The variational principle was not used to derive the equations of motion until almost 75 years later, when William Rowan Hamilton in 1834 and 1835 applied the variational principle to the Lagrangian function L = T − − V (where T is the kinetic energy and V the potential energy of an object) to obtain what are now called the Euler–Lagrange equations. Hamilton believed his results were constrained by conservation of energy, which he called conservation of living force.