"This second engine consisted as before of two vertical cylinders, which communicated directly with each pair of the four wheels that supported the engine by a cross-head and a pair of connecting-rods; but, in attempting to establish a direct communication between the cylinders and the wheels that rolled upon the rails, considerable difficulties presented themselves. The ordinary joints could not be employed to unite the engine, which was a rigid mass, with the wheels rolling upon the irregular surface of the rails; for it was evident that the two rails of the line of railway could not always be maintained at the same level with respect to each other--that one wheel at the end of the axle might be depressed into a part of the line which had subsided, while the other would be elevated. In such a position of the axle and wheels it was clear that a rigid communication between the cross-head and the wheels was impracticable. Hence it became necessary to form a joint at the top of the piston-rod where it united with the cross-head, so as to permit the cross-head always to preserve complete parallelism with the axle of the wheels with which it was in communication. "In order to obtain the flexibility combined with direct action which was essential for insuring power and avoiding needless friction and jars from irregularities in the rail, my father employed the 'ball and socket' joint for effecting a union between the ends of the cross-heads where they united with the connecting-rods, and between the end of the connecting-rods where they were united with the crank-pins attached to each driving-wheel. By this arrangement the parallelism between the cross-head and the axle was at all times maintained, it being permitted to take place without producing jar or friction upon any part of the machine.