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through the outer or boiling tubes, where it is converted into steam, which, mingled with water, ascends into the collector, being divided from the front chamber by a partition. The steam passes from the collector through a steam drier or 'superheater' on its way to the engines. This boiler is at present in use on German cruisers.

The boilers which belong to the horizontal type are all more or less suitable for continuous service on battleships and cruisers. The Express type of boilers-with

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tubes approaching the vertical *—was adopted in the first place, and still is retained almost exclusively, for torpedoboats and destroyers. The prototype of these boilers was the Du Temple, designed originally for driving the propeller of a flying machine, and improved by M. Normand, M. Guyot, and others: its earlier examples were marked by an excessive curvature of the tubes, with the idea of increasing the heating surface and affording freedom for ex

* It forms the 'accelerated circulation' class in M. Bertin's classification.

pansion. In the Du Temple-Normand design the waving of the tubes is greatly diminished. In the Du TempleGuyot it is entirely abandoned, the tubes passing from the water chambers to the steam drum in a single curve.

There are few chapters in the history of mechanical engineering more interesting than the story of the development of the Thornycroft and Yarrow boilers, both of which belong to the vertical type. There are certain resemblances, but more important differences, between the two. Both are of triangular outline, having water chambers at the lower corners, and a steam drum or collector at the apex. But

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while the Thornycroft tubes are curved, those of the Yarrow are straight. The Thornycroft tubes are brought into the steam drum above the water level, being also arched over the fire grate; and they are so disposed that the gases must pass along the whole course of the tubes before escaping. Though the straight tubes of the Yarrow boiler do not offer so large an amount of heating surface as that furnished by curved tubes, they are more easily cleaned internally, and are more readily replaced. The Yarrow boiler, moreover, has no return or circulating tubes, and yet the circulation is maintained perfectly. The explanation is that the tubes which are nearest to, and those which are

farthest from, the fire form efficient circulating elements, due to the differences in weight of the columns of mixed steam and water in the first, and of solid water in the second. Yet in the majority of boilers of this class, the down-coming or return tubes form an essential element in the circulation. The Blechynden boiler resembles the Yarrow in having tubes which are nearly but not quite straight, being slightly bent to permit of expansion; and it has no external return-tubes.

WATER
LEVEL

FURNACE

COLLECT

Fig. 8.-The Yarrow Boiler.

This brief account of the elements involved in the principal Navy types of water-tube boilers should deter one from hasty conclusions; and if we consider further what requirements have to be fulfilled at sea, where every distinct class of vessel steams under different conditions, the folly of a dogmatic attitude will be yet more apparent.

The principal requirements that must be fulfilled by an efficient Navy boiler are as follows: occupation of the minimum of space, reduction of weight as far as practicable,

capacity for rapid generation of steam, and endurance under hard firing. The less frequently repairs are needed the better; but the construction should be such that, when these are inevitable, they may be accomplished with the least possible expenditure of time and labour. In what degree do the leading types of water-tube boilers fulfil these exacting and in some cases contradictory conditions?

No type is capable of fulfilling all these requirements absolutely, and therefore none is suitable for all-round service in battleships, cruisers, and torpedo-boats. If the condition of endurance is satisfied, capacity for express steaming must be to a greater or less extent sacrificed. A familiar case for comparison is that between boilers worked in the mercantile service and those in the Royal Navy. The broad distinction between a commercial vessel and a battleship is that in the first a regular rate of speed is the chief requisite, in the second adaptability alike to slow steaming for long voyages, and to hard steaming for short periods. Durability, an essential condition in a liner, is of less value in a battleship than capacity for severe duty during brief periods, which may be of priceless value at critical moments. In the first case, facility for effecting rapid repairs is of less importance that it is in the second. The requisites of a torpedo-boat, again, differ from those of a battleship, for high speed is the cardinal consideration in the former, while durability is of secondary importance, and is in fact sacrificed in many cases. The tubes in torpedo-boat boilers are so small and their arrangements are such that it is difficult to locate any defect or to renew them while at sea; but the power generated is so enormous in proportion to the fuel consumed and the weight of the boilers that this cardinal advantage compensates for rapid wear and tear. Here it is neither a question of uniform speed and permanent service, as on a liner, nor of occasional forcing, as on a battleship, but of how to attain the highest speed with the greatest possible reduction in weight.

With regard to questions of space occupied, weight, and other matters, we cannot go beyond general statements. M. Bertin has been at great pains to compile tables of data by which comparisons can be made between boilers of various types with regard to weight of water in the boilers, weight of the boilers themselves, floor space occupied, &c. The net results in regard to the total weight, including water,

are stated by M. Bertin (p. 339): in cylindrical boilers of the Admiralty type, 1.124 tons per square foot of grate area, against only 0.539 tons in the Belleville, 0.466 in the Niclausse, and 0.453 in the Thornycroft. According to Mr Marshall's experiments,

'on a given weight the Belleville type will develope, with natural draught and for long periods, approximately the same power that the cylindrical [i.e. Scotch] type will develope for short periods of a few hours under extreme forced draught. Also, on the same weight, the Yarrow type will develope 14 per cent. more than the Belleville for long periods, and 78 per cent. more for short periods. Again, comparing the Yarrow with the cylindrical, it is seen that it will develope 50 per cent. more power for long periods, and 65 per cent. more for short periods, on a given weight.'

In time of war, the water-tube boilers, in which steam can be produced from cold water in half an hour, have a great advantage over the Scotch boilers, for which from four to six hours of firing are required to produce the same result. Battleships fitted with Scotch boilers are kept in readiness for steaming at a few minutes' notice by the constant banking up of the fires; but this involves consumption of coal and constant attendance of stokers, while with water-tube boilers no such necessity exists.

The problem of getting the largest possible quantity of heat out of the combustion of the coal burnt on the furnace grates, includes, first, the quantity of coal which can be consumed in a given area in a given time, and next, the highest utilisation of the products of combustion in their passage among the tubes. The first is a question of the volume of air supplied by natural or by forced draught; the second depends on the way in which the flame and gases are conducted from the fire grate. An abundant supply of air for combustion is secured by the use of forced draught, while the burning of the gaseous products is effected by delaying their passage among the tubes. Some boilers are better fitted to fulfil these conditions than others. In both respects they are found to differ very widely. Numerous devices for prolonging the contact of flame with tubes are embodied in the principal boilers.

But that a boiler should be capable of raising plenty of steam in a short space of time is still only one condition

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