(214.)

Fig. 126.

A provision is likewise made by which the steam escaping at the safety-valve is condensed and carried away to the feeding cistern.

(214.)One of the remedies proposed for the evil consequences arising from incrustation is the substitution of copper for iron boilers. The attraction which produces the adhesion of the calcareous matter held in solution by salt water to the surface of iron has no existence in copper, and all the saline and other alkaline matter precipitated in the boiling water in[Pg461]copper boilers is suspended in a loose form, and carried off by the process of blowing out.

Besides the injury arising from the deposition of salt and the incrustation on the inner surface of boilers, an evil of a formidable kind attends the accumulation of soot mixed with salt in the flues, which proceeds from the leaks. In the seams of the boiler there are numerous apertures, of dimensions so small as to be incapable of being rendered stanch by any practicable means, through which the water within the boiler filters, and the salt which it carries with it mixes with the soot, forming a compound which rapidly corrodes the boilers. This process of corrosion in the flues takes place not less in copper than in iron boilers. In cleansing the flues of a copper boiler, the salt and soot which was thrown out upon the iron-plates which formed the flooring of the engine-room, having remained there for some time, left behind it a permanent appearance of copper on the iron flooring, arising from the precipitation of the copper which had combined with the soot and salt in the flues.[36]In this case the leaks from whence the salt proceeded were found, on careful examination, so unimportant, that the usual means to stanch them could not be resorted to without the risk of increasing the evil.

(215.)In the application of the steam-engine to the propulsion of vessels in voyages of great extent, the economy of fuel acquires an importance greater than that which appertains to it in land-engines, even in localities the most removed from coal-mines, and where its expense is greatest. The practical limit to steam-voyages being determined by the greatest quantity of coals which a steam-vessel can carry, every expedient by which the efficiency of the fuel can be increased becomes a means, not merely of a saving of expense, but of an increased extension of steam-power to navigation. Much attention has been bestowed on the augmentation of the duty of engines in the mining districts of Cornwall, where the question of their efficiency is merely a question of economy, but far greater care should be given to this subject when the practicability of maintaining intercourse by steam between distant points of the globe will perhaps depend on the effect produced by a given quantity[Pg462]of fuel. So long as steam-navigation was confined to river and channel transport, and to coasting voyages, the speed of the vessel was a paramount consideration, at whatever expenditure of fuel it might be obtained; but since steam-navigation has been extended to ocean-voyages, where coals must be transported sufficient to keep the engine in operation for a long period of time without a fresh relay, greater attention has been bestowed upon the means of economising it.

Much of the efficiency of fuel must depend on the management of the fires, and therefore on the skill and care of the stokers. Formerly the efficiency of firemen was determined by the abundant production of steam, and so long as the steam was evolved in superabundance, however it might have blown off to waste, the duty of the stoker was considered as well performed. The regulation of the fires according to the demands of the engine were not thought of, and whether much or little steam was wanted, the duty of the stoker was to urge the fires to their extreme limit.

Since the resistance opposed by the action of the paddle-wheels of a steam-vessel varies with the state of the weather, the consumption of steam in the cylinders must undergo a corresponding variation; and if the production of steam in the boilers be not proportioned to this, the engines will either work with less efficiency than they might do under the actual circumstances of the weather, or more steam will be produced in the boilers than the cylinders can consume, and the surplus will be discharged to waste through the safety-valves. The stokers of a marine engine, therefore, to perform their duty with efficiency, and obtain from the fuel the greatest possible effect, must discharge the functions of a self-regulating furnace, such as has been already described: they must regulate the force of the fires by the amount of steam which the cylinders are capable of consuming, and they must take care that no unconsumed fuel is allowed to be carried away from the ash-pit.

(216.)Until within a few years of the present time the heat radiated from every part of the surface of the boiler was allowed to go to waste, and to produce injurious effects on those parts of the vessel to which it was transmitted. This evil,[Pg463]however, has been lately removed by coating the boilers, steam-pipes, &c. of steam-vessels with felt, by which the escape of heat from the surface of the boiler is very nearly, if not altogether, prevented. This felt is attached to the boiler-surface by a thick covering of white and red lead. This expedient was first applied in the year 1818 to a private steam-vessel of Mr. Watt's called theCaledonia, and it was subsequently adopted in another vessel, the machinery of which was constructed at Soho, called theJames Watt.

