Pascal made contributions to developments in both hydrostatics and hydrodynamics. Archimedes discovered the principles of mechanics and hydrostatics. In 1880 Lyapunov received a gold medal for a work on hydrostatics. It was there that Sauveur did his work on hydrostatics. He lectured on mechanics, hydrostatics, astronomy, optics, electricity and magnetism. This feature of hydrostatics has been called the hydrostatic paradox. The hydrostatic equilibrium pertains to hydrostatics and the principles of equilibrium of fluids. The inexhaustible bottle is a simple demonstration of hydrostatics, specifically Pascal's law. His first book was "A Treatise on Hydrostatics", Cambridge, 1830. Fluid statics or hydrostatics is the branch of fluid mechanics that studies fluids at rest. Sonics differs from hydrostatics, being based on waves, rather than pressure, in the liquid. "Fundamentals of Hydraulic Engineering" defines hydrostatics as the study of fluids at rest.