The first reference to the use of mechanical clocks to measure longitude is found in the writings of the cosmographer Reiner Gemma Frisius (1508-1555), who illustrated the method in De principiis astronomiae (1530). Mechanical clocks, however, were still very inaccurate, and there were certainly none on board ships in Vespucci's day (1454-1512). The mechanical clock operates thanks to an oscillation mechanism, which marks the flow of time, and the escapement, which counts the beats. Compared with astronomical timekeeping methods, the mechanical clock is less accurate, but it can be read at any time of day or night, even in adverse weather conditions.