Ernst Mach is best known for an 1877 paper on supersonic velocity in which he gave his name to the ratio of an object's speed to the speed of sound (an object moving at the speed of sound is at Mach 1). He worked in electricity, optics, acoustics and mechanics as a professor of physics, mathematics and philosopher in Graz, Prague and Vienna, and earned a reputation as an expert experimenter and lucid writer. A critic of Isaac Newton's model of a mechanistic universe, Mach argued that all knowledge was based on sensation, and that all scientific measurements were dependent upon the observer's perception. His scientific work influenced Albert Einstein, and his metaphysical writings influenced positivist philosophers such as Bertrand Russell. Mach's books include The Development of Mechanics (1883), The Analysis of the Sensations (1886) and Knowledge and Error (1905).