He worked hard on constructing mathematical instruments that he has promised to construct before his arrest. John Dee arrived in Louvain in 1548 and quickly became friends with Mercator. Dee, who spent three years working with Mercator, later wrote:
Siya ay nagtrabaho nang husto sa constructing matematiko instrumento na siya ay ipinangako upang bumuo ng bago ang kanyang pag-aresto. John Dee arrived in Louvain sa 1548 at mabilis na naging mga kaibigan sa Mercator. Dee, na ginugol ng tatlong taon na nagtatrabaho sa Mercator, mamaya wrote: