Daniel returned to Basel seven years later. He was the most famous of Johann’s three sons and devoted most of his energies to probability, astronomy, physics, and hydrodynamics. In probability he devised the concept of moral expectation, and in his Hydrodynamica, of 1738, appears the principle of hydrodynamics that bears his name in all present-day elementary physics texts. He wrote, on tides, established the kinetic theory of gases, studied the vibrating string,