path 'made' in such a way that vehicles shunting along it, or trains which might overrun or disobey the signals, might be prevented from entering on or crossing the path of a train passing, in obedience to signals, along the firstmentioned path.

It is manifest that in most junctions or station yards there are certain lines of road which converge so that any two engines or trains travelling on them will meet, and at such places the interlocking of points with points, mentioned in the previous paragraph, is often extremely useful. A simple example of the application of this precaution may be seen in an ordinary junction, such as shown in figs. 94, 95, 96, pp. 138 and 139, in which the apparatus should be so arranged that in addition to the interlocking of the points with the signals, the points of the branch down line should be so interlocked with the points of the branch up line, that before the branch down points could be moved to set the line for a branch down train, the points of the up branch line must be first moved to set the line parallel to the branch down line, and so divert any vehicles shunting or travelling the wrong way on the up line from crossing the path of the branch down train.

As a further instance of interlocking, the gates of level crossings (fig. 108) are often interlocked with the signals, so that when the gates are shut across the railway all the signals must be at Danger,' and the signals cannot be lowered to 'All right' till the gates are shut against the road traffic and opened for the railway traffic. One arrangement for carrying this into effect is as follows: The gates are moved by a wheel, which by a rack and pinion movement opens or shuts all the gates simultaneously, and a lever locks the gates when they have been placed in their proper position, and this lever is made to interlock with the signals.

COMPENSATION FOR TEMPERATURE.

157

Any arrangement for moving all the gates simultaneously by a lever should only be applied where the signalman can from his signal box overlook and control the road

FIG. 108. Interlocking gate at level crossings.

traffic, or in cases where a porter can be stationed at the level crossing for the same purpose, as otherwise there is considerable risk that the gates in swinging to and fro may come in collision with persons or vehicles crossing

the line.

Generally speaking the interlocking apparatus at present used more or less necessitates the concentration of a large number of point and signal levers in one frame, and under the hand of one man. The signalman is thus often of necessity placed a long way from some of the points which he works, and the movement of the signalman's hand is conveyed to the points by long iron rods, and a series of bell cranks. The expansion and contraction of these long rods, under the changes of temperature to which they are exposed, produce alterations in their lengths which have to be guarded against. The long rods are divided into approximately equal lengths, and their ends are connected to opposite ends of a short lever, which is pivoted at its centre (fig. 109), and placed at right angles to the long rod. Thus any alteration of length in one piece of the long rod, causing a movement of one end of

the compensating lever in one direction, is counterbalanced by the equal alteration in the length of the other part of

FIG. 109. Compensating lever.

the rod; this alteration of length is satisfied by the movement of the other end of the compensating lever in the opposite direction, and thus the motion of the point lever is transmitted unaltered in amount to the tongues of the points.

A truly mechanical plan, introduced on the North Eastern Railway, for imparting motion to points is shown in fig. 110. A straight rod, attached to the tongues of the points at one end, carries at its other end an upright pin, which is placed in a slot of peculiar shape in an iron plate. This iron plate is placed between guides, and is

FIG. 110. North-Eastern Company's arrangement for point rods.

capable of movement to and fro in a direction at right angles to the direction of the movement of the points.

SWITCH LOCKING BAR.

159

The ends of the slot are straight, and are in a direction parallel to the line of rails in which the points are fixed, and thus the first and last parts of a complete movement of the slotted plate do not affect the position of the points, but the necessary movement of the points is given by the curved part of the slot as it travels past the pin. This arrangement prevents any little inaccuracy of fitting from permitting unwarranted movement of the points, as any slight slackness of the rods or locks only affects the motion of the straight part of the slot past the pin. The position of the pin in any part of the straight portion of the slot secures the points in a very effective way, as no movement can take place between the surfaces so placed at right angles one to the other. A further advantage is that the mechanism which determines the adjustment of the points is placed close to the points themselves, and consequently the alterations in the length of the long rods from changes of temperature and other causes are unimportant in their consequences.

An objection arises to the plan of working points from a signal box at a long distance from them, from the danger of a signalman carelessly or unwittingly moving the points while a train is going over them; and there is also a danger of a rod breaking or becoming disconnected from the points without the knowledge of the signalman, in which case he might move the point lever, and so put in operation all the appropriate locking and unlocking in the apparatus, without any corresponding movement of the points themselves having taken place. Also, there is a danger with points worked from a distance of their not being completely shut, either from want of accuracy in adjustment of the working parts, or from a stone or some like impediment finding its way between the point and the stock rail.

The switch locking bar and the switch bolt, which are now almost always combined in one apparatus, obviate the above-named serious dangers. The switch locking bar (fig. 111) was designed to meet the danger of facing points being moved during the passing of a train. It consists of the following simple contrivance :-A bar, at least as long as the greatest distance between any two pairs of wheels of any vehicle in use on the railway, is placed at the side of one of the rails immediately in front of the facing points, and is connected with the rod

FIG. 111. Switch locking bar.

by which the points are worked. The bar is hinged on short links, so that it cannot be moved lengthways without at the same time rising. The top of the bar is level, or nearly level, with the top of the rails, when it is at either end of its stroke, but when it is in midposition the top of the bar is some inches above the top of the rails, and it cannot occupy this position if a wheel be over the bar. As the bar is longer than the greatest distance between any two pairs of wheels, it follows that from the time at which the first pair of wheels of a train comes over the bar to the time at which the last pair of wheels leaves the bar, the bar cannot be moved, and thus, as the bar is rigidly connected with the point rod, it is impossible for a signalman to impart any movement to the points during the passage of a train over the bar.