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. 2773 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.

ARTICLE 359, PAGE 410.

Governors.—Amongst various forms of marine engine governors, Dunlop's pneumatic governor has been found very serviceable for compound marine engines. In this governor the controlling power lies in the level of the water outside the ship, as this varies from time to time with the pitching of the vessel. The apparatus consists of an air vessel, which is placed in communication with the sea by a cock. The air becoming compressed, the action is communicated to the governor, which consists of a casing containing an air-tight space in connection with a pipe, and fitted with an india-rubber diaphragm. This diaphragm as it rises and falls (due to the pressure of the air, as acted upon by the varying level of the waves) acts upon a spring, which communicates with the throttle valve by means of a link.

Feed-water heaters are being introduced into boilers, with good results. The principle upon which these appliances depend is the delivery of the feed-water to the boiler at a uniformly high temperature, such as 200° F. and higher, and the freeing of the water from impurities. Exhaust steam is used, and in some cases live steam from the boiler. The exhaust steam is passed into tubes surrounded by the feedwater contained in a strong casing, and, after circulating, passes off. The water receives so much heat from the surfaces of the tubes, and is afterwards further heated by a coil of pipes containing live steam. Any matter precipitated as a deposit is received in a filter chamber containing charcoal, and the water thus purified and heated passes to the boiler.

Fuel Feeders.—Of late years many improvements have been made in connection with steam boilers, with the view of increasing the economy of the fuel, and thereby rendering the boiler a more efficient steam generator. The loss due to emission of smoke

may in many cases be met by careful firing. Appliances known as "mechanical stokers" have been introduced, whereby the fuel is gradually fed to the furnace through a hopper arrangement in front, means being adopted to work the coal to the back of the furnace. Another method of fuel feeding has been recently tried, in which the coal is charged upon a movable truck, which is pushed inside the grate, and below the fire-bars; the fresh coal is then lifted by gearing and pushed up below the burning fuel, and thus partially cokes before being consumed. Forced draught is used on board some steam-vessels, such as torpedo boats.

ARTICLE 407, PAGE 471.

Preservation of timber, for railway sleepers, &c., from decay is attempted by various processes. Kyanising consists in the immersion of the timber in a solution of corrosive sublimate, in the proportion of 1 part of sublimate to 100 parts of water, the wood being left in the solution for two days and upwards, according to thickness. In another process chloride of zinc is used. Creosoting consists in first placing the timber in a cylinder and applying superheated steam, and thereafter establishing a vacuum in the cylinder; by these operations the sap is withdrawn; thereafter a "dead oil," containing creosote heated to about 160°, is forced at a pressure of about 20 lbs. into the cylinder, and fills up the space from which the sap has been removed. This process, however, varies with the condition of the wood when used.*

Faus. Ventilation of the workings in mines is now usually accomplished by fans. Of these there are various forms, some of the pump order, and some depending on centrifugal action. The Giubal fan, belonging to the latter class, appears to be now largely used, and has been described as a "centrifugal ventilator, with eight or ten flat blades set backwards from the radius, encased with mason work, and provided with a regulating shutter at the discharge orifice." Much additional advantage is obtained from the addition of a short chimney or vertical outlet for the air, having a form enlarging outwards to the top. An efficiency of about 60 per cent. is claimed for these fans.

The Giubal fans, running about 60 revolutions per minute, with diameter of 30 feet, give about 100,000 cubic feet of air per minute; whilst fans of 37 feet diameter at the same speed give as much as 200,000 cubic feet.

Where the pits are very deep, furnace-ventilation is thought to be fully more economical than fans, as much as from 150,000 to 200,000 cubic feet of air per minute having been obtained by a single large furnace.

ARTICLE 207, PAGE 221.

Hydraulic Machinery.-Hydraulic machinery has now been applied successfully to the raising of water from deep mines. In mines of ordinary depth, a steamengine at the surface, by means of rods passing down the shaft, works a set of rams placed at different points in the shaft. When the depth is great, the weight of the pump rods becomes excessive, and other methods have to be adopted.

Recently, extensive hydraulic arrangements have been carried out in Nevada, where 1600 galls. per minute had to be raised from a mine about 2400 feet deep, and delivered at a point 800 feet above the bottom of the shaft.†

The arrangement consists of a steam-engine, with pumps and accumulator at the surface, which forces water down to a hydraulic engine placed at the foot of the shaft; the latter acts upon pumps by which the water is elevated to a height of over 800 feet.

For further information on the preservation of timber, see Trans. American Society of Civil Engineers, Vol. XI. † See Trans. Inst. Engineers and Shipbuilders in Scotland, Vol. XXV.

1

The steam-engine on the surface is compound, with a 10-feet stroke, the steam pressure being 80 lbs. The high-pressure cylinder, 35 inches in diameter, is placed before and in line with the low-pressure cylinder, of 70 inches diameter. The pumps are four in number, and have 84-inch plungers, worked direct from the cross-head of the piston rod. The water is forced through an 8-inch pipe to an air vessel 25 inches in diameter and 70 feet high, and thereafter down the shaft to the hydraulic engine at bottom. The exhaust water returns to the surface through 10-inch pipes, to be again used.

