SIGNAL (a word See also:

• COMMON

Flag signalling being comparatively slow and laborious, the See also:

• ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

The simplicity of the dot and dash principle is its best feature. As the system only requires the See also:

• EXHIBITION

In the See also:

• MIDDLE

Instead of indicating See also:

• DIFFERENCES, CALCULUS OF (Theory of Finite Differences)

Sometimes when a convenient See also:

• CLOUD (from the same root, if not the same word, as " clod," a word common in various forms to Teutonic languages for a mass or lump; it is first applied in the usual sense in the late 13th century; the Anglo-Saxon chid is only used in the sense of " a ma

(C. A. G. B.; A. F. E) Army Signalling.—Communication by visual signals between portions of an army is a comparatively See also:

• RECENT

To the See also:

• ORIGINAL

The respective sizes of these flags are as follows: large flag 3'X3', See also:

• POLE (FAMILY)
• POLE, REGINALD (1500-1558)
• POLE, RICHARD DE LA (d. 1525)
• POLE, WILLIAM (1814—1900)

The fact that the heliograph requires sunlight, as mentioned above, militates against its employment in Great See also:

• BRITAIN (Gr. Hperavuml vi7vot, Bperravia; Lat. Britannia, rarely Britannia)

The instrument is aligned by means of the sighting See also:

• VANE (formerly spelt " fane," i.e. pennon, flag; cf. Ger. Feline, Du. vaan, Fr. girouette, Ital. banderuola, Ger. Wellerfahne)
• VANE, SIR HENRY (1589-1654)
• VANE, SIR HENRY (1613-1662)

9 —. o — The semaphore code used in the army is shown below: A — J •--- S •.. B K —•— T —,. C L •—• • U ..— D M-- V...._ E N —• W •-- F 0--- X— •— G p •- -• Y —•-- H Q --•— Z --. . I • R •—• I •- /Li 11') D E p 4 S 6 7 "Numerals J o-Comif See also:

• LEH

There is an elaborate system of signals See also:

• RELATING

These usually worked in parties of 2 or 3, each party being protected by a few infantrymen or troopers. The apparatus used was heliograph by day and a very elaborate form of lamp by night, and work was carried on between posts separated by 6o and even 90 M. The signallers were employed both with the See also:

• MOBILE

These may be divided into (I) interlocking signals, used at junctions and yards, and (2) See also:

• BLOCK
• BLOCK (from the Fr. bloc, and possibly connected with an Old Ger. Block, obstruction, cf. " baulk ")
• BLOCK, MAURICE (1816–19or)

A white light is not to be regarded as a danger signal, as is sometimes erroneously stated, but rather as no signal at all; and as there is a well-known See also:

• RULE

Where more than two full-sized arms are used on a post, the See also:

• CUSTOM (from O. Fr. costume, costume or coustume; Low Lat. costuma, a shortened form of consuetudo)

When the signalman, preparatory to lowering a signal, grasps the lever at its upper end, he movesthisrodupwards, and in so doing actuates the interlocking, through the tappet N, attached at T. Lifting the tappet locks all levers which need to be locked to make it safe to move this one. In pulling over the lever the rocker R is also pulled; but the slot in it is radial to the centre on which B the lever turns, so that during the stroke N remains motionless. On the completion of the stroke and the dropping and this unlocks such levers this lever is pulled (" cleared" See also:

• WEIGHT

Fig. 5 shows a typical arrangement of interlocked signals, the principle being the same whether a yard has one set of points or a See also:

• HUNDRED

The functign of the interlocking See also:

• MACHINE
• MACHINE (through Fr. from Lat. form machina of Gr. µnxavil)

grooves N, 0, P, carry See also:

• DOGS, ISLE OF

The valves admitting air to a cylinder are controlled by electromagnets, the wires of which are laid from the cabin underground. Each switch or signal, on completing a movement, sends an electric impulse to the cabin, and the interlocking is controlled by this " return." In the machine the iSGb'armore 75 aneous with the See also:

• ADVENT (Lat. Adventus, sc. Redemptoris, " the coming of the Saviour ")

