Hints of energy being conserved, much like momentum, were showing up by the 1840s. But unlike momentum conservation, which by comparison was quickly accepted and understood (pretty much by 1687 with Newton’s Principia), energy conservation remained a mystery until 1850.
In 1644, in his Principles of Philosophy, René Descartes (1596-1650) proposed that the total motion of the universe is conserved. While this conservation of motion concept bears a certain similarity with Newton’s conservation of momentum, it’s still wrong.
