What mechanical device did Watt devise to ensure the piston rod moved in a perfectly straight line?

Answer

The parallel motion linkage.

Watt devised the parallel motion linkage in 1784 to ensure the piston rod moved in a perfectly straight line, translating the rocking beam’s circular arc into the required perpendicular action.

What mechanical device did Watt devise to ensure the piston rod moved in a perfectly straight line?
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