Again, suppose that an iron-tyred vehicle, travelling at a rapid pace, meets a large stone, what happens? Either the stone is forced into the ground or the wheel must rise over it. In either case there will be a jar to the vehicle and a loss of propulsive power. Do not all cyclists know the fatigue of riding over a bumpy road—fatigue to both muscles and nerves?
As regards motors and cycles the vibration trouble has been largely reduced by the employment of pneumatic tyres, which lap over small objects, and when they strike large ones minimise the shock by their buffer-like nature. Yet there is still a great loss of power, and if pneumatic-tyred vehicles suffer, what must happen to the solid, snorting, inelastic traction-engine? On hard roads it rattles and bumps along, pulverising stones, crushing the surface. When soft ground is encountered, in sink the wheels, because their bearing surface must be increased until it is sufficient to carry the engine's weight. But by the time that they are six inches below the surface there will be a continuous vertical belt of earth six inches deep to be crushed down incessantly by their advance.
How much more favourably situated is the railway locomotive or truck. Their wheels touch metal at a point but a fraction of an inch in length; consequently there is nothing to hamper their progression. So great is the difference between the rail and the road that experiment has shown that, whereas a pull of from 8 to 10 lbs. will move a ton on rails, an equal weight requires a tractive force of 50 to 100 lbs. on the ordinary turnpike.
In order to obviate this great wastage of power, various attempts have been made to provide a road locomotive with means for laying its own rail track as it proceeds. About forty years ago Mr. Boydell constructed a wheel which took its own rail with it, the rails being arranged about the wheel like a hexagon round a circle, so that as the wheel moved it always rested on one of the hexagon's sides, itself flat on the ground. This device had two serious drawbacks. In the first place, the plates made a rattling noise which has been compared to the reports of a Maxim gun; secondly, though the contrivance acted fairly well on level ground, it failed when uneven surfaces were encountered. Thus, if a brick lay across the path, one end of a plate rested on the brick, the other on the ground behind, and the unsupported centre had to carry a sudden, severe strain. Furthermore, the plates, being connected at the angles of the hexagon, could not tilt sideways, with the result that breakages were frequent.
Of late years another inventor, Mr. J. B. Diplock, has come forward with an invention which bids fair to revolutionise heavy road traffic. At present, though it has reached a practical stage and undergone many tests satisfactorily, it has not been made absolutely perfect, for the simple reason that no great invention jumps to finality all at once. Are not engineers still improving the locomotive?
The Pedrail, as it has been named, signifies a rail moving on feet. Mr. Diplock, observing that a horse has for its weight a tractive force much in excess of the traction-engine, took a hint from nature, and conceived the idea of copying the horse's foot action. The reader must not imagine that here is a return to the abortive and rather ludicrous attempts at a walking locomotive made many years ago, when some engineers considered it proper that a railway engine should be propelled by legs. Mr. Diplock's device not merely propels, but also steps, i.e. selects the spot on the ground which shall be the momentary point at which propulsive force shall be exerted. To make this clearer, consider the action of a wheel. First, we will suppose that the spokes, any number you please, are connected at their outer ends by flat plates. As each angle is passed the wheel falls flop on to the next plate. The greater the number of the spokes, the less will be each successive jar (or step); and consequently the perfect wheel is theoretically one in which the sides have been so much multiplied as to be infinitely short.
A horse has practically two wheels, its front legs one, its back legs the other. The shoulder and hip joints form the axles, and the legs the spokes. As the animal pulls, the leg on the ground advances at the shoulder past the vertical position, and the horse would fall forwards were it not for the other leg which has been advanced simultaneously. Each step corresponds to our many-sided wheel falling on to a flat side—and the "hammer, hammer, hammer on the hard high road" is the horsey counterpart of the metallic rattle.
On rough ground a horse has a great advantage over a wheeled tractor, because it can put its feet down on the top of objects of different elevations, and still pull. A wheel cannot do this, and, as we have seen, a loss of power results. Our inventor, therefore, created in his pedrail a compromise between the railway smoothness and ease of running and the selective and accommodating powers of a quadruped.
We must now plunge into the mechanical details of the pedrail, which is, strictly speaking, a term confined to the wheel alone. Our illustration will aid the reader to follow the working of the various parts.
In a railway we have (a) sleepers, on the ground, (b) rails attached to the sleepers, (c) wheels rolling over the rails. In the pedrail the order, reckoning upwards, is altered. On the ground is the ped, or movable sleeper, carrying wheels, over which a rail attached to the moving vehicle glides continuously. The principle is used by anyone who puts wooden rollers down to help him move heavy furniture about.