General area measurement and determining the mean height of indicator diagrams.
Notes: "This is an Amsler Type 6 planimeter that was sold by W.F. Stanley in the UK. The Type 6 differs from the Type 4 by having two spike sticking up from the tracing arm. Those spikes are used to determine the mean height of indicator diagrams. Based on the serial number, 21853, this planimeter was made in approximately 1880."
