I had John Keay's book, The Great Arc, for Christmas. It is about the surveying of India 1803-1850. George Everest had a differential counter built for his perambulator (ie surveyor's wheel). You can see a picture of it, --you probably will need to enter everest into the database search form given by the link. I will try and drop into the museum in Oxford later this week to have a close up look. The caption implies that it counts the wheel rotations giving a resolution of one thousandth of a mile.