The economy of fuel depends in a considerable degree on the arrangement of the furnaces, and the method of feeding them. In general each boiler is worked by two or more furnaces communicating with the same system of flues. While the furnace is fed, the door being open, a stream of cold air rushes in, passing over the burning fuel and lowering the temperature of the flues: this is an evil to be avoided. But, on the other hand, if the furnaces be fed at distant intervals, then each furnace will be unduly heaped with fuel, a great quantity of smoke will be evolved, and the combustion of the fuel will be proportionally imperfect. The process of coking in front of the grate, which would insure a complete combustion of the fuel, has been already described (147.). A frequent supply of coals, however, laid carefully on the front part of the grate, and gradually pushed backwards as each fresh feed is introduced, would require the fire-door to be frequently opened, and cold air to be admitted. It would also require greater vigilance on the part of the stokers than can generally be obtained in the circumstances in which they work. In steam-vessels the furnaces are therefore fed less frequently, fuel introduced in greater quantities, and a less perfect combustion produced.

When several furnaces are constructed under the same boiler, communicating with the same system of flues, the process of feeding, and consequently opening one of them, obstructs the due operation of the others, for the current of cold air which is thus admitted into the flues checks the draft and diminishes the efficiency of the furnaces in operation. It was formerly the practice in vessels exceeding one hundred horse-power, to place four furnaces under each boiler, communicating with the same system of flues. Such an arrangement[Pg464]was found to be attended with a bad draft in the furnaces, and therefore to require a greater quantity of heating surface to produce the necessary evaporation. This entailed upon the machinery the occupation of more space in the vessel in proportion to its power; it has therefore been more recently the practice to give a separate system of flues to each pair of furnaces, or, at most, to every three furnaces. When three furnaces communicate with a common flue, two will always be in operation, while the third is being cleared out; but if the same quantity of fire were divided among two furnaces, then the clearing out of one would throw out of operation half the entire quantity of fire, and during the process the evaporation would be injuriously diminished. It is found by experience, that the side plates of furnaces are liable to more rapid destruction than their roofs, owing, probably, to a greater liability to deposit. Furnaces, therefore, should not be made narrower than a certain limit. Great depth from front to back is also attended with practical inconvenience, as it renders firing tools of considerable length, and a corresponding extent of stoking room necessary. It is recommended, by those who have had much practical experience in steam-vessels, that furnaces six feet in depth from front to back should not be less than three feet in width, to afford means of firing with as little injury to the side plates as possible, and of keeping the fires in the condition necessary for the production of the greatest effect. The tops of the furnaces almost never decay, and seldom are subject to an alteration of figure, unless the level of the water be allowed to fall below them.[37]

(217.)A form of marine engine was some years since proposed and patented by Mr. Thomas Howard, possessing much novelty and ingenuity, and having pretensions to a very extraordinary economy of fuel, in addition to the advantages claimed by Mr. Hall. In Mr. Howard's engines, the steam, as in Mr. Hall's, is constantly reproduced from the same water, so that pure or distilled water may be used; but Mr. Howard dispenses altogether with the use of a boiler.

A quantity of mercury is placed in a shallow wrought-iron vessel over a coke fire, by which it is maintained at a[Pg465]temperature varying from 400° to 500°. The surface exposed to the fire was computed at three fourths of a square foot for each horse-power. The upper surface of the mercury was covered by a very thin plate of iron in contact with it, and so contrived as to present about four times as much surface as that exposed beneath the fire. Adjacent to this a vessel of water was placed, maintained nearly at the boiling point, and communicating by a nozzle and valve with the chamber immediately above the mercury. At intervals corresponding to the motion of the piston a small quantity of water was injected from this vessel, and thrown upon the plate of iron resting upon the hot mercury. From this it received not only the heat necessary to convert it into common steam, but to give it the qualities of highly superheated steam. In fact, the steam thus produced had a temperature considerably above that which corresponded to its pressure, and was, therefore, capable of being deprived of more or less of its heat without being condensed. (94.) The quantity of water injected into the steam-chamber was regulated by the power at which the engine was intended to be worked. The fire was supplied with air by a blower subject to exact regulation. The steam thus produced was conducted to a chamber surrounding the working cylinder, and this chamber itself was enclosed by another space through which the air from the furnace passed before it reached the flue. By this contrivance the air imparted its redundant heat to the steam, as the latter passed to the cylinder, and raised its temperature to about 400°, the pressure, however, not exceeding 25 lbs. per square inch. The valves, governing the admission of steam to the piston, were adapted for expansive action.

The vacuum on the opposite side was maintained by condensation in the following manner:—The condenser was a copper vessel placed in a cistern of cold water, and the steam was admitted to it from the cylinder by an eduction pipe in the usual way. A jet was introduced from an adjacent vessel filled with distilled water, and the condensing water and condensed steam were pumped from the condenser as in common engines. The warm water thus pumped out of the[Pg466]condenser was drawn through a copper worm, carried with many coils through a cistern of cold water, so that when it arrived at the end of this pipe it was reduced nearly to the temperature of the atmosphere. The pipe was thus brought to the vessel of distilled water already mentioned, and the water supplied by it replaced. The water admitted to the condenser through the condensing jet being purged of air, a small air-pump was sufficient, since it had only to exhaust the condenser and tubes at starting, and to remove the air which might be admitted by leakage. Mr. Howard stated that the condensation took place as rapidly and perfectly as in the best engines of the common kind.

