He was the author of numerous inventions, including the cagniardelle, a blowing machine, which consists essentially of an Archimedean screw set obliquely in a tank of water in such a way that its lower end is completely and its upper end partially immersed, and operated by being rotated in the opposite direction to that required for raising water.
In acoustics he invented, about 1819, the improved siren, which was named after him, using it for ascertaining the number of vibrations corresponding to a sound of any particular pitch, and he also made experiments on the mechanism of voice-production.
In 1822, he discovered the critical point of a substance in his famous cannon barrel experiments. Listening to discontinuities in the sound of a rolling flint ball in a sealed cannon filled with fluids at various temperatures, he observed the critical temperature. Above this temperature, the densities of the liquid and gas phases become equal and the distinction between them disappears, resulting in a single supercritical fluid phase.
In course of an investigation in 1822–1823 on the effects of heat and pressure on certain liquids he found that for each there was a certain temperature above which it refused to remain liquid but passed into the gaseous state, no matter what the amount of pressure to which it was subjected, and in the case of water he determined this critical temperature, with a remarkable approach to accuracy, to be 362 °C. He also studied the nature of yeast and the influence of extreme cold upon its life.