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.
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.