This instrument, which was brought from Germany by Prince Mattias de' Medici, is made up of a wooden wheel. At the centre of the hub of the wheel is a pivot. The interior of the hub contains a vertical wheel provided with a cog with 60 teeth, which meshes with a sprocket with ten spindles, in such a way that for every rotation of the wheel the sprocket rotates six times.
A second toothed wheel with 60 teeth is situated in the arm of the instrument, at the same height as the circumference of the wooden wheel, in such a way that six revolutions of the sprocket correspond to one revolution of this toothed wheel, exactly as occurs for the wooden wheel.
A system of meshing toothed wheels, of 30, 24, 24, 40, 20 and 60 teeth respectively is located inside the quadrant placed on the upper part of the arm. This quadrant is provided with three needles of different lengths, and a pointer. It is subdivided into concentric circles. The outer circle is numbered from 2 to 100, the second is numbered from 2 to 100 in the opposite direction, the third is numbered 50, 100, 150 and so on up to 1000, the fourth 5, 10, 15 and so on up to 100, and, finally, the inner circle is numbered from 1 to 6, repeated five times.
When the large wooden wheel makes a single rotation, the second toothed wheel also makes one rotation, while the three cogs of 30, 24 and 24 teeth inside the quadrant only make 1/5 of a rotation. The small needle records fifths of a rotation. The middle-sized needle, linked to the cog with 40 teeth, marks 100 geographical paces on the corresponding display. The longest needle marks the distance traversed on the display 50 to 1000, representing the geographical mile. The number of miles walked can be read through the hole made in the quadrant.
The instrument was used by the members of the Accademia del Cimento, in an attempt to measure the length of the meridian.
See G. Babbini, "Descrizione d'un antico odometro", in Annali dell'Imperial Museo di Firenze, vol. II (Florence, 1810), second part, pp. 13-20.