This is performed through the medium of a long See also:

• SHAFT
• SHAFT (O. Eng., sceaft, from scafan, to shave; the word is common to Teutonic languages)

A motor, with gearing and cranks, is fixed to the sleepers at each pair of points, the power is conveyed from the cabin by underground wires, the locking is of common mechanical types, and, in general, the system is similar to pneumatic systems except in the source of power. By using accumulators, charged by dynamos run by gasoline engines, or by a travelling power-car, the cost of power is reduced to a very low figure, so that power-interlocking becomes economical at small as well as large stations. The essence of block signalling is a simple regulation forbidding a train to start from station A until the last preceding train has Blocs passed station B; thus a space interval is maintained system . between each train, instead of the time-interval that was relied upon in the early days of railways. As the introduction of the telegraph was almost or quite contempor- clear " and " train on line " or "line blocked," the latter being the normal indication, even when there is no train in the section. The three-wire instrument has the advantage of giving three indications on the See also:

• DIAL

The function of a distant signal d has already been described; it is mechanically impossible for it to be lowered unless h has previously been lowered. The relation of the signals to the " crossover road " xx is the same in principle as is shown in fig. 5. Dwarf or disk signals such as would be used for the siding T or the crossover xx are omitted from the See also:

• SKETCH (directly adapted from Dutch schets, which was taken from Ital. schizzo, a rough draft, Lat. schedium, something hastily made, Gr. oxE&os, sudden, off-hand, axeb6v, near by; Ger. Skizze and Fr. esquisse are from the same source)

Sykes about 1876, the apparatus being so arranged that a signalman at one end of a section is physically unable to lower his signals to let a train enter that section until they have been released electrically from the cabin at the other end. The starting signal at a block section A cannot be lowered until the signalman at the next station B, by means of an electric circuit, unlocks the lever in connexion with it. In so doing he breaks the unlocking circuit at his own station, and this break is restored only on the arrival of the train for which the unlocking was performed, the wheels of the train acting through a lever or by a short rail circuit. Valuable improvements have been made in this machine by Patenall, Coleman and others, and these are in use in America, where the system is known as the " See also:

• CON

By simple "catching apparatus" on the engine, staffs or tablets may be delivered to trains moving at a good speed. The signals so far described depend for their operation, either wholly or partially, on human agency, but there are others, commonly known as " automatic," which are worked Automatic. . by the trains themselves, without human intervention. Such signals, as a rule, are so arranged that normally they are constrained to stand at " safety," instead of in the "danger" position, which, like ordinary signals, they assume if left to themselves; but as a train enters a block section the constraint on the signals that guard it is removed and they return to the danger position, which they retain till the train has passed through. To effect this result an electrical track circuit or rail circuit is employed, in See also:

• CONJUNCTION (from Lat. conjungere, to join together)

Block sections more than about r m. long are commonly divided into two or more circuits, connected together by relays; but usually they are made under r m. in length and often on See also:

• INTRA

In the electro-pneumatic system a full-size semaphore is used. Compressed air, from pumps situated at intervals of io to 20 m., is conveyed along the line in an iron pipe, and is supplied to a cylinder at each signal, exactly as in pneumatic interlocking, before described. The rail circuit, when complete, maintains pressure in a cylinder, holding the signal " off." On the entrance of a train or the failure of the current, the air is liberated and the signal arm is carried by gravity to the " stop " position. Automatic signals are sometimes made to stand normally (when no train is in the section) in the " stop " position. The local circuit is connected with the rail circuit so that it is closed only when a train is approaching within, say, i m. With the rail circuit, distant signals are controlled, without a line wire, by means of a polarized relay. Each signal, when cleared, changes the polarity of the rail circuit for the next section in its See also:

• REAR

In England, especially around See also:

• LONDON

As it is only necessary for one detonator to explode, the object of placing two on the rails being merely to guard against a See also:

• MISS

The fundamental principles of railway signalling are simple, but the development of the See also:

• SCIENCE (Lat. scientia, from scire; to learn, know)

M.