Raised in poverty in his native Holland, Stevin was one of the first people to make a living purely from his inventions.

He was an extremely talented engineer and worked for the Dutch army building windmills, locks and ports.

He also designed an extraordinary land yacht and even preceded Galileo’s work on motion by a number of years.

He wrote, in total, eleven books on a wide range of mechanical topics usually allied to water. This culminated in his pioneering work in the field of hydrostatics; he showed that the pressure exerted by a liquid upon any given surface depends on the height of the liquid and the area of the surface.

His mathematical reputation was established with the 1585 publication of De Thiende in which he presented a simple and thorough account of decimal fractions. Stevin did not invent decimals - the Chinese and Arabs had used them for many years. But he did standardise their usage and introduce the method to mathematical use.

Decimals are vital tools for engineers and scientists. Without them multiplication and division become almost impossible - all the methods that we use exploit the decimal place position of the digits that make up a number. Let us illustrate this with an example - 25 multiplied by 105.

In roman numbers this is the product of XXV and CV. But how can this be carried through? Most likely it would be carried out by repeated addition:

CV + CV + CV + … + CV | = 105 + | 105 + … | |

= | 210 + | 105 + … | |

= | 315 + 105 + … |

very tedious. In decimal notation, it is easy - we do it as * one* addition, 5x105 + 20x105:

105 + 25 _____ 525 <---- This line is 5 x 105 210 <---- This line is 20 x 105 _____ 2625

In the decimal notation we only need to remember multiplication tables up to 10. The place value system takes care of everything else.

He was also active in financial affairs. Banks had used secret tables of interest to charge merchants when they borrowed money for speculative ventures. The interest charged was either simple or compound. For example, suppose that a merchant required $1000 for five years with an interest rate of 2% every six months. Then the amount the merchant had to repay was:

**Simple interest** \qquad\mbox{Repayment}\quad\to\quad{1000.\left( 1+{2\over{100}}.10 \right)=$1200}

**Compound interest** \qquad\mbox{Repayment}\quad\to\quad{1000.\left( 1+{2\over{100}} \right)^{10}\approx$1219}

In either case it is difficult to calculate the interest (decimals make the former relatively easy; the latter is difficult even with these). Banks bought sets of tables that they could simply read off the answer. Mathematicians prepared the tables and sold copies of them - hand copies that is - to the Banks. Stevin changed that. He had the brilliant idea of printing the tables and selling them as books. Banks used books of such tables right up until the 1960s.

In another book, *Problemata geometrica* published in 1583, Stevin looked at certain aspects of geometry which were based on Archimedes and Euclid but also influenced by an artist called Durer. Durer was a draughtsman as well as an artist and he explored the geometry of paintings, diagrams and other constructions that made up the subject we might now call ‘design’. Drawing on this, Stevin’s book gives a practical account of constructions related to various polygons and polyhedra.

Stevin was an inventive, resourceful and successful mathematician, engineer, inventor and businessman.