Erasmus Darwin's improved design for steering carriages and cars

Carriage journeys in England during the eighteenth century were notoriously dangerous. Rutted and pot-holed roads exacerbated the deficiencies in steering, springing and stability. In 1758 the young Dr. Erasmus Darwin (1731-1802; F. R. S. 1761) was travelling about 10,000 miles a year in visits to patients from his house at Lichfield. To alleviate the danger and discomfort of his journey, he developed a design for improved carriage steering and stability, which he road-tested over 20,000 miles on two carriages. In 1765 Richard Lovell Edgeworth (1744-1817; F. R. S. 1718) heard about Darwin's design, urged the Society of Arts to enquire about it, and then visited Darwin himself. With the aid of manuscripts from the Archive of the Royal Society of Arts and elsewhere, I offer a reconstruction of Darwin's improved method of steering, which relies on four jointed rods, initially in the form of an isosceles trapezium. The mechanism was reinvented more than 50 years later, and came to be used widely in early modern cars.