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Clever use of a worm driving two gears one with 60 teeth and one with 59, gives a differential rotation of the two plates carrying the scales. Notice that the device could probably be read to about one foot (ie one fifth of the divisions on the thousandths scale). The lower scale rolls over after 6 miles. So it is rather like the Jones counter 140 years later, a little less precise and harder to read, but I am sure it served well during the survey for approximate measurements. The surveyor's wheel to which it was attached would be 8.8 feet in circumference, rather more than our bike wheels today.