While French theorists put serious efforts into founding and institutionalizing kinematics as a deductive science, British engineers were attacking its practical problems. Kinematically speaking, the rise of the steam engine as the motor of the Industrial Revolution was the result of a specific mechanism to “change the direction and speed of a given motion,” more precisely the (reciprocating) translational motion of the piston, into the rotational motion of the working beam. It was invented by James Watt in 1784 and was immediately patented so that it could reach the open market at the very beginning of the nineteenth century. Only then could the proliferation of cylindrical machines in the nineteenth century really begin.16
This mechanism, commonly called “Watt’s parallel motion,” changed the steam engine from a pendulum into a fully rigid mechanism. It connected the piston that rose from and was pushed into the cylinder with the beam that pivoted on a central column.
The pivoting beam was part of the early architecture of steam engines, which were primarily used to pump water. Before Watt, only the downward stroke of the engine was powered: either the rapid cooling of the steam under the piston created a vacuum that pulled the piston down, or steam was injected above the piston. In this configuration, where the piston pulled on the beam (and the beam pulled on the pumping vessel), it was enough to use chains or ropes as a connection; they were run across the ends of the beam, and the kinematic conflict between the semicircular motion of the beam and the straight motion of the piston was reconciled—just as it was in Kleist’s marionettes—by the slackness in the connection (fig. 1).
FIGURE 1. Watt’s 1774 engine. The piston (and the valve gear) are connected to the beam by a chain; the power stroke can only be downward. Reprinted from Thurston (1902, 98).
This paradigm had to be changed when Watt began to power both the up- and the downstroke of the piston by using steam as a positive (expanding) rather than as a negative medium. Now the piston was pushing up on the beam as well as pulling it down; ropes and chains did no longer work, and a simple rigid rod without a mediating mechanism would have destroyed the machines in a very short time—if the piston were pushed along a line that deviated from the cylinder’s axis, it would scrape against the inside walls, destroying its symmetry and losing the ability to seal and maintain pressure.17 Even without these difficulties, the practical problems of boring or casting accurate enough cylinders in sufficiently strong materials and of finding lubricants to minimize the inevitable friction proved very hard to overcome for most machine builders in the late eighteenth and the very early nineteenth centuries.18 One of Watt’s many advantages in the race for efficient engines was that through his partner Matthew Boulton he could intervene directly in the manufacturing of cylinders, asking for more precision in boring and for stronger alloys.19 To Boulton he first announced his discovery that a rigid linkage configured the right way could guide both the up-and the downstroke of a double-acting steam engine without stressing the materials involved.
In a formulation at once revelatory of the truly empirical process of engineering and of the stunning novelty of motion conversion, Watt wrote of the contraption he called “parallel motion”: “When I saw it work for the first time, I felt truly all the pleasure of novelty, as if I was examining the invention of another man.”20 Yet like so many engineering advances in the nineteenth century, parallel motion was an avoidance of conflict rather than an invention of something entirely new. The mechanism simply caught two semicircular movements at the point where they intersected along a seemingly straight path (fig. 2). One was the movement of the beam OA—A, which in kinematic nomenclature was called the crank (the Kurbel, of which Kleist’s Herr C. dreamed); the other was the link OB—B affixed to an opposite wall, called the follower. Both were connected by a third link A—B, the coupler. As the crank moved up and down, it led the follower into a mirror image of its own motion whereby a point M on the coupler was forced to trace out an elongated figure eight, the sign of infinity. If the proportions of the links were chosen appropriately and the movement of the crank was restricted accordingly, M traced a line that was approximately parallel to the beam’s support column. A piston rod, attached to C, could push and pull in a line extending from the cylinder’s axis. Depending on the machine’s architecture and size, Watt translated this parallel motion horizontally by means of pantographs—linkages based on the parallelograms that had long been used to translate writing and drawing across a plane—which yielded other parallel points M′ able to drive a valve train or an auxiliary pump (fig. 3).21
Watt’s mechanism not only allowed for a potentially infinite increase in power output but also universalized the use of steam engines just as much as fossil fuel rendered them independent of natural location. The four-bar linkage (the hatched line at OB and OA on the left of figure 2 indicates a fixed frame and counts as one bar, just like the “floor and datum” on the right) is the most economical way of mediating between translational and rotational motion. To repeat, such mediation is necessary because in a finite mechanism (unlike in the universe or in a gun) every translational motion needs to be “returned,” every straight motion needs to be reciprocal or oscillating.22 Using variants of the four-bar linkage, engineers could eliminate the working beam and configure machines for a hitherto unimagined variety of purposes—or else utilize the transmission as the machine’s tool, as is the case in motorized vehicles. The slider-crank mechanism—an avatar of the four-bar linkage—became the most successful of these linkages: first in the locomotive, then in the internal combustion engine, it allowed the motor to produce nothing but rotation.
FIGURE 2. (a) Watt’s “parallel-motion” linkage in schematic form: OA—A is the beam’s arm, acting as a crank; A—B is the coupler; OB—B is the follower, anchored to a wall. The point M on the coupler will trace out a figure eight, part of which is “straight” and can be used to guide the piston rod. (b) The mechanism on Watt’s engine. Point M is transposed to M′ by means of a pantograph and there guides the rod of a pump. The sun-and-planet gear on the working side of the beam would be useless without the continuous motion provided by the parallel linkage. “Floor and datum” is what Reuleaux (and Heidegger) call Gestell. Reproduced with permission of The McGraw-Hill Companies from Richard Hartenberg, Kinematic Synthesis of Linkages,© 1964.
From a kinematic point of view, it is irrelevant where the motion of a mechanism originates and where it is utilized, as kinematics is not concerned with forces or with stresses on material that might result from the impact of forces.23 Kinematic transmission functions without a fixed origin (such as straight-line motion) and without a determined destination (such as pure rotation) but is concerned (to use Walter Benjamin’s term) with translatability (Übersetzbarkeit) as such. All that linkages, and machines in general, need is a frame that determines the orientation of their movements; it generally consists in anchoring one link to an immobile part that in figure 2 is called the “floor and datum” but that in Reuleaux’s seminal terminology becomes Gestell.24
FIGURE 3. Drawing of a Watt and Boulton steam engine, after 1784. The parallel-motion linkage is on the right; on the left is the sun-and-planet gear driving a flywheel. Also visible on the left side is a governor, another of Watt’s inventions. It rotates with the engine stroke and shuts down the steam supply if the machine runs