patent boiler at the present moment.

We have now entirely overcome a great drawback mentioned before, which was the accumulation of steam in the tubes. For the last four years I have never come across a single case where we have had that. We have got the proper ingress for our water into the pipes, and the proper egress for the steam. If that were not the case would the economy that has been stated to have taken place in the Birkenhead have resulted? Would tubes now, when containing steam, be exposed to fire for three hours or six hours even, without burning through when the tubes were only ths of an inch in thickness. I think these facts will prove that the tubes really contain water and not steam. The economy that has been mentioned we also find in land boilers. By very careful experiments made by engineers we have found that with the best constructed land boilers there is about 10 per cent. saved. We do not go in for 30 or 40 per cent., but we say 10 per cent. That is certified. On the other hand in Germany our boilers are regularly tested by the German Government to work up to 150 pounds, and we never have had an accident to any of these boilers. The tubes have failed through bad workmanship, or accumulation of scale, or other defects, but we have never had damage done to anything beyond the tube itself. Cast-iron heads and caps have also been objected to; but really the fact remains that we have never had accidents; and surely if that is the case then for the Board of Trade to object to cast-iron heads and caps is not, as I submit, a just objection. As a proof of the value of the boilers there are many people in this country who had the boilers five or six years ago, and these people have been continually ordering the boilers. They have tried them and some have now as many as fifteen or sixteen boilers, and all those firms are satisfied. I therefore submit that these boilers are suitable for land purposes, and I should think that marine engineers could also make them suitable for marine purposes. We ourselves— I speak of the makers of the boilers believe that we are able to overcome, or have overcome, all the difficulties that we once had to contend with; but the fact of the matter is that we are not marine engineers who believe them valuable for marine engines, and we should be most happy if the matter were taken in hand. I thank you, Gentlemen, for the indulgence you have afforded me.

Mr. J. WIGHAM RICHARDSON: It would be interesting to the Meeting if Mr. Mac Farlane Gray would state, as we are all interested in the subject of the different pressures, what is the theoretical saving as between 75 pounds pressure of steam and say 100 pounds, supposing you expand the steam down to the same temperature as the steam enters the condenser. I believe he can give us that information.

Mr. MAC FARLANE GRAY: It is only a very small per-centage. I cannot say exactly off hand. At Stockton Mr. Blair got up to 100 pounds after having from me the table of twos. I asked him afterwards if he had expected more. He said, "The result was I did not get as much as you told me."

Mr. J. FORTESCUE FLANNERY: I do not know that there are many points, Gentlemen, I can say much about. The first speaker alluded to a well-known and fully-recognised fact, that we only get one-tenth practically of the theoretical calorific energy of the coal; and it is the earnest desire of all marine engineers to increase the practical effect from the coal. I am very glad that Mr. Gray has pointed out that the increase in the economy is not directly the same as the increase in the temperature or in the pressure of the steam. I did not at all mean to convey that, and I am very glad that Mr. Gray did not credit me with the intention of conveying it. I have desired to emphasize the fact that the higher the pressure the greater the economy. Of that there cannot be any doubt. I am also glad that Mr. Gray very frankly admits that the conclusion he drew from the failure of the Montana boilers was not quite the conclusion which on further consideration he is inclined to draw.

The CHAIRMAN: Gentlemen, I am sure we are all much indebted to Mr. Flannery for having brought before us in so very clear and interesting a form such an important subject, and I presume you will all give him your thanks for the Paper which he has read this evening.

Mr. MAC FARLANE GRAY: I did not mean to say that the cause of the failure of the Montana boilers was not what said it was. I said that I should have been inclined to pay more attention to, and to have brought more prominently forward, the other consideration of temperature.

ON SOME TRIALS OF SIMPLE AND COMPOUND ENGINES.*

By RICHARD SENNETT, Esq., F.R.S.N.A. and M. E., Assistant to Chief Engineer,
Devonport Dockyard, Member.

[Read at the Seventeenth Session of the Institution of Naval Architects, 7th April, 1876; Vice-Admiral Sir F. W. E. NICOLSON, Bart., C.B., Vice-President, in the Chair.]

