470
EXPOSITION AT VIENNA.
A is supposed to be at some distance, say a mile, from the, crossing, while B is close by. For trains going the other way, two " fixed contacts” would be required, at C and D, , but on a double track all four might be connected with one wire to the semaphore. The "fixed contacts” would, on a single-track road, have to be set, not in the centre, but at the sides of the track, so that trains going in either direction would only touch two of the four—the movable contact being also, of course, placed near the side of the engine.
If the sonnerie alone were used, there would be needed two batteries at the semaphore instead of one, and the direction of their poles would be reversed in order to give reverse signals on the magnet of the primary circuit before spoken of. A and C would be connected to the positive pole of one battery, and B and D to the negative pole of another. The additional cost would be trifling.
IY. Signals " Covering ” the Position of Teains in
Motion from Trains Following, Meeting or Crossing.
This class includes all electrical apparatus applied to the " block system,” and to the system of " interlocking points” at junctions, and is perhaps the most important division of the subject of railway signals.
The term " block,” as applied to railway signals, has become fixed in railway parlance, at least in England, and is used to designate a system under which the road is divided into sections, of greater or less length, protected by signals which allow only one train to be on a section at any given time. Mr. Preece thinks that the term arose from the necessity, in the earlier systems, of "blocking” or pinning over the signal-lever to protect the line from following trains. He suggests that the term "space” system, as opposed to that of " time” would probably be more accurate.
The "time” system, as employed in England, detained a train for five minutes after a preceding one had passed the signal-station, and exhibited a " caution” signal for five minutes more. Regard being had, however, to the difference in the speed of trains, and to the various accidents or delays which may happen between two signals, and which often cannot be notified to a following train in time to prevent a