<human>:  What was the most famous discovery of the famous physicist William Nelson Cromwell?
<bot>: That would be his discovery of the Cromwell paradox. The Cromwell paradox is a thought experiment that demonstrates that it is impossible to construct a perpetual motion machine. The paradox states that if a machine is designed so that it always runs in the same direction, then it will run forever. This is because the machine will always be running against the force of friction.