  Other accomplishments in physics included a compelling measurement of the speed of sound (showing that sound travels at the same speed, no matter the nature of its pitch), and the first satisfactory interpretation of the Pascalian barometry experiment. In his account of the Puy-de-Dôme experiment, Gassendi proposes that variations in air pressure are relative to atmospheric conditions and altitude, as the air is an elastic gas. He also suggests that this experiment (which he repeats at Toulon in 1650) shows that created vacuum is possible, at least as accumulated among part of the air particles in the instrument ‘sealed’ by the mercury column in the experimental apparatus. In establishing the elasticity of air as a gas and accumulated void as a result of particle displacement, Gassendi evokes his ontology of atoms and the void.