An engine of this construction was in the spring of 1835 placed in the government steamer called theComet. It was stated, that though the machinery was not advantageously constructed, a part of the engine being old, and not made expressly for a boiler of this kind, the vessel performed a voyage from Falmouth to Lisbon, in which the consumption of fuel did not exceed a third of her former consumption when worked by Boulton and Watt's engines, the former consumption of coals being about eight hundred pounds per hour, and the consumption of Mr. Howard's engine being less than two hundred and fifty pounds of coke per hour.

The advantages claimed for this contrivance were the following:first, the small space and weight occupied by the machinery, arising from the absence of a boiler;second, the diminished consumption of fuel;third, the reduced size of the flues;fourth, the removal of the injurious effects arising from deposit and incrustation;fifth, the absence of smoke.

(218.)The method by which the greatest quantity of practical effect can be obtained from a given quantity of fuel must, however, mainly depend on the extended application of the expansive principle. This has been the means by which an extraordinary amount of duty has been obtained from the Cornish engines. The difficulty of the application of this principle in marine engines has arisen from the objections entertained in Europe to the use of steam of high pressure under the circumstances in which the engine must be worked at sea. To apply the expansive principle, it is necessary that the moving power at the commencement of the stroke shall considerably exceed the[Pg467]resistance, its force being gradually attenuated till the completion of the stroke, when it will at length become less than the resistance. This condition may, however, be attained with steam of limited pressure, if the engine be constructed with a sufficient quantity of piston-surface. This method of rendering the expansive principle available at sea, and compatible with low-pressure steam, has recently been brought into operation by Messrs. Maudslay and Field. Their improvement consists in adapting two steam-cylinders in one engine, in such a manner that the steam shall act simultaneously on both pistons, causing them to ascend and descend together. The piston-rods are both attached to the same horizontal cross-head, whereby their combined action is applied to one crank by means of a connecting rod placed between the pistons.

Fig. 127.

A section of such an engine, made by a plane passing through the two piston-rodsP P′and cylinders, is represented infig.127.The piston-rods are attached to a cross-headC,[Pg468]which ascends and descends with them. This cross-head drives upwards and downwards an axleD, to which the lower end of the connecting rodEis attached. The other end of the connecting rod drives the crank-pinF, and imparts revolution to the paddle-shaftG. A rodHconveys motion by means of a beamIto the rodKof the air-pumpE.

(219.)Connected with this, and in the same patent, another improvement is included, consisting of the application of a hollow wrought-iron framing carried across the vessel above the machinery, to support the whole of the bearings of the crank-shaft. A plan of this, including the cylinders and paddle-wheel, is represented infig.128.The advantages proposed by these improvements are simplicity of construction, more direct action on the crank, economy of space and weight of material, combined with increased area of the piston, whereby a given evaporating power of the boiler is rendered productive, by extended application of the expansive principle, of a greater moving power than in former arrangements. Consequently, under like circumstances, greater power and economy of fuel is obtained, with the further advantage at sea, that when the engine is reduced in its speed, either by the vessel being deeply laden with coal, as is the case at the commencement of a long sea voyage, or by head winds, more steam may be given to the cylinders, and consequently more speed imparted to the vessel, all the steam produced in the boiler being usefully employed.

(220.)Another improvement, having the same objects, and analogous to the preceding, has been likewise patented by Messrs. Maudslay and Field. This consists in the adoption of a cylinder of greater diameter, having two piston-rodsP P′, as represented infig.129., of considerable length, connected at the top by a cross-headC. From this cross-head is carried downwards the connecting rodD, which drives the crank-pinE, and thereby works the paddle-shaftS. In this case the paddle-shaft is extended immediately above the piston, and the double piston-rod has sufficient length to be above the paddle-shaft when the piston is at the bottom of its stroke. This improvement is intended to be applied more particularly for engines for river navigation, the advantages resulting from[Pg469]it being that a paddle-shaft placed at a given height from the bottom of the vessel will be enabled to receive a longer stroke of piston than by any other arrangement now in use. A more[Pg470]compact and firm connection of the cylinder with the crank-shaft bearings is effected by it, and a cylinder of much greater diameter may be applied by which the expansive action of steam may be more fully brought into play; and a more direct action of the steam-power on the crank with a less weight of materials and a greater economy of space may be obtained than by any of the arrangements of marine engines hitherto used.

Fig. 128.

