Bernoulli, Daniel (February 8, 1700 - March 17, 1782) is the most distinguished of the second generation of the Bernoulli family of Swiss mathematicians. He was a Swiss physicist and mathematician who made important discoveries in hydrodynamics. He was the only member of his family to make a mark in physics. He investigated not only mathematic but also such fields as medicine, biology, physiology, mechanics, physics, astronomy, and oceanography. Bernoulli's theorem, which he derived, is named after him.
Daniel Bernoulli was the second son of Johann Bernoulli, who first taught him mathematics. After studying philosophy, logic, and medicine at the Universities of Heidelberg, Strasbourg, and Basel, he received an M.D. degree (1721); and in 1723-24 he wrote Exercitationes quaedam Mathematicae on differential equations and the physics of flowing water, which won him a position at the influential Academy of Sciences in St. Petersburg, Russia. Bernoulli lectured there until 1732 in medicine, mechanics, and physics, and he researched the properties of vibrating and rotating bodies and contributed to probability theory. In that same year he returned to Basel to accept the post in anatomy and botany. By then he was widely esteemed by scholars and also admired by the public.
Daniel's reputation was established in 1738 with Hydrodynamica, in which he considered the properties of basic importance in fluid flow, particularly pressure, density, and velocity, and set forth their fundamental relationship. The Hydrodynamica is both a theoretical and a practical study of equilibrium, pressure and velocity of fluids. He put forward what is called Bernoulli's principle, which states that the pressure in a fluid decreases as its velocity increases. He also established the basis for the kinetic theory of gases and heat by demonstrating that the impact of molecules on a surface would explain pressure and that, assuming the constant, random motion of molecules, pressure and motion increase with temperature.