Joseph Fourier (1768-1830) like many brilliant scientists and mathematicians before the 20th century and modern tendency towards narrow specialisation, excelled in many fields and combined theoretical brilliance with practical ability. An orphan at the age of ten, he was educated in a military school and an abbey, showing outstanding mathematical ability from a young age. Joseph was a man of his times and played his own role in the French Revolution, subsequently serving in Napoleons armies before taking up a position at the Governor of Lower Egypt (imagine, your mathematics lecturer being the governor of lower Egypt !). He loyalty to Napoleon continued, as he also served in his armies during Napoleon's brief return to power in 1815. Certainly, the revolution had been important in providing a person like Fourier of humble birth ( he was the son of a tailor) opportunities to excel and make a mark in French society.
After several adventures in Egypt, he returned to France and mixed his ability in administration with experimental science and mathematics. He was made a Baron in 1808 and served in senior roles in the Academy of Sciences. Fourier was particularly interested in finding mathematical methods for describing heat flow. It was in this context that Fourier developed the idea that any continuous or dis-continuous function could be expressed as a infinite series of trigonometric functions. He wasn't able to prove this to be correct or general but he did develop techniques that proved invaluable since for many wide ranging mathematical problems. It is Fourier who showed that any complex wave form could be broken down into a combination of simpler wave forms. This remains a brilliant insight that guarantees his place as one of the great figure of mathematics. Merci Monsieur Fourier !
PS. In recent times, people have argued that Fourier was also the first to correctly identify the mchanism of global warming (see http://www.aip.org/history/climate/co2.htm)
Larousse, Dictionary of Scientists, 1994, New York
Delvin, The Language of Mathematics, 1998, New York