# Marin Mersenne

Marin Mersenne
Born 8 September 1588
Died 1 September 1648 (aged 59)
Paris
Nationality French
Known for Acoustics

Marin Mersenne, Marin Mersennus or le Père Mersenne (8 September 1588 – 1 September 1648) was a French theologian, philosopher, mathematician and music theorist, often referred to as the "father of acoustics" (Bohn 1988:225). Mersenne was "the center of the world of science and mathematics during the first half of the 1600s."[1]

## Life

Marin Mersenne (pronounced Mehr-SENN) was born of peasant parents near Oizé, Maine (present day Sarthe, France). He was educated at Le Mans and at the Jesuit College of La Flèche. On 17 July 1611, he joined the Minim Friars, and, after studying theology and Hebrew in Paris received his full holy orders in 1613.

Between 1614 and 1618, he taught theology and philosophy at Nevers, but he returned to Paris and settled at the convent of L'Annonciade in 1620. There he studied mathematics and music and met with other kindred spirits such as René Descartes, Étienne Pascal, Gilles de Roberval and Nicolas-Claude Fabri de Peiresc. He corresponded with Giovanni Doni, Constantijn Huygens and other scholars in Italy, England and the Dutch Republic. He was a staunch defender of Galileo, assisting him in translations of some of his mechanical works.

For four years, Mersenne devoted himself entirely to philosophic and theological writing, and published Quaestiones celeberrimae in Genesim (1623); L'Impieté des déistes (1624); La Vérité des sciences (Truth of the Sciences against the Sceptics, 1624). It is sometimes incorrectly stated that he was a Jesuit. He was educated by Jesuits, but he never joined the Society of Jesus. He taught theology and philosophy at Nevers and Paris.

In 1635 he set up the informal Académie Parisienne (Academia Parisiensis) which had nearly 140 correspondents including astronomers and philosophers as well as mathematicians and was the precursor of the Académie des sciences established by Colbert in 1666. He was not afraid to cause disputes among his learned friends in order to compare their views, notable among which were disputes between Descartes and Fermat and Beaugrand.[2] Peter L. Bernstein in his book Against the Gods: the Remarkable story of risk writes: "The Académie des Sciences in Paris and the Royal Society in London, which were founded about twenty years after Mersenne's death, were direct descendants of Mersenne's activities."[3]

In 1635 Mersenne met with Tommaso Campanella, but concluded that he could "teach nothing in the sciences (...) but still he has a good memory and a fertile imagination." Mersenne asked if René Descartes wanted Campanella to come to Holland to meet him, but Descartes declined. He visited Italy fifteen times, in 1640, 1641 and 1645. In 1643-1644 Mersenne also corresponded with the German Socinian Marcin Ruar concerning the Copernican ideas of Pierre Gassendi, finding Ruar already a supporter of Gassendi's position.[4] Among his correspondents were Josh, Dekar, Galilei, Roberval, Pascal, Bekman and other scientists.

He died through complications arising from a lung abscess.

## Works

### Quaestiones celeberrimae in Genesim (1623)

It was written as a commentary on the Book of Genesis and comprises uneven sections headed by verses from the first three chapters of that book. At first sight the book appears to be a collection of treatises on various miscellaneous topics. However Robert Lenoble has shown that the principle of unity in the work is a diatribe against magical and divinatory arts, cabalism, animistic and pantheistic philosophies. He mentions Martin Del Rio's Investigations into Magic and criticises Marsilio Ficino for claiming power for images and characters. He condemns astral magic and astrology and the anima mundi, a concept popular amongst Renaissance neo-platonists. Whilst allowing for a mystical interpretation of the Cabala, he wholeheartedly condemned its magical application—particularly to angelology. He also criticises Pico della Mirandola, Cornelius Agrippa and Francesco Giorgio with Robert Fludd as his main target. Fludd responded with Sophia cum moria certamen (1626), wherein Fludd admits his involvement with the Rosicrucians. The anonymous Summum bonum (1629), another critique of Mersenne, is an openly Rosicrucian text. The cabalist Jacques Gaffarel joined Fludd's side, while Pierre Gassendi defended Mersenne.

### L'Harmonie Universelle (1637)

This book contains of Mersenne's laws which describe the frequency of oscillation of a stretched string. This frequency is:

1. Inversely proportional to the length of the string (this was actually known to the ancients, and is usually credited to Pythagoras himself).
2. Proportional to the square root of the stretching force, and
3. Inversely proportional to the square root of the mass per unit length.

The formula for the lowest frequency is

$f=\frac{1}{2L}\sqrt{\frac{F}{\mu}},$

where f is the frequency, L is the length, F is the force and μ is the mass per unit length.

