power of the remaining engine more than one-fifth. It is on reliable record that when the twin-screw steam ship Mary lost one of her propellers at sea in 1868 she completed her voyage without difficulty with one engine; it being only necessary to angle the rudder about five degrees to correct the one-sided thrust of the remaining propeller. She steamed up the Thames with ease, turning the sharpest bends, although her length between perpendiculars was 230 feet. As another instance it may be mentioned that both engines of the twin-screw steam ship Ruahine broke down when coming home from the West Indies; one engine was made good by borrowing from the other, and the voyage continued at 7 knots. Now compare these facts with the results of the steam ship Atrato, and many other cases of detention of mail steamers from accidents when at sea; and estimate the value of the time lost in a foreign port while waiting for duplicate parts, also the cost of dispatching other steamers at short notice to carry on the mails, and expenses and annoyances connected with transhipment of passengers and cargo, and I have no hesitation in saying that the balance will show largely to the advantage of the vessel with the longitudinal bulkhead and twin-screw engines; and this applies also to ordinary merchant steamers, though in a less degree. Another advantage is that the screw-propellers are more deeply immersed, and the feeding water besides being less broken and more free from air has greater freedom of access to them. There will be less vibration of the hull, as the water leaving the screw acts equally on both sides of the vessel; and when light and pitching heavily the engines will be less liable to race and the shafts to fracture from sudden and unequal strains. Referring to the value of deep immersion of screws as a great accessory to efficiency, I would remind you of the Paper read before this Institution in 1874 by Professor Osborne Reynolds, recording the results of a series of experiments, the summary of which is to the effect that screws should be one-third of their diameter under water; if less than this they race and froth the water; and this result was invariably obtained even at speeds varying as much as 50 per cent.

The action of the rudder will not be affected when under weigh, and if it should get damaged the vessel can be manoeuvred by the screws alone. This special feature of twinscrew propulsion commends itself especially for canal or river work, where low speeds and great nicety of steering are so necessary. When laying to from stress of weather the twin screws can keep the vessel's head up, even when she makes no progress, being independent of the rudder; but unless the single-screwed steamer is making headway, the rudder fails to act and her head will fall off. Hence, either her screw must be turned more rapidly or sail be set. If the former the engines will be more affected by racing.

Duplicate engines in vessels of large size and full power also offer advantages on account of the individual parts being little more than half the weight, and the shafts are

more likely to be free from flaws; besides only about half the usual weight of spare gear will be carried, and that will be of much handier proportions. As regards the cost and weights of duplicate engines, say each of 250 horse power and upwards, and with one set of spare gear, as compared to single-screw engines equal to their collective power and also with one set of spare gear, I have it from a firm of first-class marine engineers that there will be no difference between them in these respects.

A twin-screw vessel has also unusual facilities for avoiding collision; and if injured, and constructed on the principles advocated, will be able to assist either herself or the participator in the collision. Also, in event of war, she could render valuable assistance on account of extraordinary manoeuvring powers, and in the event of one set of machinery being injured has the other to fall back upon.

Owing to the twin-screw system affording greater immunity from accident and increased facilities in steering, rigging may be almost dispensed with in fast ocean steamers, thereby adding to the vessel's stability and saving expense in this department.

In conclusion, I may repeat that by the adoption of the twin-screw engine the cargo space is increased, as previously noted, by 196 tons at 50 feet, which at 40s. per ton-and the vessel making four round voyages to Bombay per annum-would add £3,140 to the gross yearly revenue, and much more than compensate for the cost of any additional engine staff. Also by the application of the longitudinal bulkhead system combined with the additional athwartship bulkheads, the vessel is not only much increased in strength but is also rendered practically unsinkable, which together with the greater security afforded by twin-screw engines will reduce the sea risks, and so lower the insurance premiums; consequently the shipowner can command higher freights without increasing the cost of transit to the shipper, and such vessels would undoubtedly be more largely patronised by passengers as soon as the general features of security became known to the public.

And by the adoption of the general arrangement proposed by Captain Charles Chapman and myself for large ocean steamers of the size and capacity already noted, the additional 780 tons of cargo space, if freighted as above, would add about £12,400 to the gross annual revenue.

Q Q

ON A SMALL COMPOUND STEAM ENGINE.*

By THOMAS MOY, Esq., Associate.

[Read at the Seventeenth Session of the Institution of Naval Architects, 8th April, 1876; the Right Hon. LORD HAMPTON, G.C.B., D.C.L., President, in the Chair.]

THE engine which I am about to describe is an improved arrangement of that which was brought under your notice two years ago. My first idea was to form the cylinders and reservoir in one casting, adding a steam dome of suitable dimensions, and mounting the whole upon a suitable frame, with two tube plates, generating tubes, and furnaces.

Messrs. Dudgeon entered into a contract to construct a compound engine upon this principle, and when a considerable sum of money and much time had been spent upon it, it was found that the casting, which was a heavy one, was unsound in two places; much delay thereupon took place, and that engine was for various reasons abandoned. Another design was prepared and the plans approved of, in order that Messrs. Dudgeon might carry out their contract; but funds running short with me, and Messrs. Dudgeon soon after closing their works, the engine has never been completed.

From the experience gained in the several experimental engines that I have had constructed, I have been able to so modify the several parts that all difficulties in construction are entirely eliminated, and engines of any size and power can be produced with facility.

