ODOMETERS.
In the inventory of the objects sold after the death of the Emperor Commodus, drawn up by Julius Capitolinus in the life of Pertinax, we find mentioned, among other valuable things, “vehicles that mark distances and hours.”
Vitruvius (X, 14) describes the mechanism of these vehicles, but the figures that must have served to throw light upon the text have been lost, so that his description is somewhat obscure. Fortunately, as a sequel to a manuscript of the Dioptra of Heron, there have been found two Greek fragments upon this same subject, dating back probably to the Alexandrine epoch and accompanied with figures. The following is a translation, says M. de Rochas:
To Measure Distances upon the Surface of the Earth by Means of an Apparatus called an Odometer.
Provided with this instrument, instead of being obliged to measure land slowly and laboriously with the chain or cord, it is possible in traveling in a vehicle to know the distances made, according to the number of revolutions of the wheels. Others, it is true, have, previous to us, made known certain methods of accomplishing the same object; but every one will be able to decide between the instrument described here by us and those of our predecessors.
FIG. 1.—HERON’S ODOMETER FOR VEHICLES.