Copenhagen, 1654. A young academic is packing his books in rather a hurry. He is a short, rather intense man in his early thirties. Always full of energy, the hasty move is beginning to upset his normally even temper. One item he remembers to pack is a pair of notebooks full of work about the manipulation and use of ratios. But one thing that doesn't find a place in his trunk is the little prospectus he published the previous year, outlining the contents of a book he proposed to write about the same subject. Now it doesn't seem likely that the book will ever be written.
The academic's name was Nicolaus Mercator. Not Gerard Mercator, who made maps and has the Mercator projection named after him. Our Mercator was brought up – in the region of Holstein, in Germany – as Niklaus Kauffman: as an adult he translated his surname (it means 'merchant') into Latin. Not an unreasonable thing for a scholar to do who travelled as much as he did: but he probably wasn't sorry to gain the association with his famous namesake. He studied in Rostock and Leiden, and then got a job at the University of Copenhagen.
In 1654 Mercator left Copenhagen in a hurry because plague had broken out. He had little choice: the epidemic was so bad that the university closed and he was temporarily out of a job. Plague had been spreading north and west from the Balkans since the beginning of the 1650s, and after it reached northern Poland it spread rapidly around the rest of the Baltic coast: when it reached Copenhagen, a fifth of the city's inhabitants died. Meanwhile plague was also spreading from the west: it had never been eradicated in the Low Countries after an epidemic in the 1640s. The outbreak in the '50s abated before reaching central Germany, or northern France or England. But it was to break out again in the Low Countries in 1663, and this time it reached England and caused the Great Plague of London in 1664. Mercator was not to know that, and in 1654 he fled to England.