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DURING the last Session of the Institution I had the honour of reading a Paper on Compound Engines." In it I endeavoured to show that compound engines were, both in theory and in fact, more economical and more suited to the present requirements of engineering than simple expansion engines. In the last paragraph in that Paper I referred to some engines, simple and compound, then in course of construction for the Admiralty, the trials of which would probably throw some light on the comparative merits of the two systems. Since that time three sets of engines of 360 indicated horse-power, two with simple expansion, for the gun-vessels Sheldrake and Moorhen, and one compound for the Mallard, have been completed, and the official trials made at Devonport. No special trials were made to ascertain the total quantities of water required per indicated horse-power, in the two classes of engines, as the ships proceeded to their destinations as soon as they were completed, so that there was no time for experiments. I have thought, however, that a few particulars of the engines and the results of their measured-mile trials, with the quantity of water used by each, as computed from their indicator diagrams, would perhaps be interesting to the Members of this Institution, and it will be seen that the results obtained agree very closely with those given in my Paper of last year.

The machinery and boilers of the Sheldrake and Moorhen were constructed by Messrs. Napier and Sons, of Glasgow, and the Mallard's machinery and boilers by Earle's Shipbuilding and Engineering Company, Hull. The boilers in all three ships are practically the same, the only difference being that the tubes in the Mallard's boilers are 23 inches diameter, whilst in the Sheldrake and Moorhen they are 21⁄2 inches. The boilers are cylindrical, each containing two cylindrical furnaces, 2 feet 6 inches

* Received 4th April, 1876.

diameter, and 4 feet 6 inches long. These two furnaces terminate in a common combustion chamber, 3 feet 2 inches deep. The tubes do not return over the furnaces, but are continued from the combustion chamber to the smoke box at the forward end of the boiler, the boilers being placed in a fore-and-aft direction. On the top of the combustion chamber, between the ends of the furnaces and the tubes, is suspended a bridge, which deflects the flame and heated gases before they enter the tubes, the use of which has been attended with satisfactory results.

The engines of the Sheldrake and Moorhen are simple expansion engines directacting, the connecting-rods working between the cylinders and the cranks. Both cylinders with their ends and covers are jacketed with steam of the full boiler pressure. Expansion valves, capable of cutting off the steam as early as one-fifteenth of the stroke, are fitted on the backs of the main slide valves, and are worked by means of eccentrics on the crank shaft, the different grades of expansion being regulated by links. The surface condenser tubes are horizontal, and the feed pumps take their supply direct from the hot well, no feed tank being fitted. The condensing water is circulated through the condenser around the tubes by means of a centrifugal pump, the diameter of the circulating pipes being 63 inches.

The engines of the Mallard are compound, with return connecting-rods. The piston-rod of the low-pressure cylinder is taken through the cylinder cover at the back, and a slipper guide is attached to it for the support of the piston. The barrel and both ends of the high-pressure cylinder are jacketed with steam of the full boiler pressure, and those of the low-pressure cylinder with steam at a pressure of 30 pounds per square inch. Gridiron expansion valves, arranged to cut off the steam as early as th of the stroke, are fitted to work on faces at the tops of the slide-jackets of both high and lowpressure cylinders, and are worked by eccentrics on the crank shaft, with links to regulate the grades of expansion. The surface condenser tubes are horizontal, as in the Sheldrake and Moorhen; but the condensed water passes from the hot well into a wroughtiron tank, from which the feed pumps draw their supply for the boilers. The condensing water is circulated around the tubes by means of a centrifugal pump, the diameter of the circulating pipes being 7 inches.

The weights of all three engines are practically the same, being between 77 and 78 tons; but the cost of the simple expansion engines was somewhat in excess of that of the compound engine. The principal particulars of the different engines and boilers are given in Table I., and it will be seen that the only practical difference between them is that one is a compound engine and the others simple expansion engines. Any difference in the results obtained may, therefore, be fairly attributed to the difference in the system of construction, and not so much to any difference of detail. In the cases

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under consideration the simple engine had however some advantage in this respect over the compound engine. In the Sheldrake and Moorhen the length of stroke and speed of piston were greater than in the Mallard; and the exhaustion valves of the simple engines were fitted to work directly on the backs of the main slide valves, whilst in the Mallard they were ordinary gridiron valves working on seatings on the tops of the slide-valve chests, so that the loss from clearance would be greater than if the valves had been fitted as in the Sheldrake and Moorhen.

TABLE I.-PRINCIPAL DIMENSIONS OF THE MACHINERY AND BOILERS.

The ships were all completed at the close of last year, and the performances taken for comparison are those on the measured-mile trials, when the machinery was in charge of the contractors themselves. The results of these trials, with some deductions therefrom, are given in Table II.