The pressure in the air vessel is 960 lbs. per square inch, whilst the pressure on the pipes at the bottom of the shaft is about 2000 lbs. per square inch.

The underground machinery consists essentially of two hydraulic engines fitted with four plungers, each 6 inches in diameter, and having a 10-feet stroke. Each pair of these plungers carries a second 14-inch plunger, which raises the water to a height of about 800 feet to the Sutro tunnel. The inlet and outlet valves are so arranged that the one shuts the other. Cast-iron air vessels are used and rubber valves.

Hydraulic machinery is now largely used for engineering work, such as rivetting, flanging, punching, shearing, &c., and for moving swing bridges and working cranes. The pressures used vary from 700 to 1500 lbs. per square inch, an accumulator being employed.

Hydraulic machinery is now also used on board war ships for loading the heavy guns, and in some cases the ordinary gun-carriage is done away with, the guns simply resting on blocks, which are free to slide backwards or forwards with the motion of the gun. Hydraulic cylinders are used for checking the sliding motion due to the recoil, and also serve for placing the gun again in position.

ARTICLE 233, PAGE 242.

Hastie's Differential Screw-Nut consists of a nut having two screw threads of different pitch. Thus, for fixing a piston to its rod, a nut is used with a screw cut on the inside to fit the thread on the piston rod. On the outside of the nut another screw is cut of a finer pitch than the internal thread (the difference in practice is about one-tenth). A corresponding thread to that upon the outside of the nut is cut upon the piston.

The nut being first entered on the piston rod, its outer thread is then entered into the piston block, and, being screwed home, draws the piston rod into its place at onetenth part of the speed of the nut itself. A corresponding power is in this manner obtained to make a tight fit.

Any tendency to slip back is also overcome, the angle of the thread being very fine, or equivalent to a screw having forty threads to the inch.

ARTICLE 376, PAGE 433.

Hollow shafts of Whitworth compressed steel are now frequently used for the propeller shafts of our large ocean-going steamships. In some cases the crank shafts are also made hollow. Considerable saving of weight is obtained by using these shafts, as the outermost parts of a shaft or axle are those which give the greatest resistance to the torsional action to which they are subjected. When the crank shaft is solid, it is usually built up of several pieces-that is to say, the cranks are shrunk on to the crank pin and straight part of the shaft, after being heated over a fire to induce the necessary expansion. Great care has to be taken so that all may be properly in line. Solid crank shafts for engines of 8000 or 10,000 horse-power reach to 26 inches diameter.

The diameter of a crank shaft, in terms of the indicated horse-power, may be expressed as follows:

Where ddiameter of shaft in inches, and N=number of revolutions per minute.

ARTICLE 240, PAGE 253.

Valve Gear Motions. -Various forms of link motion have been devised with the view of obtaining the necessary reciprocating motion of the valve without the intervention of eccentrics. One of these, known as Joy's valve gear, appears to carry this out with great precision.

In this arrangement the eccentric is done away with, and the necessary movement obtained from the motion of the connecting rod itself.

Fig. 1 shows the application of this arrangement to locomotive engines, and Fig. 2 to marine engines (see page 583).

A link, L (see Fig. 1), whose extremities are secured to the connecting rod, C, and to a radius bar, B, has a lever, E, attached to it, whose centre or fulcrum works in a slot, S, curved to the radius of a link, K, which connects the other end of the lever with the valve spindle, V.

The slot can be turned or rotated so as to incline on either side of the vertical at will. By giving the slot these inclined positions, forward or backward motions of the engine are obtained.

In some cases the slot is dispensed with, and the fulcrum of the lever is carried by a radius rod. The path of the centre, or fulcrum, in this case can be shifted by means of a weigh lever and hand wheel (see Fig. 2).

One great advantage of this form of valve gear, when applied to locomotives, is that the slide-valve can be placed on the top of the cylinder, and thus larger cylinders and greater bearing surfaces for the driving axles obtained. Another advantage appears to be that it gives four points of acceleration and four points of retardation in the stroke, these points of acceleration corresponding to the points of "admission" and of "cut-off ;" and by this peculiarity not only is the admission and the cut-off executed very much more smartly than by link, but the "release" occurs somewhat later and the compression very much later, and, as a result of this, an independent expansion valve is unnecessary.

The dotted lines in Fig. 1 show the paths of the various centres.

ARTICLE 381, PAGES 440-447.

Cable Tramways.-Cable tramways have now been used with successful results in some of the large cities in the United States.

In San Francisco an endless steel wire rope, 3 inches in circumference and 11,000 feet long, is supported. on sheaves 11 inches in diameter, placed 39 feet apart, the whole lying in an iron tube placed between the car rails and beneath the roadway, in which an opening of about one inch wide is left.

Through this opening the connection between the car is made, and consists of an arm, which grips the rope when the car is to be set in motion. The rope is kept constantly going by means of a powerful steam-engine. Additional sheaves are arranged at changes of direction of the rope.

The

rope passes round grip pulleys at the engine-house, 8 feet in diameter. The cars are said to be more easily started and brought to rest than when drawn by horses. The stoppage is effected by allowing the “ grip" to slacken its hold of the rope, and putting on the brakes.

In Chicago about 4 miles of these tramways are laid down, some of the ropes