mixture of steam and water subsides, and the level shown by the glass gauge will fall until it corresponds with this. As, however, any lowering of water level causes the feed-valve to open wide, and thus reduce the pressure in the feed discharge pipe, the feed-pump immediately increases speed, and pumps the water level back again to the shutting-off point, at about to glass. This lowering of level on stopping is to be expected, and does not indicate any defect. The feed-pumps will continue to work till the water level is restored.

Conversely, when getting under way quickly, as the feed-pumps will have pumped the water level back to about or glass, whereas when at work the quantity required is much less than is contained in the boiler under these circumstances, the excess of water is passed into the steam pipe to be caught at the separator, or if the increase of speed be made slowly, it is gradually evaporated, the feed inlet-valve closing while this is happening. We thus explain the usual phenomena with these boilers-i.e. feed-pumps working rapidly when slowing down, and either working slowly or stopping when the speed is being quickly increased.

Gas-mixing appliances.-The space between the fire and the lowest tubes is not large, so that unless complete combustion of gases takes place within a short distance of the fire their combustion will take

Feed
Collector

place above the lowest tubes, and much heat will pass off to the funnel, which, had the combustion taken place lower, would have been abstracted by the water in the tubes. With a properly regulated and thin fire which passes the proper amount of air there is less likelihood of this, but it has been found that the forcing of air in small streams under pressure above the fire has a beneficial effect in thoroughly mixing the gases, and so accelerating their combustion. It should be observed that the apparatus is simply a gas-mixing one, and not for forcing the rate of combustion of the coal.

FIG. 56.

The arrangement consists of a square air-pipe a, Fig. 56, about 10 or 12 inches above the fire-bars, into which air is forced in small jets by an air compressor at a pressure of 5 to 15 lbs. per square inch. A series of nozzles about 3-inch diameter, slightly inclined downwards, discharge the air among the gases above the fire with such force as to reach all parts of the furnace and thoroughly mix the gases with the oxygen. In the front casing are sight holes, two or three in number (see Fig. 49), with sliding covers, which can be moved on one side to enable the condition of the fire to be inspected and the admission of air regulated, so as to obtain the best result, which, with experience, the person in charge is able to do.

Facility for repairs.-Pipes are screwed into the steam collector to the height previously explained, as shown in Figs. 49 and 50, and the top of the elements are secured to them by flange joints. The bottom of the element is secured to the feed-box or collector by means of a conical joint, shown in detail in Fig. 52, the jointing material consisting of a thin conical nickel ring inserted between the coned nipple and the conical hole in the lower box. The single bolt shown is all that is

necessary for satisfactorily making this joint. Should it be found necessary to renew any tube, this can be accomplished by withdrawing the 'element' containing the tube from the boiler.

The bolt which makes the joint with the feed collector at the lower part, and also the four bolts making the flanged joint of the top tube with the steam collector are removed, the front of the element is then lifted sufficiently to allow the lower box to clear the cone of the feed collector, sufficient spring being allowed at the top tube of the element to enable this to be done. The element is then free and can be drawn out into the stokehold to enable the new tube to be fitted.

Economiser type of Belleville boiler. The preceding description refers to the Belleville boiler as fitted in 'Sharpshooter,' 'Powerful,' 'Terrible,' and four other cruisers, but the later vessels building for the British Navy are provided with a modified and improved variety described as the economiser' type. In this type the boiler is divided into two parts, viz. (a) a lower part consisting of elements of exactly the same construction and with the same fittings as in the previouslydescribed boiler, except that the height is considerably reduced, each element containing fourteen tubes, and sometimes only twelve, instead of twenty-this part is called the 'generator'; and (b) a series of smaller elements termed the economiser,' placed 4 or 5 feet above the top tube of the generator, in the boiler uptake, and through which the feed-water is first discharged and raised in temperature before being admitted to the steam collector.

The space between the generator and the economiser acts as a combustion chamber, in which any unconsumed gases that may have passed the lower parts of the boiler are provided with additional air to assist their combustion, and given room to expand, prior to traversing the spaces between the economiser tubes and giving up heat to the feed-water contained therein.

The economiser generally consists of one or two less number of elements than contained in the generator, but of smaller diameter, viz. 2 inches instead of 4 inches, as in the generator. The number of tubes in each economiser element varies from 12 to 20, depending on the height available. The length of the economiser is less than that of the generator portion, and in most boilers building is about 6 ft. The economiser is formed similarly to the lower elements, and consists of straight zigzagged tubes screwed into malleable cast-iron end boxes.

The sketch, Fig. 57, shows a boiler with the elements in the generator each seven pairs of tubes high, each economiser consisting of elements nine pairs of tubes high, a considerable space being allowed between the generator and economiser. In this arrangement the feedwater proceeds from the pump to the valve A at the feed regulator chamber, exactly as previously described, but on leaving this valve, instead of entering the steam collector direct, it proceeds through a non-return valve to the pipe B, called the 'cold water collector,' at the bottom of the economiser, and then enters the element through an orifice in each lower front box, and is pumped upwards to and fro through the elements and abstracts heat from the gases. It issues into a pipe c, called the 'hot water collector,' at the top of the elements, and is thence led to the non-return valve D on the steam collector. The fronts of the economiser tubes are fitted

with cleaning doors, and the economiser casing is provided with doors at the front for access to the tubes.

A few small nozzles P are supplied with compressed air from the furnace air pumps, and inject air into the combustion chamber space to supplement that admitted by the gas-mixing jets above the fire. A small safety valve is fitted on the discharge pipe from the economiser.

Very high results have been obtained with this modified type of Belleville boiler, as will be seen from the following table, giving the results of trials made by Admiralty officers on one boiler erected on shore. Good Welsh hand-picked coal was used, and a large separator fitted to ascertain if any water was carried over with the steam.

The Thornycroft boiler ('Speedy' type), Fig. 58, consists essentially of a central upper steam cylinder A, and two smaller lower water cylinders B, these latter being fitted about the level of the fire-bars. A series of steam generating tubes of small diameter are fitted between the upper cylinder and each of the lower water cylinders. They are secured at each end by being simply rolled into the cylinder plating by means of the roller expander, the parts of the cylinders into which they are rolled. being made thick enough for this purpose. These tubes form practically the whole of the heating surface of the boiler, and the inner row on each side is curved in such a manner that they are close together at the top and form the roof of the furnace or combustion chamber. Each of these two rows is made into a wall of tubes, through which the gases cannot penetrate except through orifices left for this purpose at the bottom of the tubes. This is effected by bending the tubes just above the lower cylinder and fitting each alternate tube into the

space between its neighbours, so that it forms a closed wall, except for the spaces E F at the bottom. It will be seen that the curvature of the tubes allows considerable freedom of expansion.

The outer row on each side is similarly made into a wall of tubes through which the gases cannot escape except through openings G H

left for this purpose at the top. The gases, therefore, generated in the furnace enter among the tubes through the opening left at the bottom of the inner walls of tubes; they ascend, traversing the whole of the tube surface between the inner and outer walls, and emerge on their