of the various forms are the screw-brake, chain-brake, vacuumbrake, hydraulic-brake, and compressed-air-brake, in all of which, by means of mechanism extending below the carriages and actuated by the engine-driver or guard, the whole or part of the wheels of the train can be braked. In the first two methods, rigid or flexible bodies are employed to transmit the power required, whilst, in the others, the same object is accomplished through the medium of fluids. In the hydraulic-brake, water at a high pressure from a pump on the engine is conveyed by a pipe; in the vacuum-brake the air is removed, and in the airbrake the air is forced under pressure to the points required. In the automatic arrangements, whether of air or vacuum, there are reservoirs. It has been found desirable to adopt reservoirs or vessels having pistons immediately in connection with the brake blocks, the object in the automatic arrangements being to keep up a certain condition in the chambers, whether of pressure or vacuum, by which, if destroyed either intentionally or accidentally (as through the breakage of a pipe), the braking action may at once take place.

In some cases 1 seconds is sufficient to apply the brakes, and fast trains can be stopped in about 300 yards.

From experiments made by Capt. Douglas Galton, C.B., F.R.S., on the effect of brakes upon railway trains, it appears that

(1.) The retarding effect of a wheel sliding upon a rail is not much less than when braked with such a force as would just allow it to continue to revolve, the distance due to friction of the wheel on the rail being only about of the friction between the wheel and the brake blocks.

(2.) The coefficient of friction between the brake blocks and the wheels varies inversely according to the speed of the train; thus, with cast-iron brake blocks on steel tires, the coefficient of friction when just moving was 330,

Further it was found that this coefficient was affected by time; thus, starting at 27 miles per hour, the coefficient was ∙171, after 5 secs. 130, after 10 secs. 119, after 15 secs. 081, and after 20 secs. 072; at 47 miles per hour the coefficient at starting

was 132, falling after 10 secs. to 070; and at 60 miles per hour falling from 072 to 058.

These coefficients are further influenced by material and weather. It was found that the distance run by a train on the level at 50 miles per hour, varied with the percentage of the total weight of train used for retardation, as follows-with 5 per cent. 555 yds., 10 per cent. 277 yds., 20 per cent. 139 yds. 30 per cent. 92 yds.

The author points out among other conditions that a perfect continuous brake should be fitted to act upon all the wheels of engine and carriages; that it should exert upon the brakes of each pair of wheels within two seconds a force of about twice the load on these wheels; that the brake block pressure should be such that the friction between the block and wheel may not be greater than the adhesion between the wheel and the rail; and that the action should be automatic, in the event of a separation of the train or failure of connections.

ADDENDUM TO ARTICLE 108, PAGE 112.

From a calculation of the co-efficient of friction of the flow in the 4-foot main pipes of the Loch Katrine Waterworks, made by the late Professor Rankine from data supplied by Mr. J. M. Gale, C.E.,* the following was obtained:

where ƒ co-efficient of friction, g = gravity, i = rate of declivity, m = hydraulic mean depth, and v velocity of flow in feet per second. Professor Rankine further states that Darcy's co-efficient of friction, reduced to British measure, would give

ADDENDUM TO ARTICLES 261 AND 353,
PAGES 334 AND 501.

Compound Engines (called Double Cylinder Engines in the text) are now generally used for marine purposes.

* See Trans. Inst. Engineers and Shipbuilders in Scotland, Vol. XII.

The arrangement is that of two cylinders, a smaller and a larger, placed vertically and side by side. The steam passes from the boiler into the smaller cylinder, and, after doing work there, it passes into the larger cylinder, where the remainder of its energy is utilised. The capacity of the large cylinder is usually about three times that of the smaller, and the steam is cut off in the small or high pressure cylinder at about half stroke. A considerable range of expansion is thus obtained, reaching in some cases to ten times. About half of the work of the steam is done in each cylinder.

The cranks of such engines are usually set at or about right angles, and the exhaust-steam from the high pressure cylinder passes first into a "receiver," from which it enters the low pressure cylinder at the proper time. This receiver may be differently arranged, but it is virtually an enlargement of the steam passages from one cylinder to another, the space being sufficiently large to receive the exhaust-steam driven out by the piston of the small cylinder until the larger piston is in such a position that this steam can act upon it.

Where, as in some cases, compound-engines are constructed with their cranks nearly opposite each other, a receiver is not required, as the steam passes directly from the one cylinder to the other.

The advantage of the compound engine lies in the economical use of steam through high expansion, the lessening of excessive variation of strain on the moving parts through the distribution of the pressure on the two pistons, and the more uniform temperature at which the cylinders can be maintained, as the low pressure cylinder alone is in communication with the condenser.

In some cases, two low pressure cylinders are used, and the steam is expanded from the small cylinder into these larger cylinders. The principle of action is, however, the same, as the quantity of steam originally received from the boiler, when expanded, will theoretically perform the same amount of work, whether this expansion takes place in one cylinder, or in two, or

more.

