Fig. 70 is a delineation of the launch engines and boiler exhibited by Messrs. Rennie in Paris in 1867, and which combination is employed to drive a pair of screws 2 feet diameter, 3 feet pitch, 34 inches long, and with four blades. One engine is employed

to give motion to each screw, and the steam is condensed by surface condensers so as to obviate the necessity of carrying fresh water for supplying the boiler. The weight of the whole machinery and boiler with water was 3 tons, and without water

21 tons. On the trial of a launch of 10 tons displacement fitted with this machinery, the pressure of steam on the boiler being 80 lbs., about 33 indicated horse power was exerted, and the speed attained was 8 knots per hour. The circulation of the water through the surface condensers is maintained by small centrifugal pumps driven by gearing from the screw shafts.

Here, then, I close my remarks on marine engines of recent construction; and notwithstanding the innumerable and incessant efforts which have been made to introduce new improvements, I do not see that any considerable improvement has yet been introduced. Superheating, from which such exaggerated benefits were at one time expected, has collapsed to its proper dimensions; and it is now found that about the same amount of superheating as obtained in the old flue boilers is the most beneficial. The pressure has been gradually increasing, and that no doubt is a benefit if adequate measures be simultaneously adopted to increase the strength of the boiler. But the existing marine boiler is ill adapted to withstand any considerable pressure; and, as things now stand, to increase the pressure is to increase the risks of explosion. The method of surface condensation now so generally employed in steam vessels I do not believe will be permanently retained, at least in its present cumbrous form; and, on the whole, there is very little that is new in marine engines which can be characterised as a permanent amelioration. The

introduction of the governor and the use of steel for shafts are valid steps of improvement, though not very momentous ones; and, indeed, the use of a steel shaft is only tantamount to the employment of an iron one so much larger than before. We now require marine boilers capable of enduring high pressures of steam with safety, and if salt water is used in the boilers, we require the introduction of some arrangement which will prevent the sulphate of lime from being precipitated on the heating surfaces, which takes place at a temperature answering to 40 lbs. pressure of steam without any concentration of the water at all. We also require the introduction of some simple and effectual mechanism for firing the furnaces, especially in the case of large vessels employed in warm climates. It would also be an advantage, especially in the case of vessels performing long voyages, if some really effectual and unobjectionable method could be introduced of burning the smoke.

LOCOMOTIVE ENGINES.

There are two main objects of aspiration which are set forth in the designs of many of the modern locomotives; the one to burn the smoke so as to enable coal instead of coke to be either wholly or partly used in the furnace; the other to realise great tractive power, so as to enable each goods engine to draw heavier trains than heretofore. Neither of these indications can be said to have been very

perfectly fulfilled by any of the plans hitherto propounded for that purpose; and in seeking to increase the power, various forms of monstrosity have been produced, promising neither eminent success nor great longevity. In particular, the recent goods engines on some of the continental railways are remarkable examples of retrograde improvement; and it does not appear probable that the use of such cumbrous and gouty structures can long be retained after the stimulus of novelty attending their creation has passed away.

The various plans which have been propounded for burning smoke in locomotives are mostly reproductions of old plans long since tried in land and marine boilers, and gradually abandoned. The principle on which these various arrangements are founded is either that of admitting air above the fuel, to burn the smoke, or that of using a sufficient area of fire bars, and a sufficiently thin fire, to enable the quantity of air required to burn the smoke to pass through the fire; and the smoke is conducted either among hot bricks and tiles, or over incandescent embers, to induce the more effectual union of the uncombined oxygen in the air with the unconsumed carbon in the smoke. All these methods, however, are only methods of approximation, which, though they diminish the smoke, by no means prevent it; and the consequence is, that locomotives pretending to burn the smoke, or unlawfully using coal even without this plea, are now spreading such

large volumes of smoke over the face of the country as to constitute a new and serious nuisance. Heretofore coke only was used in locomotives, when of course no smoke was created. But, of late years, they have been gradually sliding into the use of coal; and the probability is, that the nuisance will go on increasing until it becomes intolerable, and is finally subverted by the strong hand of power. Some of the smoke-burning expedients employed are merely hollow pretexts for the evasion of the obvious duty of burning the smoke.

COAL-BURNING LOCOMOTIVES.

It would be impossible to enumerate within the limits to which these remarks have to be restricted, the numerous projects which have been propounded at different times for burning the smoke in steam boilers. Among those who have directed their attention to burning smoke in locomotives, the plans of Gray, Dewrance, Yarrow, M'Connell, Beattie, Cubworth, and Tembrinck, and especially the four last, have attracted most attention, and some of these expedients have obtained a pretty wide introduction. In M'Connell's arrangement the fire-box is divided longitudinally by a water space, so as in reality to form two furnaces like the furnaces of a marine boiler. Air is admitted at sundry openings at the front and sides of the fire-box, and the tubes are considerably shortened in the barrel of the boiler, so as

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