In the engine before you the cylinders and valve-box are cast in one with the base plate which forms the bottom of the water and steam reservoir. The cylinders, &c., are bored truly, and the top covers inserted. The base plate is faced up, and the steam and water dome is bolted down upon it, the tube-plate fitted with tubes is bolted to the side of the dome.

By this, which appears at first sight but a slight alteration in the arrangement, all the parts can be cheaply made, the work being nearly all lathe work, and for economy in weight, space, first cost, and working it is difficult to see where there is much room for improvement.

* Received 14th March, 1876.

This engine is worked at from 120 pounds to 200 pounds to the square inch, and as excellence of workmanship combined with cheapness and simplicity are aimed at, the valve motion is of a very simple construction. A semi-circular main valve with a stroke of only half an inch, and a semi-circular cut-off valve with a stroke of of an inch, effect together the distribution of the steam, which is cut off at one-third of the stroke in the high-pressure cylinder, and by the action of the main valve passes direct from the high to the low-pressure cylinder, and as the cylinders are in area as one to three the steam is expanded nine times.

The tubes are made of solid drawn metal for internal pressure, the substance being sufficiently thin to allow of the rapid passage of the heat to the water, and yet of sufficient strength to allow of an ample margin of safety; and each tube is tested to half a ton to the square inch. The very rapid circulation of the water which takes place in the tubes is a perfect preventative to burning the metal, and as a matter of practice they seem to be imperishable; and, as was well pointed out by Mr. Charles Wye Williams more than twenty years ago, the rapid circulation of the water is the best thing for rapid production of steam. I have also found in practice that by placing the tubes very near together little heat can escape, the tubes being kept comparatively cool by the circulating water, and absorbing the heat as a dry sponge absorbs water.

A tube plate is on the table which has had some months' work, and which shows that there is no amount of deposit taking place in the tubes beyond a very thin scale.

The arrangement now brought under your notice is intended to utilize common gas as the fuel; and for this purpose I use Wallace's gas burners, which burn entirely without smoke. But the furnace can be arranged for any other fuel. The first heat goes of course to the steam-generating tubes. I am arranging a peculiar form of feed apparatus which will utilize the surrounding heat outside the steamgenerating tubes. Surrounding this feed apparatus I have a cylinder of non-conducting material, and outside this a cistern containing the feed water: so that the products of combustion are pretty well utilized.

As I am just now conducting experiments with this feed apparatus, I would rather reserve my remarks upon it to a future time. In the meantime the engine can be worked with an ordinary feed pump or injector, and there is no difficulty whatever in preserving the water level.

In conclusion I think I may say that I have invented a steam engine which for simplicity, economy, safety and lightness, will stand comparison with any engine that has yet been produced, and for facility of transport will compete with any other engine and boiler in existence.

DISCUSSION.

Mr. HENRY LIGGINS: My Lord and Gentlemen, I should like to make one remark only in reference to this particular steam engine standing before me. It appears to me to have been constructed for the use of a steam launch. I would ask the inventor whether I am right in saying so?

Mr. Moy: No; I should make a lighter one for a steam launch. That is a stationary engine of what I call my heaviest class.

Mr. LIGGINS: I am merely objecting to the form of it for a steam launch. We always try in steam launches to get the weight of the boiler as low as possible, that is to say, a horizontal boiler is preferred to a perpendicular one. With regard to steam-generating powers, I should think it is a very rapid one, but that is a matter of experience and not perhaps of private opinion. It seems a very elegant little apparatus, and extremely suitable for a steam launch, if you can place the boiler horizontal instead of perpendicular.

Admiral J. H. SELWYN: I spoke last night on Mr. Flannery's Paper, with reference to the enormous advantages to be gained from tubulous boilers, as compared with the existing type in the service which we call tubular. Here is a specimen of what can be done by the rapid circulation of water. Whereas now for boilers we carry an enormous weight of water, and evaporate slowly and inefficiently; this system permitting, as it does, of a rapid circulation of water through the tubes, which may be arranged vertically instead of horizontally, or in any other shape; yet if there be a rapid circulation of water you do not need to carry more water in the boiler, and therefore you do not need to expend more heat in heating that water, which you only bring out again when the boiler is cooled down, than is absolutely necessary for evaporation per pound of fuel per hour for work to be done. That is one of the great advantages of tubulous boilers. The advantage of the application of fuel also is a very large one, since the sides of the furnace may be coated with any material like fire-brick, and thereby more perfect combustion will be obtained; whereas in our present boilers we can do nothing of the kind without sacrificing largely the heating surface of the boiler. The time also in which the steam is got up in all boilers constructed on the tubulous system is an important element of success for naval purposes, since, as we shall rarely go about under full steam, and may very suddenly be called upon after a foggy day or a dark night to employ our full power, it is essential to the efficiency of the ship that she should be able to get her steam to a full pressure in the shortest time. This can never be done if large bodies of water are to be heated. If we have got by tubular boilers to something like three-quarters of an hour for raising steam instead of an hour and a half or two hours as used to be the case before tubular boilers came into use, that is a large measure of improvement, but it is nothing in any way approaching to the improvement which is promised by this system. The tubulous system offers the advantage (I am not speaking now with reference to any particular model which is before us, because there will be naturally modifications which no doubt the author of this Paper would import into it, if he were asked to provide for salt water and its encrustations, as well as for the ordinary work of a small steam engine) that you can carry all your weights very low down. I think it is not quite understood that here the weight of water is not that of the old system of boiler, and consequently the objections do not obtain to the placing of the boiler as it is here. The principal weight I take it in the old system is of the reservoir of water,