(1700-1782) Dutch/Swiss
Theoretical and Experimental Physicist (Hydrodynamics), Mathematical Physicist
Daniel Bernoulli was an early pioneer of hydrodynamics, who also laid the groundwork for the later development of the kinetic theory of gases. He is famous for discovering what has come to be called the Bernoulli effect, the relation between the pressure in a steadily flowing fluid and its velocity, which is crucial to an understanding of how airplanes fly.
He was born in Groningen, the Netherlands, on February 8, 1800, into a family of brilliant, fiercely competitive mathematicians, which included his father, Johann; his uncle; and two brothers. When Daniel was five, the family returned to Basel, Switzerland, their native city, where Johann took over the chair in mathematics after the death of its previous occupant, his brother Jakob. Johann was determined to steer Daniel away from a career in mathematics, allegedly because it paid poorly, and planned to prepare him to be a merchant. First, however, he sent him at age 13 to the University of Basel to study logic and philosophy. When his son’s passion for mathematics persisted, Johann agreed to tutor him. It was Johann who introduced him to the theory of conservation of energy, which would later lead Daniel to his important work on the mathematical theory of fluid flow. When he had given up on making his son a merchant, Johann ordered him to study medicine. Daniel dutifully pursued a medical degree in Heidelberg, Strasbourg, and Basel, where he obtained a doctorate in 1721 with a thesis on respiration, in which he applied mathematical physics to medicine. Attracted to the work of the British physician William Harvey on blood as a fluid, he did research on blood flow and pressure, which combined his interests in mathematics and fluids.
Upon graduation, Bernoulli hoped to be an academician as his father was but was unable to obtain a position. Instead, at age 23, he set out for Padua, Italy, to study practical medicine. When illness led to enforced solitude, he spent his time studying mathematics and in 1724 published his first mathematical work, on probability. The following year he designed an hourglass for a ship that would flow even in stormy weather and won the first of a series of 10 prizes awarded by the Paris Academy for papers on such diverse topics as marine technology, navigation, oceanography, astrology, and medicine. He returned home to Basel that year and found a letter from Empress Catherine the Great, inviting him to become professor of mathematics in Saint Petersburg, Russia. To make the proposal more attractive, she offered a second position for his brother Nicolaus. The brothers accepted, but within eight months, Nicolaus was dead of tuberculosis. Grieving and oppressed by the harsh climate, Bernoulli wrote to his father, expressing his intention to return home. At this point Johann sent him his star student Leonard Euler to assist him in his work. Bernoulli remained in Russia, embarking on what would be a lifelong collaboration with the brilliant young mathematician.
From 1727, when Euler arrived, until 1733, when Bernoulli left Saint Petersburg, he enjoyed his most fruitful period. Probing the relationship between the speed at which blood flows and its pressure, he performed experiments that involved puncturing the wall of a pipe with a small open-ended straw and noting that the height to which the fluid rose in the straw was related to the fluid’s pressure in the pipe. Soon European physicians were measuring blood pressure by sticking point-ended glass directly into their patients’ arteries. In 1896 an Italian doctor would discover the less painful blood pressure cuff used today. However, Bernoulli’s original method of measuring the pressure in a flowing fluid is still used in modern aircraft to measure the speed of the air passing the plane, that is, its air speed.
In Saint Petersburg, Bernoulli completed his most famous work, Hydrodynamica, a theoretical and practical study of equilibrium, pressure, and velocity in fluids, which relied on his earlier work on conservation of energy; it would be published, after years of polishing, in 1738. When Bernoulli wrote his magnum opus, scientists knew that a moving body exchanges its kinetic energy for potential energy when it gains altitude. Bernoulli had the insight that in a similar way, a moving fluid exchanges its kinetic energy for pressure. From these principles he developed the principle of what is now known as the Bernoulli effect, which states that the pressure of a fluid depends inversely on its velocity: the pressure decreases as the velocity increases. Thus, the Bernoulli effect, which governs fluid flow and has many applications, is a consequence of conservation of energy. Applied to aerodynamics, it explains how a moving wing whose cross section has the shape of an airfoil (curved on the top, flat on the bottom) experiences the lifting force that allows an airplane to fly. The curve of the wing is designed to create a faster flow of air over the top of the wing than over the bottom. As a result of the Bernoulli effect, the air pressure over the top of the wing is lower than the air pressure beneath the wing. In this way, the wing is pushed upward, enabling an airplane to fly.
In his Hydrodynamica Bernoulli also attempted to construct a mathematical description of the behavior of gases, based on the assumption that they are composed of tiny particles. By producing an equation of state, that is, an equation that relates the pressure, temperature, and volume of a gas, he was able to relate atmospheric pressure to altitude. This was the first step toward the kinetic theory of gases that would be developed a century later.
Despite his productive collaboration with Euler, Bernoulli was unhappy in Saint Petersburg. In 1734, he returned to Basel to lecture, first on botany and later on physiology—the only post he could get at the time. He would continue to correspond with Euler, who put many of Bernoulli’s physical insights into rigorous mathematical form. When Bernoulli and his father were declared joint winners of the Paris Academy’s Grand Prize that year, the father, enraged that his son had been judged his equal, broke off relations with him. Bernoulli stayed in Basel, barred from his father’s house. A year after publication of Hydrodynamica, his father published Hydraulica, based on his son’s work but written as if his son’s work had been based on his. Europe, however, gave the younger Bernoulli the credit he was due, electing him to most of the leading scientific societies of his day. In 1750, Bernoulli at last became professor of physics at Basel, where he would remain for the next 26 years, giving a series of memorable physics lectures during which he performed actual experiments. In research that advanced the field of mathematical physics, he developed sir isaac newton’s theories and used them together with the more powerful calculus of Gottfried Leibnitz.
Bernoulli died on March 17, 1782, in Basel, where he was buried. An imaginative scientist with broad interests, Bernoulli helped launch the field of hydrodynamics, anticipated the kinetic theory of gases, and discovered a fundamental principle of aerodynamics.
