Figure 9.—Watt's mechanisms for guiding the upper end of the piston rod of a double-acting engine (British Patent 1432, April 28, 1784). Top left, straight-line linkage; top right, crosshead and guide arrangement; lower left, piston rod A is guided by sectors D and E, suspended by flexible cords. From James P. Muirhead, The Origin and Progress of the Mechanical Inventions of James Watt (London, 1854, vol. 3, pls. 21, 22).

Brilliant as was the conception of this linkage, it was followed up by a synthesis that is very little short of incredible. In order to make the linkage attached to the beam of his engines more compact, Watt had plumbed his experience for ideas; his experience had yielded up the work done much earlier on a drafting machine that made use of a pantograph.[20] Watt combined his straight-line linkage with a pantograph, one link becoming a member of the pantograph.

[20] "It . . . Read More