war vessel, and what has hampered the use of oil fuel by the British navy is, no doubt, the fact that while there is an ample coal supply in these islands, oil fuel is geographically out of reach. Latterly, however, naval authorities are seriously grappling with the oil fuel problem, with results which are likely, it is thought, to lead to important developments. It is thought that liquid fuel ought to be successfully used on torpedo boats. The evaporative duty required from the boilers of destroyers is greater than that required from the boilers of any other type, and, as the author reminds his readers, while it is possible to burn enough liquid fuel to produce the required duty in boilers hitherto using coal at natural draught, or even coal at moderate forced draught, difficulty has been found in burning enough oil-fuel with burners of the destroyer type to produce the same duty as that realised under coal at great air pressure. The question of economy in fuel in destroyers when at full power is of comparatively little importance; but the production of the maximum power is essential, and the further experiments now in progress will probably solve the difficulty. The experiments conducted by the Liquid Fuel Board has shown that it is now possible to force the combustion of oil, and that the greatest evaporation per square foot of heating surface secured with coal can be greatly exceeded by an oil installation of modern

design. The problem of successfully installing an oil fuel appliance on board a warship, as Mr. North points out, grows more complicated as it is further investigated. The arrangement of Holden's burner fitted to a marine boiler is shown in fig. 2.

The appendix contains a series of tables showing the results of tests made by the United States Liquid Fuel Board in a Hohenstein water-tube marine boiler under forced and natural draught, and using air burners and steam burners.

A word may be said in conclusion as to the use of oil fuel for metallurgical and other purposes. It should, the author believes, form an unequalled medium for smelting operations; the chief objection being that of cost. Various forms of smelting furnaces designed for the use of oil fuel are described, but, as the author points out, although the smelting of iron ore by the use of oil fuel has often been suggested, the attempts made have so far been nei ther mechanically nor commercially successful. In the blast furnace, if coke or charcoal should be entirely replaced by oil, the charge would, in all probablity, become too dense to allow the combustion gases to escape freely.*

"Oil Fuel: Its Supply, Evaporation, and Application," by Sydney H. North. With numerous illustrations."-Charles Griffin and Co. Price 5s. net.

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N anticipation of the more strenuous conditions caused by the shorter headway planned, the District Railway Company has had installed a system of signalling for which great improvements over all earlier systems are claimed. This plant presents features of more than ordinary interest, and is, moreover, the first installation of its kind in this country.

The section of line between Hanger Lane Junction and South Harrow, has now been worked electrically by the District Railway for over a year, with results that augur well for its success on the more important sections of the line. The length of this trial section is about five miles of double track, and it is divided into block sections, varying in length from 1,400 ft. to about 3,500 ft. On the District Railway generally there are both positive and negative insulated traction rails; the running rails not being used for the traction current. The space between stations is usually divided into two blocks, that is, a "starting" and a "berth" block, and the signalling is on the "normal clear principle.

The signals themselves are of the ordinary mechanical type, and in no way specially adapted for power or automatic working; consequently, they are being replaced by special arms with Westinghouse electriopneumatic signal motors, fitted close under them, and the ordinary counterweight abolished for one directly on the signal arm as shown in the illustration (fig. 1). The motors are enclosed in cast-iron casings, and, being essentially strong and simple, are not affected by climatic conditions. They are controlled by small pin valves, worked by means of electro-magnets in the local signal circuit. Signal cabins are required only at each end of the branch, or where there is a cross-over road; in the latter case they are normally closed, being opened only when required.

An automatic stop prevents trains from over-running home signals (fig. 2). This consists of an iron arm between the track rails actuated by a compressed air motor acting in unison with the adjoining signal

motor. When the signal goes to danger" this arm is elevated to a position in which it engages with a cock on the air brake system of the train; thus the brakes are instantly and automatically applied if, for any reason, the driver should run past the signal. No great saving in first cost need be expected from an automatic installation, as the initial expenditure would probably be quiet as great as that incurred in laying down the mechanical system. The working expenses, on the other hand, should be very much reduced, the saving in labour being a most important item, and the cost of renewals and repairs trifling. But, undoubtedly, the greatest advantage of an automatic system is its suitability to the requirements of a very frequent and quick service. The blocks may with facility be made as short as is compatible with the running speed and brake power of the trains. The strain on the signalmen, inseparable from such a service, is abolished, and the safety of passengers is thus independent of the personal equation.