Fig. 129.

(221.)Mr. Francis Humphrys has obtained a patent for a form of marine engine, by which some simplification of the machinery is attained, and the same power comprised within more limited dimensions. In this engine there is attached to the piston of the cylinder, instead of a piston-rod, a hollow casingD D(fig.130.), which moves through a stuffing-boxG, constructed in a manner similar to the stuffing-box of a piston-rod. In the figure, this casing is presented in section, but[Pg471]its form is that of a long narrow slit, or opening, rounded at either end as exhibited in the plan (fig.131) of the cylinder-cover. The crankCis driven by the other end of the connecting rodH, the crank-shaft being immediately above the centre of the piston and the connecting rod passing through the oblong openingD, and descending into the hollow piston-rod it is attached to an axisIat the bottom of the piston. A box or coverK Kencloses the cross-piece or axisIwith its bearings, and is[Pg472]attached so as to be steam-tight to the bottom of the piston. A hollow spaceL Lis cast in the bottom of the cylinder for the reception of the boxK K, when the piston is at the bottom of the cylinder.

Fig. 130.

Fig. 131.

By this arrangement the force by which the piston is driven in its ascent and descent is communicated to the connecting rod, not, as usual, through the intervention of a piston-rod, but directly from the piston itself by the cross-pinI, and from thence to the crankC, which it drives without the intervention of beams, cross-heads, or any similar appendage.

The slide-valves regulating the admission and eduction of steam are represented ata; the rod of the air-pump is shown atd, being worked by a crank placed on the centre of the great crank shaft.[38]

(222.)To obtain from the moving power its full amount of mechanical effect in propelling the vessel, it would be necessary that its force should propel, by constantly acting against the water in a horizontal direction, and with a motion contrary to the course of the vessel. No system of mechanical propellers has, however, yet been contrived capable of perfectly accomplishing this. Patents have been granted for many ingenious mechanical combinations to impart to the propelling surfaces such angles as appeared to the respective contrivers most advantageous. In most of these the mechanical complexity has formed a fatal objection. No part of the machinery of a steam-vessel is so liable to become deranged at sea as the paddle-wheels; and, therefore, that simplicity of construction which is compatible with those repairs which are possible on such emergencies is quite essential for safe practical use.

Fig. 132.

The ordinary paddle-wheel, as has been already stated, is a wheel revolving upon a shaft driven by the engine, and carrying upon its circumference a number of flat boards, called paddle-boards, which are secured by nuts and braces in a fixed position; and that position is such that the planes[Pg473]of the paddle-boards diverge nearly from the centre of the shaft on which the wheel turns. The consequence of this arrangement is that each paddle-board can only act in that direction which is most advantageous for the propulsion of the vessel when it arrives near the lowest point of the wheel. Infig.132.let O be the shaft on which the common paddle-wheel revolves; the position of the paddle-boards are represented atA,B,C, &c.;X,Yrepresents the water line, the course of the vessel being supposed to be fromXtoY; the arrows represent the direction in which the paddle-wheel revolves. The wheel is immersed to the depth of the lowest paddle-board, since a less degree of immersion would render a portion of the surface of each paddle-board mechanically useless. In the positionAthe whole force of the paddle-board is efficient for propelling the vessel; but as the paddle enters the water in the positionH, its action upon the water, not being horizontal, is only partially effective for propulsion: a part of the force which drives the paddle is expended in depressing the water, and the remainder in driving it contrary to the course of the vessel, and, therefore, by its re-action producing a certain propelling effect. The tendency, however, of the paddle entering the water atH, is to form a hollow or trough, which the water, by its ordinary property, has a continual tendency to fill up. After passing the lowest pointA, as the paddle approaches the positionB, where it[Pg474]emerges from the water, its action again becomes oblique, a part only having a propelling effect, and the remainder having a tendency to raise the water, and throw up a wave and spray behind the paddle-wheel. It is evident that the more deeply the paddle-wheel becomes immersed, the greater will be the proportion of the propelling power thus wasted in elevating and depressing the water; and if the wheel were immersed to its axis, the whole force of the paddle-boards, on entering and leaving the water, would be lost, no part of it having a tendency to propel. If a still deeper immersion take place, the paddle-boards above the axis would have a tendency to retard the course of the vessel. When the vessel is, therefore, in proper trim, the immersion should not exceed nor fall short of the depth of the lowest paddle; but for various reasons it is impossible in practice to maintain this fixed immersion: the agitation of the surface of the sea, causing the vessel to roll, will necessarily produce a great variation in the immersion of the paddle-wheels, one becoming frequently immersed to its axle, while the other is raised altogether out of the water. Also the draught of water of the vessel is liable to change, by the variation in her cargo; this will necessarily happen in steamers which take long voyages. At starting they are heavily laden with fuel, which as they proceed is gradually consumed, whereby the vessel is lightened.

