Simon Stevin

Simon Stevin (1548 – 1620) was a Flemish-Dutch engineer and mathematician, who was one the first to write rational numbers as decimal fractions (although he did not yet use the decimal point as place holder). He decomposed forces by using diagrams that were equivalent to what we now call "the parallelogram of forces" (see this article) and he did experiments—eighteen years before Galileo Galilei—that refuted Aristotle's law of free fall, that is, he found that heavy bodies do not fall faster than light ones. This is only strictly true in vacuum; when there is friction with air, lighter bodies experience theoretically a larger upward force than heavy bodies. .

Life
Simon Stevin was born in Bruges, one of the important cities of Flanders, the Dutch speaking part of Belgium. He worked as a merchant's bookkeeper in Antwerp, also in Flanders, and moved around 1580 to heatur Netherlands]], first to Middelburg and then in 1581 to Leiden. He was matriculated as a student in the University of Leiden on February 16th, 1583. Simon Stevin had four children. As to his marriage we only know of a notice of marriage with Catherina Cray at Leyden on April 10th 1616, at the age of 68, but Simon Stevin had children already before 1616). Stevin died in 1620; the exact date nor the place is known but he passed away between February 20th and April 8th and most probably in the Hague.

Work
In 1585 Stevin published a booklet, De Thiende [The Tenth], in which he presented an elementary and thorough account of decimal fractions and their daily use. Although he did not invent decimal fractions and his notation was rather unwieldy, he established their use in day-to-day mathematics. He declared that the universal introduction of decimal coinage, measures, and weights would be only a question of time. A year later he published De beghinselen der weeghconst [The principles of the art of weighing] (1586). With weeghconst Stevin means statics and in this work he continues the theoretical work of Archimedes on forces that keep bodies in equilibrium. He treats a solid body on a sloping plane and decomposes the gravitational force in one perpendicular to the plane and one that keeps the body in place, thus introducing the "the parallelogram of forces".

Reference
E. J. Dijksterhuis, Simon Stevin, Martinus Nijhoff, The Hague (1943) (In Dutch) He was the n