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CHAPTER 4

The Cylinder as Motor

Cylinders appear in the steam engine in all three of its traditional parts: there is the cylinder in which the pressure of expanding steam lifts and pushes a second, inserted cylinder, the piston; there is the transmission, which is based on “cylinder chains”; and there is the cylinder as a tool in the all-important process of rolling. In addition, the boiler, one of the many cylindrical storage devices of the time, allows for the initial generation and compression of steam.

From a kinematic point of view, the cylinder-piston assembly in the motor achieves the isolation of translational motion along the central axis; since the cylinder wraps around the piston completely, it is an instance of “pair-closure,” which Reuleaux heralded as the negentropic ideal that would overcome the “force-closure” of, say, a wheel on a straight rail. To minimize friction and wear, the piston rod must take the position of the central axis, and the piston itself must be fitted as tightly as possible into the cylinder.

In the cylinder-piston couple, as in many other mechanical devices, the cosmic—to use Reuleaux’s term for interferences of unforced motions—coincides with the practical. Why cylinders as expansion or combustion chambers, and not another shape with a central axis, like a cube? The cosmic answer is that a shape without corners allows for a more complete utilization of energy, since steam or combustible fuel (the “flame front”) expands in spherical fashion. (The same phenomenon, slowed down considerably, led the builders of silos to abandon rectangular shapes.) It is unlikely that this consideration was much on the minds of the early steam engineers, but a more practical one certainly was: cylinders can be bored by tools in continuous rotation, thereby achieving precision and uniformity and minimizing the loss of energy. One reason for the superiority of James Watt’s early engines was the accuracy with which his partner Matthew Boulton first cast and then bored his cylinders by means of machine tools that, as we will see, were crucial for the production and reproduction of cylindrical devices.¹