Dissertation on a number of new types of tautochrone curves
According to Eneström: "Intrigued by a clock invented by D. Sully, Euler busied himself with the construction of curves which 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.