this instance owing to the construction of the donkey engine the pump cannot be worked at less than 40 revolutions per minute, at which speed it is fully capable of supplying a 100 horse power boiler at ordinary working pressures, instead of one of only 60 horse power. With high pressure steam superheated and worked expansively, the pump is large enough for a 150 horse power boiler, in which case 3rd per cent. orth of the whole power produced is all that is required for working the circulating pump; and with the improved circular bends that have now been adopted for uniting the ends of the tubes in the boiler there is reason to expect the circulation can be maintained with much less power. No more power is required to work the pump with 80 and 100 lbs. steam than with 20 lbs., since the pressure is the same on both sides of the piston and the only resistance to be overcome is the friction of the water in the tubes, which of course is increased in proportion to the speed; with the boiler now at work the resistance on the piston at the proper speed does not exceed 7 to 10 lbs. per square inch. Originally the delivery pipe G, Fig. 2, into which the steam and water from the tubes are discharged, was only 5 inches diameter inside, which was found too small; in the present boiler it has been made 10 inches diameter. The receiver C is supplied with feed water by one of Giffard's injectors L, Fig. 1, instead of an ordinary feed pump.

It was originally supposed that the mechanical circulation of the water with 9 to 11 times more water forced through the tubes than is evaporated would be sufficient to prevent deposit, by keeping them washed out clean; and this is the case to a certain extent, as all loose matter is washed by the circulation from the tubes into the receiver. Some incrustation however does take place, but not sufficient to present any practical difficulty or cause any damage to the tubes. One of the tubes from the first boiler is exhibited as a specimen, showing the amount of deposit that has been formed during the ten months it has been in use. The deposit is greatest in the lower tubes of the boiler, and decreases in the upper rows: practically it is prevented from accumulating so thick as to cause the tubes to be injured by the heat, since it becomes cracked and loosened from the tubes by their alternate expansion and contraction under the varying temperature of the fire.

At times also nearly all the water is worked out of the tubes so as to let them get quite hot, but not hot enough to cause injury by over

heating; and when the deposit is thus loosened in the tubes it is washed out into the receiver by the circulation of the water.

The dirt

and scale are cleared out of the receiver by a blow-off cock, which is opened for blowing off two or three times a day. It takes about a quarter of a minute to free the blow-off cock from the deposit lodged in the receiver before a full body of water issues from it. Pieces of deposit are blown off which have a circular form, showing that they have been formed in the tubes and then scaled off and washed into the receiver. The semicircular form of the bends uniting the ends of the tubes prevents any incrustation lodging in them by giving an unobstructed passage.

The mode of uniting the tubes together in the former boilers of this construction was with right and left handed screws cut on the ends of the tubes and screwed into the bends: but this make required an entire section of the boiler to be taken out when a new tube had to be put in; and with large boilers this is too much trouble, owing to weight, difficulty of handling, and the impossibility of unscrewing many of the tubes in the bends after they have once been screwed up and put to work. To meet these difficulties a new form of bend has been made in the present boiler, which admits of any one of the tubes in any part of the boiler being taken out, without removing that section of the boiler or interfering with any other joints than those of the tube to be removed. Figs. 4, 5, 8, and 9, Plate 8, show enlarged views of the improved bends. Instead of screwing the ends of the tubes they are made with collars of suitable size welded on, and the ends of the bends are recessed out to receive them the bends are brought up tight against the collars on the tubes by the centre screw bolt M, Fig. 8, which passes through a hole in the bend in line with the centres of the two tubes, and is screwed into the crossbar N bearing against the outside face of the collars. The passage through the bend is made on one side of the fixing bolt, Fig. 9, to prevent it By this plan any of

from obstructing the flow of steam and water. the bends can be taken off through the doorways at the front and back

G

of the boiler, and any tube can be taken out and replaced. The ends of the tubes are passed through the end bearing plates PP, Figs. 4 and 5, which serve also as shield plates to protect the cast iron bends from the heat of the fire; these plates rest on the walls of the furnace, or are suspended at the top from the girders Q, as in Fig. 2. Figs. 6 and 7, Plate 8, show the mode of joining the tubes to the main supply and delivery pipes, which is done in a similar manner by collars upon the ends of the tubes fitting into recesses in the main pipes and held up tight by a crossbar N and stud bolt. By having valves for cutting off the communication between the receiver and tubes, the steam and water can be retained in the receiver during the time of removing a tube; and when distilled water from a surface condenser is used in the boiler, the water can by this means be saved if a tube should burst, and shut off from the boiler while the repairs are done.

