riously diminished the amount of the moving power, whereas condensation by jet was almost instantaneous, and the efficiency of the piston throughout the entire stroke was more uniform.

Mr. Watt also found that a fur collected around the tubes of the condenser, so as to obstruct the free passage of heat from the steam to the water of the cold cistern; and that, consequently, the efficiency of the condenser was gradually impaired, and could only be restored by frequent cleansing.

It is stated by Mr. Hall that a vacuum is preserved in his condensers as perfect as that which is maintained in the ordinary condensers by injection. It is objected, on the other hand, that without the injection water and the air which accompanies it being introduced into his condensers, Mr. Hall uses as large and powerful an air-pump as those which are used in engines of equal power condensing by injection; that, consequently, the vacuum which is maintained is produced, not as it ought to be altogether by the condensation of steam, but by the air-pump drawing off the uncondensed steam. To whatever extent this may be true, the efficacy of the machine, as indicated by the barometer-gauge, is only apparent; since as much power is necessary to pump away any portion of uncondensed vapour as is obtained by the vacuum produced by the absence of that vapour.

A tubular condenser of the form proposed by Mr. Hall is represented in fig. 126.; a is the upper part of the condenser to which steam is admitted from the slide after having worked the piston; k is the section of a thin plate, forming the top of the condenser, perforated with small holes, in which the tubes are inserted so as to be steam-tight and water-tight. Water is admitted to flow around these tubes between the top k and the bottom d of the condenser, so as to keep them constantly at a low temperature. The steam passes from a through the tubes to the lower chamber ƒ of the condenser, where it is reduced to water by the cold to which it has been exposed. A supply of cold water is constantly pumped through the condenser, so as to keep the tubes at a low temperature. The air-pump g is of the usual construction, having valves in the piston opening upwards, and

similar valves in the cover of the pump also opening upwards. The water formed by the condensed steam in ƒ is drawn

Fig. 126,

through the foot-valve, and after passing through the pistonvalves, is discharged by the up-stroke of the piston into the hot well. Any air, or other permanent gas, which may be admitted by leakage through the tubes of the condenser, or by any other means, is likewise drawn out by this pump, and when drawn into the hot well is carried from thence to the feeding apparatus of the boiler, to which it is transferred by the feed-pump.

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

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.* 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 coalmines, 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 quan

Appendix I., on Marine Boilers, by J, Dinnen; Tredgold on the Steam Engine, second edition.

tity 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 paddlewheels 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, how

ever, has been lately removed by coating the boilers, steampipes, &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 the Caledonia, and it was subsequently adopted in another vessel, the machinery of which was constructed at Soho, called the James 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 arrange