The principles of the system may be best understood by reference to the diagram (fig. 3). One of the track rails is electrically continuous through the

whole length of the plant, and constitutes the positive conductor from the generator to the individual track sections. The other rail is cut up into block sections by means of special rail joints insulated with fibre. All uninsulated rail joints are bonded to ensure electrical continuity. Power for the operation of the system is generated at 65 volts pressure by motor generators placed centrally, the negative terminals of which are connected to an insulated negative main running the entire length of the system. This main is connected to each section of the sectionalised track rail at a point near the latter end of the block, i.e., the end at which the train leaves, as distinct from the end at which it enters. It will be seen, then, that there is a potential difference of approximately 65 volts. between the continuous-positive-rail and the insulated negative

main.

Resistances are interposed in the connections between the negative main and the sectioned rail, which reduce the potential difference between the rails to, in the present instance, from 3 to 6 volts, according to the length of the block and various local conditions.

Let us now consider the circuit for a single block unoccupied by a train. The current from the positive brush of the dynamo flows along the continuous rail, and thence through the two relays, one at each end of the block, and through the ballast between the rails, all in parallel, to the sectioned rail. From this it flows through a relatively large resistance to the negative main and back to the machine.

Now, suppose a train enters the block.

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the current now flows through the practically negligible resistance of the car wheels and axles from one rail to the other, and the relays are shunted, with the result that the signal is allowed to go to danger. The " track battery" resistances connected between the negative main and the sections of the sectionalised rail prevent the generator being short-circuited when the track circuit is shunted by the axles of the train. In fact, these resistances bear such a relation to the combined resistance of the road bed from rail to rail and the two relays, all in parallel, that the shunting of the track cuts out only a small percentage of the total resistance of the circuit. Thus the current increase in a circuit, when shunted, is not great; this is important, as it is advisable to keep the track potential as nearly as possible constant. An increase of the total current, resulting from the blocks being occupied by trains, affects the potential between the rails of unoccupied sections, increasing the transmission loss in the negative main.

The loss in the continuous track rail may generally be neglected on account of its large section. Another circumstance directly affecting this track potential is the variation of the resistance of road bed according to weather conditions. Broken stone forms much the best ballast from an electrical point of view, and cinders the worst. It may here be mentioned, however, that though on several occasions recently the track rails on this system have been flooded, the operation of the signals has, we are informed, been in no wise interrupted.

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Next we come to the consideration of the circuits of the track relays, which control the signal circuits. Fig. 3 is a diagram of the relay and signal magnet circuits for a single block. The track coils (see fig. 3) of the relays are permanently connected across the rails at that end of the block at which the relay is placed. Between the pole pieces a polarised armature is suspended from a pivot. This armature bears a winding of considerable resistance, and is connected between the positive rail and the negative main through a contact-operated by the track coil armature—which is closed when the track coils are energised. To the polarised armature is rigidly connected an arm which actuates a contact, the function of which is to open or close the local circuits controlling the signal motors The operation of the relays is then as follows: When a difference of potential exists in the normal direction between the rails, i.e., when there is no train on the block, the relay track coils are excited and draw up the armature, which closes the circuit through the polarised armature. The polarised armature is then attracted to one of the poles of the relay, and, swinging over, closes the contact in the local signal circuit.

As already mentioned, there are two relays in each block, one at each end: these are duplicates, and operate normally in a precisely similar manner, each working a contact in the local signal circuit. These contacts are in series, as shown clearly in fig. 3-relays a and band, unless they are both closed, no current can flow through the signal magnet, and the signal will therefore remain at danger" by gravity.

The position of the apparatus when the block is empty has been indicated above: both relays energised, the local signal circuit closed, and the electromagnetic valve operating the pneumatic signal motor consequently open, admitting compressed air to the motor, which holds the signal arm "off." As soon as a train enters the block, the relays are short-circuited by the car axles, and thereby de-energised, permitting their armatures to drop, and thus breaking the circuit through the polarised armature coils. The polarised armatures then swing back from their position in contact with one of the track-coil pole-pieces, and in doing so break the signal circuit at two points in series. The electro-magnet operating the admission and exhaust valves of the pneumatic signal motor is deenergised, and the exhaust is opened, permitting the signal to return to danger" by gravity. It is, of

The main source of extraneous currents affecting the signals is the 500-volt traction power circuit, and when, as is the case at Ealing, the track is not used as a return, the presence on the block sections of current from this source is abnormal. Faulty insulation of the train equipment, positive and negative rails, positive cables, etc., is the most frequent cause of leakage to the track rails. Whether currents other than the signalling current on the track rails are normal, as would be the case were one of the track rails used as a power return, or abnormal, as in the present case, is immaterial, such currents would be equally disastrous to a susceptible system, and are, it is said, impotent to affect the system considered. Faults in the main power circuit occur with sufficient frequency to demand the closest attention on account of the disorganisation of the traffic which they may cause, but even if they were rare, the least liability to give false signal indications of safety would forfeit all claims for consideration.

