinder. There is, therefore, no junk ring, and
should the rings wear slack sideways, new ones must be substituted.
The air pump valves are of brass, yet they are so constructed as to
make very little noise, even when the engine is working at a speed
of 120 revolutions, per minute. This end is attained by making the
lower part of the valve a cone, so that the ascending water meets
it gradually, and opens it by degrees. As the valve opens it has to
compress a spiral spring placed in a tube cast on the top of the
valve, and this has the effect of closing the valve gradually, and
without shock, as the pressure is withdrawn. The recommendation
to make the valves of pumps of cones instead of discs, so as to take
off the shock, is given in Leupold's "Theatrum Machinarum,"
published in 1727, and the device probably is of great antiquity;
but except in the engines of this vessel, there are not many instances
in which the idea has been practically applied. The wheel shown
on the screw shaft and its accompanying pinion, are merely for the
purpose of turning the engines round slowly by hand.
Engines of the Princeton. - These engines were constructed by
Merrick and Towne, after the designs of Ericsson for the United
States war steamer Princeton. Engines of this kind were con-
templated by Mr. Watt, and are described in the specification of one
of his patents taken out in the last century, as will be seen by a
reference to page 22 of the present work. But the engines of the
Princeton were the first actual engines made upon this plan
which have performed efficient service. With so much novelty in
the design, and apparent difficulty in the execution, even a very
inferior performance might fairly have been accepted as a success.
But the performance of the engines of the Princeton will bear a
very favourable comparison with the performance of common engines,
and the old hull of the Princeton having been worn out, a new
hull has been built for the engines, which after the lapse of ten
years were still found to be in excellent condition. The Princeton
was the first vessel ever built with the engines below the water line,
and Ericsson's object in fixing upon this species of engine, was to
remove the apprehensions of those who were fearful that the mo-
mentum of the moving parts would, at so great a speed as was indis-
pensable to any engine in direct connection with the screw, speedily
work the destruction of the machine.

It will be obvious, on an inspection of the Plate, that in this engine the piston moves like a door on its hinges, and the piston shaft, which answers to the hinge, by being prolonged beyond the steam chamber, gives motion, by a reciprocating crank or short lever and accompanying connecting rod, to the crank upon the screw shaft. The air pumps are at the opposite end of the engine, and derive their motion by means of short levers from the reciprocating piston shafts. The piston is made tight on all the four sides by metallic packing, for it will be observed that the piston shaft is not at the very edge of the piston, but only near the edge, and the small projecting portion works steam tight on the interior of the pipe which covers the piston shaft.

Engines of the Étoile. - These engines are also, in all their material features, after the designs of Ericsson; and numerous barges and other vessels for canals have been constructed by Ericsson, in America, after this plan, most of them with high pressure engines. There is only one cylinder in this engine, and the rods, which answer to side rods in ordinary engines provided with a cross head, act here as connecting rods, and turn round the shafts with which they are connected, in opposite directions. The air pump is wrought in precisely the same way by cranks in the shafts

315

that the cylinder uses to work cranks in the shaft. This is the simplest form of engine we have met with for giving motion to two screws, and for all vessels of shallow draft, and probably for all vessels whatever, two screws are better than one. If the engine is to be high pressure, the air pump and its accompanying gearing have only to be discarded, the rest of the engine remaining as before.

Engines of the Minx. These engines are high pressure. They were constructed by Messrs. Seaward and Capel, for H. M. S. Minx, and resemble their larger engines with four cylinders, but with the alternate cylinders on the opposite sides left out. These engines are so simple, and all their arrangements are made so obvious by the drawing, that they do not stand in need of further explanation.

Screw Engines by Mr. James Whitelaw. These engines are substantially land engines with an unequally divided beam, the object of which is to reconcile the realisation of a long crank with a short stroke of the piston, whereby any inconvenient momentum of the moving parts will be obviated, while at the same time there will be no inconvenient amount of pressure thrown upon the screw shaft.

Engines of the Pomone.

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These engines, designed by Mr. Holm, are steeple engines laid upon their side; but there are two piston rods instead of one, and the bottom of the angular frame is rested upon a slide to take its weight off the piston rods. The air pump of each engine, which is also horizontal, is worked by a projecting arm from one end of the cross head, and it is made double acting. The Pomone is the first vessel that was constructed with double acting air pumps laid in a horizontal position, and she is the second vessel constructed with the engines below the water line, the Princeton having been the first. The engines of this vessel are a very excellent specimen of direct acting screw engines. They are furnished with a very ingenious species of expansion valve; and a ring moving steam tight on the back of the valve casing, is fixed to to the back of the valve to take the pressure off the valve face. The cylinders of these engines are of 46 inches diameter, and 46 inches stroke, and the engines make 40 revolutions per minute.

Disc Engine. This engine, though a good many years have elapsed since it was first brought forward, has not hitherto come into very extensive use; and, indeed, very few engines of this class are in actual practical operation. The manner in which the engine works is not made readily intelligible to any one seeking to investigate the subject for the first time; but we shall endeavour to explain its mode of action in a few words:

the

A flat disc is set in a short horizontal cylinder with conical ends, and the apexes of the two cones meet in a large ball in the middle of the cylinder by which the disc is supported. A diaphragm rises from the lower side of the cylinder up to the ball, and a slit is cut from the circumference to the centre of the disc, so as to enable it to enter the cylinder, notwithstanding the existence of the diaphragm. The steam enters the cylinder upon one side of the diaphragm, and escapes at the other side, and when the disc bears upon upper side of one cone it will bear upon the under side of the other cone. From the centre of the ball an arm extends to the exterior of the cylinder, which arm communicates the motion of the disc to a crank placed to receive it. Now if we suppose the disc drawn up against the steam port, and that steam is then admitted, the steam will press on one side upon the diaphragm, and on the other side it will seek to pass the point of contact of the disc and cone; but as it cannot do this, it will force itself forward like a wedge, continually shifting the point of contact of the disc further back, and enlarging the space filled by the steam. An oscillating motion will thus be given to the disc, which is communicated to the crank, and the action continues so long as the engine is supplied with steam. The first disc engines were both leaky and noisy, but these defects have been completely surmounted, and disc engines now work with quite as great economy of fuel as other engines, and with a more equable motion, which for some purposes is an advantage. For marine purposes, however, we do not think they are likely to come into use. engine of large power, and working with the speed proper for the screw, would be of inconvenient diameter, and a nicety of workmanship is required in the construction of disc engines, which, even

An

Description of the Engines of the Steamers Iona, Persia, and Jura.

with all the aids afforded by modern tools and improved modes of working, it will be very difficult to obtain.