row were continually bulging and giving out from overheating, and the vessel was frequently off duty for repair. The exit of the steam was no doubt retarded by the turnings and counterturnings it underwent before reaching the surface. The greater vertical distance through which the steam rises from the surface of the water before rushing to the engine the less is the tendency to carry water away with it; not only because there is more time given for the water to fall back from the ascending steam, but because the pull caused by each stroke of the engine is less severely felt at the bottom tubes if they are some distance below the steam outlet. The diagram shows that in this design a very small height was between the stop valve and the bottom tubes, and the engines being paddle engines with large cylinders and slow stroke, there is no doubt that the tendency of the water to forsake the bottom tubes was greatly enhanced from this cause, although primarily due to imperfect circulation. This defect would

also be increased by the stoppages and lying-to of the vessel in her work as a ferry steamer, allowing accumulations of the steam pressure. Not only so, but the deficient room rendered it impossible to get in so much boiler power as was afterwards thought necessary; and the manufacturers allege that consequent forcing of the fire contributed in no small degree to the failure of the tubes. The sister ships to the Birkenhead, working on the same station and with very similar engines, but boilers of the ordinary type, offered an excellent standard of the average consumption of coal, and the saving in the Birkenhead, in comparison with one of the boats, amounted to 17 per cent., and by comparison with the other, to 25 per cent. This result is the more remarkable when it is remembered that the pressure was only 40 pounds, and that the expansion in the engines of the Birkenhead could not be greater than in the sister ships; the comparative economy was therefore entirely due to the more effective arrangement of the parts, the thinness of the metal in the tubes, and the direct action of the flame upon them

favouring the evaporation; had the pressure been higher and the expansion greater the economy would no doubt have been largely increased. Notwithstanding the excellent result in point of economy, the Ferry management condemned the boilers after nearly three years' work, on account of the continual breakdowns, and the vessel is now fitted with boilers of the ordinary type made by Messrs. James Taylor and Co. The cause of the comparative failure of these boilers has already been indicated; the twisted passages for the exit of the steam and the low vertical height, added to heavy firing, caused the bottom tubes to be occasionally full of steam, and being exposed to the full action of the fire they were quickly destroyed.

Another vessel fitted with boilers on Root's patent, and of very similar design, but still more contracted passages for circulation, was the steam ship Malta, of 2,000

tons, belonging to the Merchants' Trading Company, Liverpool. They were in this case found to give off steam with great rapidity, and steam could be raised in them in about half the time necessary in boilers of the ordinary type. The working pressure was 50 pounds. The boilers went on about twelve months, during which time the vessel visited the Mediterranean and the Baltic. Very little priming took place, but

the same defect as in the other cases developed itself; the circulation of the water was insufficient, some of the bottom tubes nearest the fire bulged and gave out, and the boilers were finally taken out to convert the vessel into a sailing ship. The engines were very old before the Root boiler was fitted, and their worn-out condition contributed to the owners' decision to alter the vessel.

I am informed that many of these Root boilers of the same manufacture are working successfully on land, and where large space may be given, and leisurely delivery of the steam is possible, they have been found highly successful.

FIG. 12.

Figs. 10, 11, and 12 represent the system of boilers fitted to the steam ships Amalia and Palm under Ramsden's patent. These boilers worked very satisfactorily for about four years; but at the end of that time it was found that the construction of the boilers not fully providing means for the removal of the incrustation, the plates had seriously deteriorated, and the boilers were consequently removed.

Trials were made of the Howard patent safety boiler in the steam ships Fairy Dell, Meredith, and Marc Antony. These boilers were all constructed about the same date, 1870. Their working on board ship in all these cases was quite unsatisfactory; the lower tubes burst, others leaked, and very great priming took place. These results were traced to the same general causes that led to the difficulties experienced with the Birkenhead and Malta, and it is unnecessary to discuss them in detail.

66

After discussing so many failures of the water-tube system, I cannot close this Paper without describing the most recent working example of the tubulous boiler for marine purposes. One of the earliest advocates of the tubulous boiler is Mr. John Watt, of Birkenhead, and his system is shown in Fig. 13 which represents the boiler of the steam flat Gertrude. The boiler consists of a series of inclined tubes connected at "their ends to rectangular water chambers. To the top of the chamber is a steam "receiver from whence the steam is taken to the engines. The rectangular chambers are stayed like the ordinary locomotive fire-box, by means of stays through the door " and tube plates, one end of each stay being left sufficiently long to enable the tube

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"doors to be secured. The tubes are in diagonal rows, and the course of the flame is "in zigzag direction amongst them." A very brief consideration of the natural action

of the steam and water in this boiler will show that it possesses features superior to some of the boilers already described. As the steam is generated in the tubes, the angle at which they are inclined will facilitate its ascent to the upper rectangular water space a, and once there, its ascent to the steam receiver is easy and certain, its upward passage being unobstructed; at the same time the water to take the place of the steam generated contained in the lower rectangular chamber b will by a natural action ascend from it, and so a good circulation is secured. The necessary feature in a boiler of this arrangement is a separate

FIG. 13.

outlet for the steam, and a separate inlet for the circulating water; any contention between the ascending steam and the descending water must be fatal.

The shorter and wider the tubes are made the greater will be their safety, because the amount of evaporation will vary as the heating surface of the tube. Now the surface of the tube increases directly as the diameter, but the volume contained increases in the square of the diameter, and this fact goes far to explain the danger of long narrow tubes, the length still further increasing the difficulty of the exit of the steam. In the case of the Propontis some of the smaller vertical pipes were so far attenuated as to be 8 feet long and 2 inches diameter. I am informed that the Gertrude's boiler has worked for some months, and with so perfect a circulation that no deposit or scale has been formed, although salt water is partially used and the engine is not fitted with a surface condenser.

Another design strongly advocated by the inventor is that of Mr. Wigzell's, shown in Figs. 14 and 15. It will be noticed that this is a compromise between the tubulous boiler and the boiler of ordinary type; the flame will surround the inclined cylinders, and the cylinders being perforated with tubes it will pass through these on its way to the chimney, its exit through them being made compulsory by blocking up the outer spaces between the cylinders by diaphragm plates. No boilers of this design have yet been tried at sea, though the inventor assures me that most satisfactory results are obtained by their use for land purposes.

I set out with the statement that, so far as present lights enable us to judge, greater economy in the use of coal at sea can only be obtained by increased boiler pressure; that cylindrical vessels of small diameter are the only practical generators of high-pressure steam; that up to the present time no water-tube boiler on a large scale has given perfectly satisfactory results at sea; and that the practical trials already made are in themselves sufficient to teach useful lessons, and perhaps to indicate to some extent the practice of the future. If there is some specific feature in the design of the

boilers of the above-mentioned ships which is common to them all, and has contributed to their failure, then a valuable lesson will have been drawn from them. Insufficient circulation of the water has been more or less a characteristic of the above cases; and to get at the root of the matter it is necessary to discover why the circulation has been insufficient. In the ordinary boiler the reservoir for steam is vertically above the surface upon which it is generated, and no obstruction to its ascent is offered except the friction through the superincumbent water.