Jean-Jacques d'Ortous de Mairan (26 November 1678 – 20 February 1771) was a French natural philosopher (physicist), born in the town of Béziers on 26 November 1678. De Mairan lost his father, François d'Ortous, at age four and his mother twelve years later at age sixteen. Over the course of his life, de Mairan was elected into numerous scientific societies and made key discoveries in a variety of fields including ancient texts and astronomy. His observations and experiments also inspired the beginning of what is now known as the study of biological circadian rhythms. At the age of 92, de Mairan died of pneumonia in Paris on 20 February 1771.
Biography
De Mairan attended college in Toulouse from 1694–1697 with a focus on ancient Greek. In 1698 he went to Paris to study mathematics and physics under the teachings of Nicolas Malebranche. In 1702, he returned home to Béziers and began his lifelong study of several fields, most notably astronomy and plant rhythms. Furthermore, during his time in Béziers, he ate almost every day with the Bishop, Louis-Charles des Alris de Rousset. In 1723, de Mairan, who had become a member of the Académie Royale des Sciences, co-founded the Académie de Béziers under the protection of Cardinal de Fleury, acting prime minister of Louis XV (and himself a protégé of a former Bishop of Béziers, Pierre de Bonsi). Eventually, de Mairan received official lodging in the Louvre where he remained pensionnaire until 1743 and served as secretary from 1741 to 1743. In 1746, he was reinstated as pensionnaire géomètre, or full-time boarding surveyor. It is reported that the Prince of Conti and other great lords heaped extravagant gifts upon him. He was also secretary to the Duke of Orléans.
Observations and notable experiments
In 1719, De Mairan discussed the varying obliquity of light that causes cold in winter and heat in summer. He postulated that the sun's heating effect was related to the square of the sine of its elevation. He neglected the effects of atmosphere, admitting that he did not know how much the sun's heat would be absorbed by it. Two and a half years later, he presented a paper to the Academie Royale des Sciences in Paris: "Problem: the ratio of two degrees or quantities of sunlight seen through the atmosphere at two different known angular elevations being given, to find what part of the absolute light of the sun is intercepted by the atmosphere at any desired elevation." In this paper, de Mairan made a hypothesis based on mere observations, supposing that the ratio had been measured, even though it had not. The significance of de Mairan's work, although incorrect, led his protégé, Pierre Bouguer, to invent the photometer. In 1729, de Mairan constructed an experiment showing the existence of a circadian rhythm in plants, presumably originating from an endogenous clock (See 'Experiment on circadian rhythms in plants' below). In 1731, he also observed a nebulosity around a star near the Orion nebula. This was later designated M43 by Charles Messier. In 1731, he published "Traite Physique et Historique De l'Aurore Boreale" (a short summary appeared in the Philosophical Transactions ) in which he put up a novel hypothesis that the Northern Lights are caused by the Sun, as the interaction of the atmosphere with the zodiacal light. At the time, the aurorae were thought to be 'flames' caused by sulfurous effluvia emanating from the Earth.
Experiment on circadian rhythms in plants
In 1729, de Mairan performed an experiment that demonstrated the existence of circadian rhythms in plants, specifically the Mimosa pudica. He was intrigued by the daily opening and closing of the heliotrope plant and performed a simple experiment where he exposed the plants to constant darkness and recorded the behaviour. De Mairan's key conclusion was that the daily rhythmic opening and closing of the leaves persisted even in the absence of sunlight. However, de Mairan did not infer that heliotropes have internal clocks driving leaf rhythms, but rather that they were able "to sense the Sun without ever seeing it". The concept of an internal clock was actually not formulated until much later, although de Mairan did suggest that "it would be curious to test [...] whether, using kilns heated to higher or lower temperatures, one could artificially recreate a day & night perceptible to [plants]; and whether in doing so one could reverse the order of the phenomena of true day & true night."