The special advantage of this boiler is that steam of high pressure is generated in it with greater safety than steam of low pressure in ordinary boilers. Its construction ensures almost perfect safety: for the receiver C, Fig. 1, Plate 7, the only portion containing any quantity of steam and water capable of causing damage by explosion, is of the strongest form for resisting pressure, of simple construction, and removed from the action of the fire, so that it is entirely free from the injurious effects of overheating and the alterations of expansion and contraction, which are considered to be the cause of so many injuries and explosions of ordinary boilers. The only portion of the boiler exposed to the fire is the tubes, which are of such small capacity that their explosion is incapable of doing any damage and can only cause the fire to be put out by the water escaping from them. This has been confirmed by the experience with the boiler at the writer's works, where a tube has burst on more than one occasion, whilst the boiler and engine were at work; and the effect was so small that the accident was not immediately perceived, until shown by the loss of steam pressure, the steam and water blowing out upon the fire through the leak in the split tube and putting it out. The advantages of high pressure steam are now generally recognised: but a much higher pressure than can be obtained in ordinary boilers and superheating of the steam are required to develop these advantages fully, by cutting off

the steam earlier with a higher degree of expansion. The economy of expansion is now limited by the weakness of boilers in general use; and a large increase of economy may be obtained if much higher pressures can be safely used.

The leading feature of this boiler is the use of the circulating pump, to maintain a constant and regular circulation of the water through the entire set of tubes forming the heating surface of the boiler. This principle of mechanical circulation is found essential in order to carry out completely the idea of a tubular boiler, in which the heating surface consists entirely of the tubes having the pressure internal, and thereby attaining a maximum of strength and safety with a minimum of material. The rapid generation of steam in the lower portion of such a boiler would so far choke the passage of the tubes as to check the natural circulation of the water and cause the tubes to be rapidly burnt out. The objection arising at first against the adoption of artificial or forced circulation instead of natural,-that it is not self-acting and may therefore be liable to cause interruption to the working of the boiler, has been satisfactorily proved by the results of the continued working of this boiler to be practically met by the simplicity of construction of the circulating pump, as shown in Fig. 10, Plate 9, previously described. During the ten months that the boiler has been in continual work the circulating pump has always worked well, and never given any trouble except from causes foreign to its principle of working; such as the water freezing in it and breaking it, which occurred once during the late severe frost. In first raising steam in the boiler no difficulty is experienced from the circulating pump not being at work, since the tubes do not require circulation of the water until steam is raised, and the pump then starts with a small pressure of steam, so little power being required to work it.

The portability of this boiler is an important practical advantage for several cases of application. The largest piece, the receiver, is only one tenth the size of an ordinary boiler of the same power; and the tubes can be packed in bundles, giving great advantage for shipping over other boilers both in the reduction of total weight and in the

increased facility for stowage. The economy of space is very great, and an important advantage in many situations where space is limited and valuable; the space occupied being only one sixth to one fourth of that required for ordinary Cornish or cylindrical boilers of the same power.

Owing to the duplication of parts in its construction, the cost of the boiler is but little more than that of ordinary boilers above 25 horse power, including the circulating pump and all the mountings. A small boiler of the kind costs more in proportion than a large one; for in all cases it is best to have an independent circulating pump, and a small pump costs nearly as much as a large one. In this comparison it is supposed that the steam is worked at the ordinary pressures in both cases, say from 25 to 50 lbs. per square inch; but the suitable working pressure for the new boiler is 100 to 150 lbs. per square inch, with the steam superheated and worked expansively; when thus worked and compared with other boilers in first cost per horse power, the new boiler is much cheaper, and in all cases far cheaper for transporting and setting in masonry. The average thickness of the boiler tubes is not more than inch, and their whole surface is effective heating surface; this results in a great saving of weight compared with ordinary boilers with plates to inch thick. In comparison with marine boilers the new boiler can be made much cheaper than those on the ordinary mode of construction, while the facility for repair gives a decided advantage.

Though the steam and water from the tubes are discharged together into the receiver, there is a complete separation of them, and there has not been the least trouble from priming. More fully to prove the fact of their separation, cocks have been placed on the upper and lower sides of the delivery pipe G, Fig. 1, Plate 7, leading from the tubes to the receiver: from the upper cock nothing but steam was found to issue, and from the lower nothing but water; and supposing priming to be caused by taking steam from boilers exposed to the direct action of the fire, it is effectually prevented in this boiler for the reason that no fire acts upon the receiver containing the water, from which the steam is taken off, and consequently the water remains in a quiet state.