148 MOTIVE POWER AND DISPOSITION

to describe the inode by which the recti-linear motion of the piston is converted intothe rotatory one of the wheels, and the pro-gressive motion of the carriage thereby affect-ed : a slight inspection of the drawing will con-vey to those who have the slightest knowledgeof machinery, the manner in which it is done,as nothing can be more simple and effective.

The sliding, or steam valve, is opened and shut, at theproper periods, by the following contrivance; a, Fig.V.Plate IV., represents the aide of the travelling wheels ofthe carriage; a b is, a lever fastened upon, and turninground, at the same time with it; b c is a circular openingin the eccentric circle d e, within which a pin, attached to theend of the lever a b, is at liberty to move; this eccentric circleis loose upon the axle of the carriage, and is only turnedround when the pin, at the end of the lever a b, arrives atb ac, according to the direction in which it is moving; acircular hoop, or strap of iron, fits the circumference of theeccentric motion, connected to the lever fgh, which is movedbackwards and forwards as the axle turns round; as thislever is moved, its motion is communicated to the arm i k,as shewn m Fig. 1. Plate V. also, and through it, by thelever k l and rod l m, to the cross-head m n, and so to therod n o, of the sliding or steam valve, which, as the car-riage is moved forward, is thus slided up and down to openand shut the communication between the two sides of thecylinder and the boiler, at the proper periods.

Before the application of this mode the mea-sure was effected by a square box or tumbler.It seemed to me, from the irregularity of the