Of course, the peds cannot be put on the ground and left behind; they must accompany their rollers and rails. We will endeavour to explain in simple words how this is effected.
To the axles of the locomotive is attached firmly a flat, vertical plate, parallel to the sides of the fire-box. Pivoted to it, top and bottom, at their centres, are two horizontal rocking arms; and these have their extremities connected by two bow-shaped bars, or cams, their convex edges pointing outwards, away from the axle. Powerful springs also join the rocking arms, and tend to keep them in a horizontal position. Thus we have a powerful frame, which can oscillate up and down at either end. The bottom arm is the rail on which the whole weight of the axle rests.
The rotating and moving parts consist of a large, flat, circular case, the sides of which are a few inches apart. Its circumference is pierced by fourteen openings, provided with guides, to accommodate as many short sliding spokes, which are in no way attached to the main axle. Each spoke is shaped somewhat like a tuning-fork. In the V is a roller-wheel, and at the tip is a "ped," or foot. As the case revolves, the tuning-fork spokes pass, as it were, with a leg on each side of the framework referred to above; the wheel of each spoke being the only part which comes into contact with the frame. Strong springs hold the spokes and rollers normally at an equal distance from the wheel's centre.
It must now be stated that the object of the framework is to thrust the rollers outwards as they approach the ground, and slide them below the rail. The side-pieces of the frame are, as will be noticed (see [Fig. 3]), eccentric, i.e. points on their surfaces are at different distances from the axle centre. This is to meet the fact that the distance from the axle to the ground is greater in an oblique direction than it is vertically, and therefore for three spokes to be carrying the weight at once, two of them must be more extended than the third. So then a spoke is moved outward by the frame till its roller gets under the rail, and as it passes off it it gradually slides inwards again.
It will be obvious to the reader that, if the "peds" were attached inflexibly to the ends of their spokes they would strike the ground at an angle, and, of course, be badly strained. Now, Mr. Diplock meant his "peds" to be as like feet as possible, and come down flat. He therefore furnished them with ankles, that is, ball-and-socket joints, so that they could move loosely on their spokes in all directions; and as such a contrivance must be protected from dust and dirt, the inventor produced what has been called a "crustacean joint," on account of the resemblance it bears to the overlapping armour-plates of a lobster's tail. The plates, which suggest very thin quoits, are made of copper, and can be renewed at small cost when badly worn. An elastic spring collar at the top takes up all wear automatically, and renders the plates noiseless. This detail cost its inventor much work. The first joint made represented an expenditure of £6; but now, thanks to automatic machinery, any number can be turned out at 3s. 6d. each.
A word about the feet. A wheel has fourteen of these. They are eleven inches in diameter at the tread, and soled with rubber in eight segments, with strips of wood between the segments to prevent suction in clay soil. The segments are held together by a malleable cast-iron ring around the periphery of the feet and a tightening core in the centre. These wearing parts, being separate from the rest of the foot, are easily and cheaply renewed, and repairs can be quickly effected, if necessary, when on the road. The surface in contact with the ground being composed of the three substances—metal, wood, and rubber, which all take a bearing, provides a combination of materials adapted to the best adhesion and wear on any class of road, or even on no road at all.
Fig. 3
Motive power is transmitted by the machinery to the wheel axle, from that to the casing, from the casing to the sliding spokes. As there are alternately two and three feet simultaneously in contact with the ground, the power of adhesion is very great—much greater than that of an ordinary traction-engine. This is what Professor Hele-Shaw says in a report on a pedrail tractor: "The weight of the engine is spread over no less than twelve feet, each one of which presses upon the ground with an area immensely greater—probably as much as ten times greater—than that of all the wheels (of an ordinary traction-engine) taken together on a hard road. Upon a soft road all comparison between wheels and the action of these feet ceases. The contact of each of the feet of the Pedrail is absolutely free from all slipping action, and attains the absolute ideal of working, being merely placed in position without sliding to take up the load, and then lifted up again without any sliding to be carried to a new position on the road."