John Herapath (30 May 1790 – 24 February 1868) was an English physicist who gave a partial account of the kinetic theory of gases in 1820 though it was neglected by the scientific community at the time.
Motivated by his search for a mechanical explanation of gravitation, he started to consider how a system of colliding particles could give rise to action at a distance.
He used momentum, rather than the kinetic energy on which the later established theory is based, as it seemed to him to avoid some difficulties around whether elastic collisions were possible between indivisible atoms.
Apparently ignorant of Daniel Bernoulli's work, he was led to the incorrect, but suggestive, relationship that expresses the product of pressure P and volume V as proportional to the square of his true temperature.
He revised his theories in the 1840s, largely based on the experimental work of Thomas Graham and Henri Victor Regnault.