Animation of Watt's Linkage Dimensions (unit lengths a, b):
Link 3: a + a
Links 2 & 4: b
Vertical distance between ground joints ≈ 2a Horizontal distance between ground joints ≈ 2b Thus, link 1 (total distance between ground joints): Hand-drawn diagram by James Watt (1808) in a letter to his son, describing how he arrived at the design.[1]
A Watt's linkage is a type of mechanical linkage invented by James Watt in which the central moving point of the linkage is constrained to travel a nearly straight path. Watt's described the linkage in his patent specification of 1784 for the Watt steam engine.
Today it is used in automobile suspensions, where it is key to a suspension's kinematics, i.e., its motion properties, constraining the vehicle axle's movement to nearly vertical travel while also limiting horizontal motion.
^Franz Reuleaux, The Kinematics of Machinery (1876), page 4.