Denis Papin, an associate of Boyle's, built in 1679 a bone digester, which is a closed vessel with a tightly fitting lid that confines steam until a high pressure is generated. Later designs implemented a steam release valve to keep the machine from exploding. By watching the valve rhythmically move up and down, Papin conceived of the idea of a piston and cylinder engine. He did not however follow through with his design. Nevertheless, in 1697, based on Papin's designs, Thomas Newcomen greatly improved upon engineer Thomas Savery 's earlier "fire engine" by incorporating a piston. This made it suitable for mechanical work in addition to pumping to heights beyond 30 feet, and is thus often considered the first true steam engine.