When first introduced the bar was connected directly with the lever which worked the points, so that, as the points were moved backwards or forwards, the bar was also moved at the same time on its hinges. But recently instead of connecting the switch locking bar directly with the points, so that one cannot be moved without the other, it has been found to be more desirable to work the switch locking bar by a lever in connection with the second appliance above mentioned, viz. with the 'switch bolt.'

The purpose of the switch bolt (fig. 112), which is designed to counteract the second of the two dangers to which I referred in a previous paragragh, is to ensure that facing points are in their proper position after they have been moved by the point lever, and before the signal can be given for a train to pass over them; also to securely and firmly lock the points in their proper position, when they have been adjusted by the point lever, thus guarding against the points being disturbed by the vibration of a passing train. A transverse connecting bar, with two holes in it, is fixed at right angles to, and between the points, and a long bolt is fixed on the sleepers, between the rails, so that the bolt is shot parallel to the line of railway, through the holes in the connecting bar. When either of the holes in the connecting bar is opposite to the bolt, the bolt can be shot, but in any intermediate position of the points the bolt cannot be shot, because there would be no hole opposite to it. Thus, if the points are not put thoroughly home by the action of the point lever, the lever working the switch bolt cannot be moved; and, as the latter lever interlocks with the signal levers, no train can be signalled to approach until the points are accurately adjusted and locked in their proper position. At the same time the switch locking

M

bar, which is connected to the lever working the switch. bolt, prevents the signalman from altering the position of the bolt while any wheel of a train is passing over the bar.

Thus, when the switch bolt and the switch locking

FIG. 112. Switch bolt and switch locking bar.

bar are in use, a signalman has, in order to adjust a pair of points for a train to pass over them, first to put the points in their proper position, and then to shoot the locking bolt. When these two operations are complete and not before he can give the signal to allow an approaching train to pass over the points. The switch locking bar at the same time prevents the bolt from being withdrawn until the whole train has passed over the bar.

There are many other contrivances and arrangements of mechanism for producing much the same results as those described in this chapter, but it is only possible

GENERAL APPLICATION OF INTERLOCKING.

163

within the limits of these lectures to describe those mostly in use, and to refer to the characteristics of the more complete apparatus and to the principles observed in their construction. The signalling apparatus which have been of late adopted on our railways have attained a considerable degree of efficiency in dealing with a most complicated problem, and no doubt tend to greatly reduce the results of human fallibility.

I have especially, and at length, directed your attention to the leading principles of interlocking, because these principles are not only valuable for ensuring the safety of railway trains, but may be applied to ensure the correct working of almost all such mechanical combinations as have in part to depend on the will of other intelligences than that represented by the machine.

A clock, a steam engine, a calculating engine, may be made self-acting, that is to say, may be made to represent with almost perfect exactness the will of the person who contrives the machine; but the working of a railway, a lift, a drawbridge, the machinery for tipping coals from trucks into ships, the machinery for loading and training heavy guns, and many other kindred operations, depend on actions into which the fallible human element enters.

In these cases a variety of successive movements have to be performed, but for the initiation of each it is essential that some man in charge should decide when it is expedient that the movement in question should take place when it should begin and when cease.

If left mechanically uncontrolled, a man in charge of any machinery, such as that to which I am referring, may by mistake initiate some movement that ought not to be begun until some other movement has been absolutely completed, and this mistake may be one which will infal

For examples, among

libly cause disastrous results. many which might be cited, a lift may be started while a passenger is entering it, or a gun turret may be revolved before the rammer is withdrawn, or a heavy gun may be fired when the compresser is slack. Loss of life or injury to machinery may result from such mistakeswhich are by no means unknown to those who have had charge of such kinds of machinery.

These mistakes may in most cases be guarded against by simple applications of the interlocking principle, under which system, while perfect freedom is given to the men in charge of the machinery to carry on their duty, they can be not only warned against, but be absolutely debarred from, committing any act which may lead to injury to the mechanism or to man.

LECTURE V.

WEIGHTS ON WHEELS OF ROLLING STOCK-NUMBER OF WHEELS TO A VEHICLE-DEAD WEIGHT OF VEHICLES- UNDERFRAMESSPRINGS-BUFFERS-COUPLING OF VEHICLES-AXLES-TIRESWHEEL BODIES-TIRE FASTENINGS-AXLE BOXES -LUBRICANTSBOGIES-AMERICAN CARRIAGES-BREAKS FRICTION AT DIFFERENT VELOCITIES-RETARDING FORCE OF BREAKS CONTINUOUS BREAKS

-BREAK EXPERIMENTS.

ROLLING STOCK of a railway, as the term is generally accepted, may or may not include the locomotive engines. I shall not, however, attempt to describe the engines, which will be dealt with by my friend Mr. Bramwell in the lectures which follow; nor shall I be able to refer otherwise than briefly to the leading characteristics of railway carriage and waggon stock. All I can hope to do, in the time at our disposal, is to give you a general view of the main features of rolling stock and to direct attention to some of the principles of its proper design and construction. Any one who wishes to follow out the subject and to study its details thoroughly must apply himself to some of the well-known published works on this important part of railway engineering.

The constructional parts of railway rolling stock, under which I comprise the wheels, the axles, and the underframes which rest on the axles, are much the same in all cases, and are not materially affected by the modifications by which the stock is adapted to special purposes. The dimensions and designs of these constructional parts, of course, vary somewhat, but speaking generally their design is much the same in all cases. The differences which meet