sideration for fuel, oil, time saved, overloading, &c., the following differences will be found :—

Among the advantages claimed for the compound system are:1. Facility in starting. The Author believes, however, that the contrary may be said in many cases. Nevertheless, French locomotives have valve-gears of such design that they readily start their trains, and very soon attain full speed. The inconvenience of sluggishness in starting met with in certain types of locomotives, can be avoided; it is a question of gear.

2. The exhaust-pressure being diminished, it results therefrom that sparks are not thrown out. But the same results may be obtained by adopting variable exhaust, which is not uncommon in France, and by giving in every case sufficient section to the blastpipes. Trials have been made on the Orleans Railway with the fixed exhaust of Mr. Wassner, which seem to give good results: the exhaust is into a hollow ring provided with a series of small taps placed round it. The blast, under these conditions, is more regular, and it seems to facilitate the employment of slack coal; it does not make the fuel scatter in the furnace. A good result also follows with the Lencauchez system, which reconveys to the boiler a portion of the exhaust steam, and diminishes the quantity discharged into the atmosphere, thus promoting also economy of fuel. Some goods locomotives on the Paris and Orleans Railway are provided with the Lencauchez apparatus, whereby an economy of 5 per cent., perhaps more, is gained; for passenger locomotives this system has been given up because of its complexity.

3. Compound engines are more steady. So far compound engines have outside cylinders, or outside cylinders and one or two inside cylinders; now engines with outside cylinders will always be less steady than those with inside cylinders. The Author has had in use, and still employs engines on the two systems, and there are such considerable advantages in having inside cylinders that, except under special circumstances, he finds it preferable to use

them. They are at least as steady as any form of compound engine.

4. The compound engine diminishes the difficulties due to the wire-drawing of the steam.

In ordinary locomotives, wire-drawing induces the superheating of the steam, which occasions the seizing of the friction surfaces of the slide-valve and of the piston. This inconvenience can be remedied by lubrication, if need be, from the foot-plate when running, and also by slightly opening the water-cock of the Le Chatelier apparatus. The advantage of the compound engine in this respect does not give it any absolute value; there remains the fall of pressure from the wire-drawing of the steam; but its importance does not seem to be very great, and may be overcome by special arrangements.

5. The compound engine diminishes condensation and reevaporation.

To compare the influence of condensation in compound engines with that in engines with single cylinders, the result is all in favour of compound engines; but it has been observed that the steam moves so rapidly in the cylinder of locomotives that this is less noticeable than in engines where the speed is less. Steam at the initial temperature coming in contact with the cylinder at the exhaust would encounter theoretically a surface at a temperature of 212° Fahrenheit, and would be rapidly condensed while raising the temperature of the cylinder, which in its turn would produce an evaporation of the water during expansion, which is useful, and evaporation during exhaust, which is a loss. Another phenomenon must then intervene, particularly in locomotives, namely, the thermic influence of compression. There succeeds then a series of phenomena in a space of time which is worthy of notice. At high velocities locomotive driving-wheels attain a speed of four or five revolutions a second, or eight or ten cylinder strokes. The series of phenomena under consideration ought then to be developed in one-eighth or one-tenth of a second. It follows that practically the cylinder of a locomotive takes a mean temperature, the variations from which are very slight, and that the results of condensation are very different from those which are produced in a low-speed stationary engine. The compound engine, besides, fully makes up for these considerations by the dimensions of the large low-pressure cylinders, which in Mr. Webb's "Dreadnought" attain 23 to 26 inches in diameter, and which consequently present a considerable cooling-surface. Very great condensation results therefrom, which is disadvantageous.

But it does not therefore follow that compound engines should be abandoned. When in Austria, the Author constructed a compound locomotive on the Webb system for the Austro-Hungarian State Railways; and had not many of his colleagues done so too, he would have been one of the first to have made experiments with it, but under present circumstances he awaits the results of trials on a large scale made in various quarters.

