Evangelista Torricelli was an Italian physicist and mathematician who is best known for inventing the barometer, a device used to measure atmospheric pressure. He was a student and assistant to Galileo Galilei and made significant contributions to the understanding of the principles of mechanics and the behavior of gases. Torricelli's work laid the foundation for the development of the science of hydrodynamics and the study of atmospheric pressure. He is considered one of the great scientists of the 17th century.