"General area measurement, mean height of steam engine indicator diagrams"
Notes: "The Amsler Type 6 is the same as the Type 4 but with two steel pointers added to the top of the tracing arm. These pointers are used to help determine the mean height of indicator diagrams. Thus it makes perfect sense then that this planimeter was sold through the Crosby Steam Gage & Valve Co. of Boston. Their name is beautifully engraved on the pole arm. The Type 6 was introduced in the early 1880s, and with a serial number of 11021, this is an early one, probably from 1890-1910."