The compound engine is the solution of a difficulty felt in countries where fuel is very dear, and under special circumstances; but Watt has said: "In all things, and especially in mechanics, it is necessary to seek simplicity," and compounding is a complication in the locomotive. High pressures cannot be actually used in locomotives, because of the character of the valve-gear, which is not adapted for prolonged expansion. With the compound system the latter can be prolonged further. In the express engines of the Paris and Orleans Railway, adapted to a pressure of 142 lbs. per square inch, the exhaust commences at 52 per cent. of the stroke, and the steam escapes at 42 lbs. If the pressure is much higher than 142 lbs., steam will be exhausted at 70 or 85 lbs. per square inch, which would hardly be economical. At speeds of 47 to 50 miles an hour, this exhaust or 52 per cent. of the stroke is less inconvenient, for it has been proved by diagrams that the steam has not time to escape fast enough; the fall of pressure is not rapid, as in engines of low speed, and the effective expansion is greater than the normal expansion. The compound locomotive would then certainly be theoretically an economical solution; but it would be necessary, as in stationary or marine engines, to be able to give the cylinders the dimensions desired. But, between the sole-bars there is insufficient room, and moreover, space is wanting on account of the gauge. Practically, the Author does not believe that the economy realized makes up for the difficulties of maintenance arising from mechanical complication, and the supplementary expenses of lubrication, even for two-cylinder compound engines; for, despite all precautions, the work is unequal on each side of the engine, and the result is evidently loss of power, and dislocations more or less rapid and injurious. The double-expansion compound locomotive is thus not economical. Recourse must therefore be had to the threeor four-cylinder compound engine; but in this case, in the Author's opinion, the economy in fuel will be almost counterbalanced by the increase of expenditure of construction, of lubrication, and of maintenance of the machinery. Finally, he may cite the conclusion submitted to the Thirteenth Congress of the Chief

Engineers of the Steam-Users Association, held in Paris on the 11th, 12th, and 13th of November, 1888, by Messrs. Coste and Bour:

1. The compound system applied to locomotives has little elasticity, and the normal performance of an engine designed for special conditions of work may become very defective as soon as these conditions are departed from.

2. The compound engine is less adapted to regular work than a single-cylinder engine, when both are applied to variable work.

3. The compound system does not lend itself easily to the performance of dual functions, in the sense that the conditions of work of a condensing engine will differ from those of a noncondensing one. Thus if, in a particular engine, any departure is made from a certain average performance, one or other of such performances is likely to be very defective.

4. Non-condensing compound engines present, in an exaggerated degree, all the defects found in condensing compound engines.

5. The compound engine can hardly be considered an industrial motor, susceptible of being established according to fixed types capable of meeting the general requirements of workshops. Good in certain cases, in others it may give rise to serious disappointment, unless special precautions have been observed. There are, moreover, cases where it ought never to be adopted. The singlecylinder engine, on the contrary, admits of types being established of more general use.

Point 2 applies in a special manner to locomotive engines; and the general result of these conclusions entirely confirms the Author's opinion on the application of the compound principle to loco

motives.

(Paper No. 2416.)

"The Bore of the Tsien-tang-Kiang."

By W. USBORNE MOORE, Commander R.N.

AMONG the periodical phenomena of Nature, none are more deserving of attention than the tides; and of the various phases of the tides, none are more worthy of study than the phenomenon of the bore. To Civil Engineers, especially those engaged in works on the seacoast, it will probably have a peculiar interest, as it presents a marked instance of wasted power. Possibly, the day is not far distant when the energy exerted by the tides generally, and by this crowning development in particular, may be brought under control in practical affairs.

The bore, called also the "Hygre" and "Eagre" in England, "Mascaret" in France, and "Pororóca or "Prororóca" in Brazil, is well known in six or eight rivers in the British Islands, in two or three of those in France, in some of the Indian rivers, in the branches which compose the mouth of the Amazon, and in one river at least in China. It is rarely observed except at spring-tides, and, as a rule, shows itself on the days of full and of new moon, appearing with the first of every flood for three or four days. succeeding those phases, after which the tide comes in with only a swift rush without noise or violent commotion.

The conditions necessary for its creation appear to be three:(1) A swiftly flowing river; (2) An extensive bar of sand, dry at low-water, except in certain narrow channels kept open by the outgoing stream; (3) The estuary into which the river discharges must be funnel-shaped with wide mouth, open to receive the tidalwave from the ocean.

When either of these conditions is absent, the bore is not known. Thus, in the Thames, although the third condition is present, the first and second conditions are absent; for the stream is not swift, and there is no bar dry at low-water. In the Severn, all three conditions are present, and there is a bore, not a very large one, but the highest in these islands.

1 Derived probably from the Icelandic “báva," a "billow"; "eagre," from the French "eau-guerre," or "water-war.