Guillaume Amontons

Among his contributions to scientific instrumentation were improvements to the barometer (1695), hygrometer (1687), and thermometer (1695), particularly for use of these instruments at sea. He also demonstrated an optical telegraph and proposed the use of his clepsydra[1] (water clock) for keeping time on a ship at sea.

Amontons investigated the relationship between pressure and temperature in gases though he lacked accurate and precise thermometers. Though his results were at best semi-quantitative, he established that the pressure of a gas increases by roughly one-third between the temperatures of cold and the boiling point of water.[2] This was a substantial step towards the subsequent gas laws and, in particular, Gay-Lussac's law. His work led him to speculate that a sufficient reduction in temperature would lead to the disappearance of pressure. Though he came close to finding absolute zero - the theoretical temperature by which the volume of air in his air-thermometer will be reduced to nothing (estimated by him as −240° on the Celsius scale),[3] the discovery would not be complete until at least a century later.

Guillaume Amontons is also the inventor of the hot air engine.[4] In 1699, he built his first engine, more than a century earlier than the well-known Stirling engine.[5] This engine, named by Amontons a "fire mill" (moulin à feu) followed a new thermodynamic cycle, which later became known as the Stirling cycle.

The fire mill is a wheel that makes use of the expansion of heated air to generate motive power. The calculated power of Amontons' fire mill was 39 HP, equal to the power of the most powerful hot air engines of the 19th century (with the exception of the "caloric engine" of Ericsson[6]). The main difference between Amontons' engine and the hot air engines of the 19th century was the nature of the piston (Amontons used water) and the use of rotational motion instead of alternating motion.

Free-body diagram for a block on a ramp. Arrows are vectors indicating directions and magnitudes of forces. N is the normal force, mg is the force of gravity, and F_f is the force of friction.

In 1699, Amontons published his rediscovery of the laws of friction first put forward by Leonardo da Vinci.[7] Though they were received with some skepticism, the laws were verified by Charles-Augustin de Coulomb in 1781.[8] For this contribution, Amontons was named as one of the 23 "Men of Tribology" by Duncan Dowson.[9]