(223.)To remove this defect, and economise as much as possible the propelling effect of the paddle-boards, it would be necessary so to construct them that they may enter and leave the water edgeways, or as nearly so as possible; such an arrangement would be, in effect, equivalent to the process called feathering, as applied to oars. Any mechanism which would perfectly accomplish this would cause the paddles to work in almost perfect silence, and would very nearly remove the inconvenient and injurious vibration which is produced by the action of the common paddles. But the construction of feathering paddles is attended with great difficulty, under the peculiar circumstances in which such wheels work. Any mechanism so complex that it could not be easily repaired when deranged, with such engineering implements and skill[Pg475]as can be obtained at sea, would be attended with great objections; and the efficiency of its propelling action would not compensate for the dangers which must attend upon the helpless state of a steamer, deprived of her propelling agents.

Feathering paddle-boards must necessarily have a motion independently of the motion of the wheel, since any fixed position which could be given to them, though it might be most favourable to their action in one position would not be so in their whole course through the water. Thus the paddle-board when at the lowest point should be in a vertical position, or so placed that its plane, if continued upwards, would pass through the axis of the wheel. In other positions, however, as it passes through the water, it should present its upper edge, not towards the axle of the wheel, but towards a point above the highest point of the wheel. The precise point to which the edge of the paddle-board should be directed is capable of mathematical determination. But it will vary according to circumstances, which depend on the motion of the vessel. The progressive motion of the vessel, independently of the wind or current, must obviously be slower than the motion of the paddle-boards round the axle of the wheel; since it is by the difference of these velocities that the re-action of the water is produced by which the vessel is propelled. The proportion, however, between the progressive speed of the vessel and the rotative speed of the paddle-boards is not fixed: it will vary with the shape and structure of the vessel, and with its depth of immersion; nevertheless it is upon this proportion that the manner in which the paddle-boards should shift their position must be determined. If the progressive speed of the vessel were nearly equal to the rotative speed of the paddle-boards, the latter should so shift their position that their upper edges should be presented to a point very little above the highest point of the wheel. This is a state of things which could only take place in the case of a steamer of a small draught of water, shallop-shaped, and so constructed as to suffer little resistance from the fluid. On the other hand, the greater the depth of immersion, and the less fine the lines of the[Pg476]vessel, the greater will be the resistance in passing through the water, and the greater will be the proportion which the rotative speed of the paddle-boards will bear to the progressive speed of the vessel. In this latter case the independent motion of the paddle-boards should be such that their edges, while in the water, shall be presented towards a point considerably above the highest point of the paddle-wheel.

A vast number of ingenious mechanical contrivances have been invented and patented for accomplishing the object just explained. Some of these have failed from the circumstance of their inventors not clearly understanding what precise motion it was necessary to impart to the paddle-board: others have failed from the complexity of the mechanism by which the desired effect was produced.

(224.)In the year 1829 a patent was granted to Elijah Galloway for a paddle-wheel with movable paddles, which patent was purchased by Mr. William Morgan, who made various alterations in the mechanism, not very materially departing from the principle of the invention.

Fig. 133.

This paddle-wheel is represented infig.133.The contrivance may be shortly stated to consist in causing the wheel which bears the paddles to revolve on one centre, and the radial arms which move the paddles to revolve on another centre. LetA B C D E F G H I K Lbe the polygonal circumference of the paddle-wheel, formed of straight bars, securely connected together at the extremities of the spokes or radii of the wheel which turns on the shaft which is worked by the engine; the centre of this wheel being atO. So far this wheel is similar to the common paddle-wheel; but the paddle-boards are not, as in the common wheel, fixed atA B C, &c., so as to be always directed to the centreO, but are so placed that they are capable of turning on axles which are always horizontal, so that they can take any angle with respect to the water which may be given to them. From the centres, or the line joining the pivots on which these paddle-boards turn, there proceed short armsK, firmly fixed to the paddle-boards at an angle of about 120°. On a motion given to this armK, it will therefore give a corresponding angular motion to the paddle-board, so as to make it turn on its pivots. At[Pg477]the extremities of the several arms markedKis a pin or pivot, to which the extremities of the radial armsLare severally attached, so that the angle between each radial armLand the short paddle-armKis capable of being changed by any motion imparted toL; the radial arms are connected at the other end with a centre, round which they are capable of revolving. Now, since the pointsA B C, &c., which are the pivots on which the paddle-boards turn, are moved in the circumference of a circle, of which the centre isO, they are always at the same distance from that point; consequently they will continually vary their distance from the other centreP. Thus, when a paddle-board arrives at that point of its revolution at which the centre round which it revolves lies precisely between it and the centreO, its distance from the former centre is less than in any other position. As it departs from that point, its distance from that centre gradually increases until it arrives at the opposite point of its revolution, where the centreOis exactly between it and the former centre; then the distance of the paddle-board from the former centre is greatest.[Pg478]This constant change of distance between each paddle-board and the centrePis accommodated by the variation of the angle between the radial armLand the short paddle-board armK; as the paddle-board approaches the centrePthis gradually diminishes; and as the distance of the paddle-board increases, the angle is likewise augmented. This change in the magnitude of the angle, which thus accommodates the varying position of the paddle-board with respect to the centreP, will be observed in the figure. The paddle-boardDis nearest toP; and it will be observed that the angle contained betweenLandKis there very acute; atEthe angle betweenLandKincreases, but is still acute; atGit increases to a right angle; atHit becomes obtuse; and atK, where it is most distant from the centreP, it becomes most obtuse. It again diminishes atK, and becomes a right angle betweenAandB. Now this continual shifting of the direction of the short armKis necessarily accompanied by an equivalent change of position in the paddle-board to which it is attached; and the position of the second centrePis, or may be, so adjusted that this paddle-board, as it enters the water and emerges from it, shall be such as shall be most advantageous for propelling the vessel, and therefore attended with less of that vibration which arises chiefly from the alternate depression and elevation of the water, owing to the oblique action of the paddle-boards.