In the compound-engine, we have thus a similar action to the single engine, working with same ratio of expansion where, for a part of the stroke, the pressure on the piston is from steam in direct communication with the boiler, and, for the rest of the stroke, the pressure is that due to the expansive action of the

steam.

The castings required for large compound cylinders are complicated and heavy, such diameters as 60 inches and 104 inches.

for the high and low pressure cylinders respectively, being now required for large vessels.

Of late years the principal advance made in the marine engine has lain in the use of higher pressures, and consequent greater range of expansion. To obtain the latter, the compound system has lent itself in a variety of ways. Starting from two cylinders, high and low pressure, we have now commonly three cylinders in our large ocean-going steam-vessels, and these cylinders again have been variously arranged. In the case notably of the City of Rome, there are six cylinders arranged on what is known as the tandem form, where three large and low-pressure cylinders have three small and high-pressure cylinders placed above them, the piston-rods passing right through each pair of cylinders, and connected to three separate cranks. A usual form of late has been one high-pressure cylinder, with two low-pressure cylinders placed on each side of it, and in line of the ship's length, the pistons of these three cylinders being connected to three cranks on the shaft.

The use of steel plates, together with the corrugated furnaces now used in marine boilers, has enabled the engineer to use higher pressure of steam than formerly, and thus the advantages to be obtained by using steam expansively have been realised in practice.

The advantage of surface condensation, whereby fresh water is used in the boiler instead of salt water, as was common at one time, is also on the side of economy of fuel.

The tendency of modern improvements in the marine steam engine and boiler is towards economy of fuel, and to increased efficiency in proportion to weight of machinery and boiler, and thus render available more of the ship's hull for cargo or passengers.

Tubulous boilers, in which the water is contained in the tubes, have been tried from time to time, but have not as yet been practically successful, difficulties arising from defective circulation, and the tendency for the tubes to be affected by deposit from impurities in the water.

The Servia, added in 1881 to the Cunard fleet, has engines similarly arranged to the above, the high-pressure cylinder being 72 in. diameter, and each of the two low-pressure cylinders are 100 in. diameter, the stroke being 6 ft. 6 in. The high-pressure cylinder is fitted with piston valves, and the low-pressure cylinders with slide-valves. There are seven boilers, having a total grate area of 1,050 square feet, and heating surface of 27,000 square feet. The indicated horse-power is 10,500.

In the above type of three-cylinder engine the steam is passed from the central high-pressure cylinder right and left to the two low-pressure cylinders.

The latest arrangement of the three-cylinder engine, and one which is found to give great economy of working, is to pass the steam from the high-pressure cylinder into an adjoining lowpressure cylinder, which in turn exhausts into another lowpressure cylinder, and finally from that to the condenser. This appears to have been first tried in the "Propontis," engined by John Elder & Co. in 1874. In the Aberdeen, built in 1882 by R. Napier & Sons, three cylinders are adopted, the steam passing from one to the other as just described. In this arrangement there are three cranks placed at 120° on the shaft. The diameters of the cylinders are 30 in., 45 in., and 70 in., the stroke of piston being 4 ft. 6 in. The horse-power, as indicated on trial trip, was 2631, the engines working at 70 revolutions per minute. The steam pressure is 125 lbs. per square inch, and the consumption of coal on trial appears to be as low as 1.28 lb. per indicated horse-power per hour.

Since the successful introduction of the triple-expansion engine in the ocean-going steamer Aberdeen, the more recent large and powerful steamers added from time to time by the great shipping companies have been fitted with these engines, and in some cases with a still further extension of the principle in the quadruple expansion engine. The Admiralty have also adopted the tripleexpansion system for the Navy.

The two most powerful war cruisers at present afloat (1897), the Powerful and Terrible, just completed for the British Navy, are vessels measuring fully 500 feet long, by 71 feet beam and 43 feet deep. The displacement is 14,200 tons, and the engines, tripleexpansion driving twin screws, work up to 25,000 I.H.P.

From a thirty hours' trial of the Powerful at 18,000 I.H.P., the coal consumption was about 15 tons per hour, or at the rate of 1.838 lb. per I.H.P. per hour. The boiler pressure was 232 lbs., vacuum 26.6 inches, revolutions 103, and the speed at this was 21 knots. On a thirty hours' trial at 5,000 I.H.P. the speed was about 14 knots. At full power the speeds of these vessels were fully 22 knots.

power

The immense power developed by such marine engines necessitates crank-shafts of large diameter; hence in some cases these are made hollow, and of steel. Propeller shafts are also made hollow.

ADDENDA TO ARTICLES 312 AND 313, PAGES 459 AND 461.

The pressures now commonly used at sea with triple- or quadruple-expansion engines are about 150 lbs. per square inch and upwards, and the consumption of coal under 2 lbs. per indicated horse-power per hour.

To withstand such pressures, the shell-plating of boilers is