Gasometer is an instrument capable of controlling and measuring the uniform flow and volumes of the gases it dispensed. Gas flow was controlled by mechanical balance. Lavoisier was using these instruments by working volumetrically and had to combine gas volumes with the weight of solids converting volumes via densities. Constant pressure was kept on the gas in the main reservoir by means of weights on a mechanical balance using rolling bearings to minimise friction. Pressure and the height of gas in the reservoir could be measured by manometer.
Antoine Laurent de Lavoisier was the real founder of modern chemistry. He invested most of his fortune, accumulated thanks to his office as Farmer General, in having the best manufacturers of his time make the scientific instruments that won renown for his laboratory in the Paris Arsenal.