(225.)In the year 1833, Mr. Field, of the firm of Maudslay and Field, constructed a paddle-wheel with fixed paddle-boards, but each board being divided into several narrow slips arranged one a little behind the other, as represented infig.134. These divided boards he proposed to arrange in such cycloidal curves that they must all enter the water at the same place in immediate succession, avoiding the shock produced by the entrance of the common board. These split paddle-boards are as efficient in propelling when at the lowest point as the common paddle-boards, and when they emerge the water escapes simultaneously from each narrow board, and is not thrown up, as is the case with common paddle-boards.[39]

Fig. 134.

[Pg479]The theoretical effect of this wheel is the same as that of the common wheel, and experience alone, the result of which has not yet been obtained, can prove its efficiency. The number of bars, or separate parts into which each paddle-board is divided, has been very various. When first introduced by Mr. Galloway each board was divided into six or seven parts: this was subsequently reduced, and in the more recent wheels of this form constructed for the government vessels the paddle-boards consist only of two parts, coming as near to the common wheel as is possible, without altogether abandoning the principle of the split paddle.

(226.)To obtain an approximate estimate of the extent to which steam-power is applicable to long sea-voyages, it would be necessary to investigate the mutual relation which, in the existing state of this application of steam-power, exists between the capacity or tonnage of the vessel, the magnitude, weight, and power, of the machinery, the available stowage for fuel, and the average speed attainable in all[Pg480]weathers, as well as the general purposes to which the vessel is to be appropriated, whether for the transport of goods or merchandise, or merely for despatches and passengers, or for both of these combined. That portion of the capacity of the vessel which is appropriated to the moving power consists of the space occupied by the machinery and the fuel. The distribution of it between these must mainly depend on the length of the voyage which the vessel must make without receiving a fresh supply of coals. If the trips be short, and frequent relays of fuel can be obtained, then the space allotted to the machinery may bear a greater proportion to that assigned to the fuel; but in proportion as each uninterrupted stage of the voyage is increased, a greater stock of coals will be necessary, and a proportionally less space left for the machinery. Other things being the same, therefore, steam-vessels intended for long sea-voyages must be less powerful in proportion to their tonnage.

It will be apparent that every improvement which takes place in the application of the steam-engine to navigation will modify all these data on which such an investigation must depend. Every increased efficiency of fuel, from whatever cause it may be derived, will either increase the useful tonnage of the vessel, or increase the length of the voyage of which it is capable. Various improvements have been and are still in progress, by which this efficiency has undergone continual augmentation, and voyages may now be accomplished with moderate economy and profit, to which a few years since marine engines could not be applied with permanent advantage. The average speed of steam-vessels has also undergone a gradual increase by such improvements. During the four years ending June, 1834, it was found that the average rate of steaming obtained from fifty-one voyages made by the Admiralty steamers between Falmouth and Corfu, exclusive of stoppages, was seven miles and a quarter an hour direct distance between port and port. The vessels which performed this voyage varied from 350 to 700 tons measured burden, and were provided with engines varying from 100 to 200 horse-power, with stowage for coals varying from 80 to 240 tons. The proportion of the power to the[Pg481]tonnage varied from one horse to three tons to one horse to four tons. Thus theMessengerhad a power of 200 horses and measured 730 tons; theFlamerhad a power of 120 horses, and measured 500 tons; theColumbiahad a power of 120 horses, and measured 360 tons. In general it may be assumed that for the shortest class of trips, such as those of the Channel steamers, the proportion of the power to the tonnage should be about one horse for every two tons; but for the longer class of voyages, the proportion of power to tonnage should be about one horse-power to from three to four tons measured tonnage. These data, however, must be received as very rough approximations, subject to considerable modifications in their application to particular vessels. We have already stated that the nominal horse-power is itself extremely indefinite; and if, as is now customary in the longer class of voyages, the steam be worked expansively, then the nominal power almost ceases to have any definite relation to the actual performance of the vessel. It is usual to calculate the horse-power by assuming a uniform pressure of steam upon the piston, and, consequently, by excluding the consideration of the effect of expansion. The most certain test of the amount of mechanical power exerted by the machinery would be obtained from the quantity of water actually transmitted in the form of steam from the boiler to the cylinder. But the effect of this would also be influenced by the extent to which the expansive principle has been brought into operation.