In this book, Mersenne, a disciple of René Descartes, also introduced several innovative concepts that can be considered as the basis of modern reflecting telescopes:

• Much earlier than Laurent Cassegrain, he found the fundamental arrangement of the two-mirrors telescope combination, a concave primary mirror associated with a convex secondary mirror and discovered the telephoto effect that is critical in reflecting telescopes, although it is obvious that he was far from having understood all the implications of that discovery.
• Mersenne invented the afocal telescope and the beam compressor that is useful in many multiple-mirrors telescope designs.[citation needed]
• He recognized also that he could correct the spherical aberration of the telescope by using nonspherical mirrors and that in the particular case of the afocal arrangement he could do this correction by using two parabolic mirrors, though in fact a hyperboloid is required.[5]

Because of criticism that he encountered, especially that of Descartes, Mersenne made no attempt to build a telescope of his own.

### Other

Mersenne is also remembered today thanks to his association with the Mersenne primes. The Mersenne twister, named for him, is frequently used in computer engineering, and is central to fields such as cryptography.

However, Mersenne was not primarily a mathematician; he wrote about music theory and other subjects. He edited works of Euclid, Apollonius, Archimedes, and other Greek mathematicians. But perhaps his most important contribution to the advance of learning was his extensive correspondence (in Latin) with mathematicians and other scientists in many countries. At a time when the scientific journal had not yet come into being, Mersenne was the centre of a network for exchange of information.

It has been argued that Mersenne used his lack of mathematical competence, his ties to the print world, his legal acumen, and his friendship with the French mathematician and philosopher René Descartes (1596-1650) to manifest his international network of mathematicians.[6]

Mersenne's philosophical works are characterized by wide scholarship and the narrowest theological orthodoxy. His greatest service to philosophy was his enthusiastic defence of Descartes, whose agent he was in Paris and whom he visited in exile in the Netherlands. He submitted to various eminent Parisian thinkers a manuscript copy of the Meditations on First Philosophy, and defended its orthodoxy against numerous clerical critics.

In later life, he gave up speculative thought and turned to scientific research, especially in mathematics, physics and astronomy. In this connection, his best known work is Traité de l'harmonie universelle (also referred to as Harmonie universelle) of 1636, dealing with the theory of music and musical instruments. It is regarded as a source of information on 17th-century music, especially French music and musicians, to rival even the works of Pietro Cerone.

One of his many contributions to musical tuning theory was the suggestion of

$\sqrt[4]{\frac{2}{3-\sqrt{2}}}$

as the ratio for an equally-tempered semitone ($\sqrt[12]{2}$). It was more accurate (0.44 cents sharp) than Vincenzo Galilei's 18/17 (1.05 cents flat), and could be constructed using straightedge and compass. Mersenne's description in the 1636 Harmonie universelle of the first absolute determination of the frequency of an audible tone (at 84 Hz) implies that he had already demonstrated that the absolute-frequency ratio of two vibrating strings, radiating a musical tone and its octave, is 1 : 2. The perceived harmony (consonance) of two such notes would be explained if the ratio of the air oscillation frequencies is also 1 : 2, which in turn is consistent with the source-air-motion-frequency-equivalence hypothesis.

He also performed extensive experiments to determine the acceleration of falling objects by comparing them with the swing of pendulums, reported in his Cogitata Physico-Mathematica in 1644. He was the first to measure the length of the seconds pendulum, that is a pendulum whose swing takes one second, and the first to observe that a pendulum's swings are not isochronous as Galileo thought, but that large swings take longer than small swings.[7]

## Music

An air attributed to Mersenne was used by Ottorino Respighi in his second suite of Ancient Airs and Dances.

## References

1. ^ Bernstein, Peter L. (1996). Against the Gods: The Remarkable Story of Risk. John Wiley & Sons. p. 59. ISBN 978-0-471-12104-6.
2. ^ Sergescu, Pierre (1948). "Mersenne l'Animateur". Revue de l'Histoire des sciences et leur applications 2 (2:1): 5–12.
3. ^ Bernstein 1996, p. 59
4. ^ Murr, Sylvia, ed. (1997) (in French), Gassendi et l'Europe, Paris: Vrin, ISBN 978-2-7116-1306-9.
5. ^ Pendergrast, Mark (2003). Mirror Mirror: A History of the Human Love Affair with Reflection. Basic Books. pp. 88–89. ISBN 0786729902.
6. ^ Grosslight, Justin (2013) "Small Skills, Big Networks: Marin Mersenne as Mathematical Intelligencer", History of Science 51, 337-374.
7. ^ Koyre, Alexander (1992). Metaphysics and Measurement. Taylor & Francis. p. 100. ISBN 2-88124-575-7.
• Bohn, Dennis A. (1988). "Environmental Effects on the Speed of Sound". Journal of the Audio Engineering Society 36 (4): 223–231.

## Bibliography

### Works by Mersenne

• Euclidis elementorum libri, etc. (Paris, 1626)
• Les Mécaniques de Galilée (Paris, 1634)
• Questions inouies ou récréation des savants (1634)
• Questions théologiques, physiques, etc. (1634)
• Harmonie universelle First edition on line from Gallica. (Paris, 1636-7). Translation to English by Roger E. Chapman (The Hague, 1957)
• Nouvelles découvertes de Galilée (1639)
• Cogitata physico-mathematica (1644)
• Universae geometriae synopsis (1644)