Left: plots of the digitised geomagnetic measurements from August and September 1859. From top to bottom the declination, inclination and horizontal force are shown in scale parts. The red and black lines denote the measurements in Utrecht and Den Helder respectively. Right: the same measurements scaled to modern day units (Sect. 4) with the Den Helder declination and inclination measurements offset to 1 degree (black), and the Utrecht declination and inclination measurements offset to 0.5 degrees (red). The horizontal intensity measurements have been offset to 300 and 600 nT for the Utrecht and Den Helder measurements respectively.