From the reported performances of the larger class of steam-ships within the last few years, it would appear that the average speed has been increased since the estimate above mentioned, which was obtained in 1834; and on comparing the consumption of fuel with the actual performance, it would appear that the efficiency of fuel has also been considerably augmented. No extensive course of accurate experiments or observations have, however, been obtained from which correct inferences may be drawn of the probable limits to which steam-navigation, in its present state, is capable of being extended. The jealousy of rival companies has obstructed the inquiries of those who, solicitous more[Pg482]for the general advancement of the art than for the success of individual enterprises, have directed their attention to this question; and it is hardly to be expected that sufficiently correct and extensive data can be obtained for this purpose.

(227.)Increased facility in the extension and application of steam-navigation is expected to arise from the substitution of iron for wood, in the construction of vessels. Hitherto iron steamers have been chiefly confined to river-navigation; but there appears no sufficient reason why their use should be thus limited. For sea-voyages they offer many advantages; they are not half the weight of vessels of equal tonnage constructed of wood; and, consequently, with the same tonnage they will have less draught of water, and therefore less resistance to the propelling power; or, with the same draught of water and the same resistance, they will carry a proportionally heavier cargo. The nature of their material renders them more stiff and unyielding than timber; and they do not suffer that effect which is calledhogging, which arises from a slight alteration which takes place in the figure of a timber vessel in rolling, accompanied by an alternate opening and closing of the seams. Iron vessels have the further advantage of being more proof against fracture upon rocks. If a timber vessel strike, a plank is broken, and a chasm opened in her many times greater than the point of rock which produces the concussion. If an iron vessel strike, she will either merely receive a dinge, or be pierced by a hole equal in size to the point of rock which she encounters. Some examples of the strength of iron vessels were given by Mr. Macgregor Laird, in his evidence before the Committee of the Commons on Steam Navigation, among which the following may be mentioned:—An iron vessel, called theAlburkah, in one of their experimental trials got aground, and lay upon her anchor: in a wooden vessel the anchor would probably have pierced her bottom; in this case, however, the bottom was only dinged. An iron vessel, built for the Irish Inland Navigation Company, was being towed across Lough Derg in a gale of wind, when the towing rope broke, and she was driven upon rocks, on which she bumped for a considerable time[Pg483]without any injury. A wooden vessel would in this case have gone to pieces. A further advantage of iron vessels (which in warm climates is deserving of consideration) is their greater coolness and perfect freedom from vermin.

Iron steam-vessels on a very large scale are now in preparation in the ports of Liverpool and Bristol, intended for long sea-voyages. The largest vessel of this description which has yet been projected is stated to be in preparation for the voyage between Bristol and New York, by the company who have established the steam-ship called the Great Western, plying between these places.

Several projects for the extension of steam-navigation to voyages of considerable length have lately been entertained both by the public and by the legislature, and have imparted to every attempt to improve steam-navigation increased interest. A committee of the House of Commons collected evidence and made a report in the last session in favour of an experiment to establish a line of steam-communication between Great Britain and India. Two routes have been suggested by the committee, each being a continuation of the line of Admiralty steam-packets already established to Malta and the Ionian Isles. One of the routes proposed is through Egypt, the Red Sea, and across the Indian Ocean to Bombay, or some of the other presidencies; the other across the north part of Syria to the banks of the Euphrates, by that river to the Persian Gulf, and from thence to Bombay. Each of these routes will be attended with peculiar difficulties, and in both a long sea-voyage will be encountered.