Although in the present system, it is possible for the extraneous currents to energise either one or both relays while the train is in the block, these latter are so interconnected that is is not possible for them both to be energised in the normal direction at the same time by extraneous currents.

A few words may be added with regard to reliability. The only parts of the mechanism which can be described as in any way delicate are the relays, and these are enclosed in weatherproof boxes, where they can readily be examined at intervals. The air pressure used is about 70 lb. per square inch, which enables compact motors to be used for signals, points, stops, etc. The air motors employed, work, we are told, year in year out without giving the least trouble. The piping is compact, the electrical wires few and small, and the alterations to the track of a trifling nature. The system is applicable where track return is used with either third rail or overhead trolley wire.

The signal motors, relays, and other special apparatus were manufactured at the London works of the Westinghouse Brake Company, and supplied direct to the Underground Electric Railways Company of London, Ltd., who installed the apparatus themselves.

HE boilers of locomotive engines, although subject to various restrictions, in the way of dimensions and weight, are, nevertheless, beyond all comparison most energetic steam producers. The principal reason for this is, of course, the intensity of the steam blast in the chimney, which enables them to burn usefully an enormous amount of coal per unit of grate surface, although there are two other causes-already touched upon-which in lesser degree contribute to the efficiency of this type of boiler. First, the relatively clean heating surfaces, as compared with boilers having a more sluggish rate of combustion : there is little soot deposited in the firebox plates, or tube surfaces, of an express locomotive travelling at full speed; and, secondly, the shaking to which these boilers are subject while in active use, which has the effect of promoting the escape and discharge of the globules of steam as they form on the heated surfaces.

It must not be forgotten that the fierce combustion in the locomotive firebox is not maintained for any great length of time; and is, as a rule, alternated with more or less lengthened periods of inaction, during which the grate-bars and air spaces may be thoroughly cleansed, and the fire skilfully rebuilt in preparation for the next spell of high-pressure activity. And here we come to the real distinction betwen the locomotive, and the loco-type boilers. The latter, differing little in outward appearance, is commonly in use for ten or twelve hours at a stretch upon six days of the week; and with less skilled attendance, and inferior fuel, demands a very different scale of proportions from its fire-eating brother of the railway locomotive.

In proportioning a locomotive boiler so as to give the best efficiency, regard must be had to the conditions under which the boiler is to work. If

we want a boiler which is to develop the maximum of power in a limited space, if we are prepared to put a highly-skilled fireman in charge, and to allow the requisite intervals for cleaning the fire, and washing out the boiler, we turn unhesitatingly to railway practice, and put in a boiler whose tube surface is nearly or quite sixty times the grate area, knowing that the tremendous energy of the blast demands a corresponding increase in the amount of tube-surface required to absorb the heat on its way to the chimney. If, on the contrary, we are designing a boiler for stationary purposes, we fall under a different set of conditions at once. In order to compare the different classes and sizes of boilers, it is desirable to fix upon a unit to which all the dimensions of any boiler may be referred; and this unit may conveniently be taken as one square foot of grate area. From this, as a basis, we may proceed to determine the extent of surface in firebox and tubes, which will best fulfil the requirements of the service for which the boiler is intended.

If we examine the proportions of ordinary stationary, or loco-type, boilers by several leading makers of this class, we shall find that taking all-round sizes, the average of a large number of examples will give the Firebox heating surface ratio Grate-area neighbourhood of 5, and the ratio

as very closely in the Tube heating surface Grate-area

averages about 23. Whereas, in the case of thirty locomotives boilers taken from the leading British railways the ratio of firebox heating surface to grate-area averages 5.86, which does not differ widely in itself from the first-named range of boilers, but is accompanied by an average ratio of tube-heating surface to grate-area, of more than double that found necessary for the stationary loco-type, viz., 58.35, and similar ratios taken from the locomotives of nine American railroads show 566, and 5961 respectively.