continuous contact between the wheels c and d he employed exactly the kinematic pairing which we described in § 43. The idea present to his mind seems evidently to have been that a mechanism was "simple" just in proportion to the fewness of its parts. It was later on that he adopted the arrangement of Fig. 175. In this the chain does indeed contain one link more, the bar e, but the required motions are obtained with greater precision, and the fly-wheel with its force-closure and the accompanying destructive wear have disappeared.42
In our various means of transport the change from force- to pair-closure has continued to the present time. After all had been done in improving the construction of the vehicle itself, furnishing it with a suitable fore-carriage, making better roads for it to move upon, etc., force-closure still remained, if nowhere else at least in the preservation of the direction of motion, which still demanded accustomed animals and an intelligent driver. Men naturally attempted to replace this force-closure by pair-closure. In the Railway the rails are paired with the wheels,—force-closure is used only to neutralise vertical disturbing forces. The step thus made in the direction of machinal completeness,—which it required half a century to make,[1]—was a most important one;—it was in reality no other than the uniting of the carriage and the road into a machine. The rail forms a part of this machine, it is the fixed element of the kinematic chain of which the mechanism really consists. The further improvement of the pair-closure, the removal of any remaining disturbing force-closure whether in the rails, in the axle-boxes, in the arrangement of the springs of carriages and of locomotives and so on, still engages most careful attention. In opposition to this we have the problem of steam locomotion on common roads, which has been so feverishly taken up again within the last few years, but the solutions of which seem doomed to eternal incompleteness, for they are self-contradictory. It is desired to make something which shall be a machine, but in which at the same time the special characteristic of the machine,—the pairing of elements,—may be disregarded. On the other hand, attempts have been made,—as in Boydell's Traction Engine,—to carry with the machine at least a portion of a transportable element which
- ↑ Wooden rails were in use at pits near Newcastle as early as 1676,—the first iron rails were laid down in 1738.