In the route by the Red Sea it is proposed to establish steamers between Malta and Alexandria (eight hundred and sixty miles). A steamer of four hundred tons' burden and one hundred horse-power would perform this voyage, upon an average of all weathers incident to the situation, in from five to six days, consuming ten tons of coal per day. But it is probable that it might be found more advantageous to establish a higher ratio between the power and the tonnage. From Alexandria the transit might be effected by land across the isthmus to Suez—a journey of from four to five days—by caravan and camels; or the transit might be made either[Pg484]by land or water from Alexandria to Cairo, a distance of one hundred and seventy-three miles; and from Cairo to Suez, ninety-three miles, across the desert, in about five days. At Suez would be a station for steamers, and the Red Sea would be traversed in three runs or more. If necessary, stations for coals might be established at Cosseir, Judda, Mocha, and finally at Aden or at Socatra—an island immediately beyond the mouth of the Red Sea, in the Indian Ocean; the run from Suez to Cosseir would be three hundred miles—somewhat more than twice the distance from Liverpool to Dublin. From Cosseir to Judda, four hundred and fifty miles; from Judda to Mocha, five hundred and seventeen miles; and from Mocha to Socatra, six hundred and thirty-two miles. It is evident that all this would, without difficulty, in the most unfavourable weather, fall within the present powers of steam-navigation. If the terminus of the passage be Bombay, the run from Socatra to Bombay will be twelve hundred miles, which would be from six to eight days' steaming. The whole passage from Alexandria to Bombay, allowing three days for delay between Suez and Bombay, would be twenty-six days: the time from Bombay to Malta would therefore be about thirty-three days; and adding fourteen days to this for the transit from Malta to England, we should have a total of forty-seven days from London to Bombay, or about seven weeks.

If the terminus proposed were Calcutta, the course from Socatra would be one thousand two hundred and fifty miles south-east to the Maldives, where a station for coals would be established. This distance would be equal to that from Socatra to Bombay. From the Maldives, a run of four hundred miles would reach the southern point of Ceylon, called the Point de Galle, which is the best harbour (Bombay excepted) in British India: from the Point de Galle, a run of six hundred miles will reach Madras, and from Madras to Calcutta would be a run of about six hundred miles. The voyage from London to Calcutta would be performed in about sixty days.

At a certain season of the year there exists a powerful physical opponent to the transit from India to Suez: from[Pg485]the middle of June until the end of September, the south-west monsoon blows with unabated force across the Indian Ocean, and more particularly between Socatra and Bombay. This wind is so violent as to leave it barely possible for the most powerful steam-packet to make head against it, and the voyage could not be accomplished without serious wear and tear upon the vessels during these months.

The attention of parliament has therefore been directed to another line of communication, not liable to this difficulty: it is proposed to establish a line of steamers from Bombay through the Persian Gulf to the Euphrates.

The run from Bombay to a place called Muscat, on the southern shore of the gulf, would be eight hundred and forty miles in a north-west direction, and therefore not opposed to the south-west monsoon. From Muscat to Bassidore, a point upon the northern coast of the strait at the mouth of the Persian Gulf, would be a run of two hundred and fifty-five miles; from Bassidore to Bushire, another point on the eastern coast of the Persian Gulf, would be a run of three hundred miles; and from Bushire to the mouth of the Euphrates, would be one hundred and twenty miles. It is evident that the longest of these runs would offer no more difficulty than the passage from Malta to Alexandria. From Bussora, near the mouth of the Euphrates, to Bir, a town upon its left bank near Aleppo, would be one thousand one hundred and forty-three miles, throughout which there are no physical obstacles to the river-navigation which may not be overcome. Some difficulties arise from the wild and savage character of the tribes who occupy its banks. It is, however, thought that by proper measures, and securing the co-operation of the pacha of Egypt, any serious obstruction from this cause may be removed. From Bir, by Aleppo, to Scanderoon, a port upon the Mediterranean, opposite Cyprus, is a land-journey, said to be attended with some difficulty, but not of great length; and from Scanderoon to Malta is about the same distance as between the latter place and Alexandria. It is calculated that the time from London to Bombay by the Euphrates—supposing the passage to be successfully[Pg486]established—would be a few days shorter than by Egypt and the Red Sea.

Whichever of these courses may be adopted, it is clear that the difficulties, so far as the powers of the steam engine are concerned, lie in the one case between Socatra and Bombay, or between Socatra and the Maldives, and in the other case between Bombay and Muscat. This, however, has already been encountered and overcome on four several voyages by theHugh Lindsaysteamer from Bombay to Suez: that vessel encountered a still longer run on these several trips, by going, not to Socatra, but to Aden, a point on the coast of Arabia, near the Straits of Babel Mandeb, being a run of one thousand six hundred and forty-one miles, which she performed in ten days and nineteen hours. The same trip has since been repeatedly made by other steamers; and, in the present improved state of steam navigation, no insurmountable obstacles are opposed to their passage.

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