Dissertation on a number of new types of tautochrone curves
According to Eneström, Euler was inspired by a clock invented by D. Sully, and sought to construct curves that cause a pendulum to swing isochronally.
Original Source Citation
Commentarii academiae scientiarum Petropolitanae, Volume 2, pp. 126-138.
Opera Omnia Citation
Series 2, Volume 6, pp.4-14.