libly cause disastrous results. For examples, among 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be cited, a lift may be started while a passenger is entering it, or a gun turret may be revolved before the rammer is withdrawn, or a heavy gun may be fired when the compresser is slack.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 mistakes which 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 Α 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

the eye in ordinary railway rolling stock are in the upper work which is placed on the underframe in order to adapt the carriage or waggon to its particular function.

The weights resting on the wheels of carriage and waggon stock are very much less than the weights on the wheels of locomotives which have from 11 to 16 tons (varying with the description of engine) supported on a pair of driving wheels. The following table will give some idea of the weights on the wheels of rolling stock in ordinary use at the present time.

Weights on each pair of wheels of carriage and waggon stock.

The design of the underwork of railway rolling stock differs from that of other wheeled vehicles, from the fact that the path along which the wheels of railway vehicles travel is accurately defined, and because the curves on which it travels are laid out with special reference to the nature of the rolling stock, and to the speed at which the trains will travel. A coachman can regulate the speed of the horses he is driving and can change the direction in which the carriage on which he is seated is travelling, but an engine-driver can only control the speed of his train, and can exercise no influence on the direction in which it is to travel. All changes of direction are prescribed by the pointsman or platelayer, and consequently there is no absolute need in a railway vehicle for any such provision for turning as we see in the front part of the under-carriage of all roadway vehicles.

In ordinary rolling stock, as has been explained in a

previous lecture (p. 94), the wheels cannot even adjust themselves to the slight extent required to set themselves tangentially to the curves of the line. Railway vehicles as generally used are therefore in a mechanical point of view undoubtedly defective in the respect above mentioned, and the want of power of adjustment involves an unnecessary expenditure of the power of the locomotive in overcoming the friction of the wheel both in sliding sideways and in slipping circumferentially on the rail. A further evil ensues in the consequent and unnecessary wear and tear both to the permanent way and to many parts of the vehicles themselves.

Certain descriptions of rolling stock have, however, been constructed, and especially in the United States, on other principles, which mitigate the evils involved in wheels rigidly fixed to parallel axles. The remedy consists in the use of what are called bogies, which are, as a rule, low four-wheeled trucks, with a very short wheel base, and carrying a pivot. The ends of the body of the railway vehicle are supported on the pivots on the bogie trucks, in a way similar to that in which the forward part of an ordinary roadway carriage is supported on a pivot between the front pair of wheels. The bogie form of construction will be referred to in due course, but the rolling stock in general use on English railways, which has parallel rotating axles on which the wheels are fixed, will be first considered.

The practice of railway companies differs as to the number of wheels placed under passenger carriages, some preferring four-wheeled, and some six-wheeled carriages. As a general rule, goods and other waggons have four wheels, and the same is the case with the majority of carriages, but many of the carriages, especially for express trains, have six wheels.

There can be little doubt that in the case of an

accident happening to the springs, wheels, or axles of a carriage, a six-wheeled vehicle is much safer than one with four wheels. If any one of the springs, wheels, or axles of a four-wheeled carriage is disabled, the corner or end of the carriage over the disabled wheel or axle will probably drop; but with a six-wheeled carriage this need not happen, as the carriage would still be supported on five or four wheels, and might run a long distance without serious disaster. The disadvantage of six-wheeled carriages is that they cannot accommodate themselves to the curves of the line quite so readily as four-wheeled carriages, for the curve of the line demands that the three wheels on each side should be in a curved line, whereas the attachment of the wheels to the frame of the carriage tends to keep them in a straight line. The necessary accommodation is in practice provided by the clearance between the wheels and the rails, by a certain amount of slackness or play in the several parts of the connection between the wheels and the frames of the carriages, and perhaps by some bending of the frames themselves. There is, thus, no difficulty in constructing six-wheeled carriages properly, or in working them on railways with reasonably good curves; in fact on many lines the long six-wheeled carriages are specially selected for express trains on account of their steadiness, in addition to their other advantages referred to above. It is to be borne in mind. in considering the relative safety of the different forms of carriage, that ordinary stopping trains often travel at some portions of their journey at a speed as high as that of express trains, and that an accident to an ordinary train at such a time would be as disastrous as one to an express. In four-wheeled vehicles with parallel axles the desirable length of wheel base is governed by the sharpness of the curves of the line, since on that and on the