Patent Model for Estimator Slide Rule Invented by Fredric Maurice Stapff

In 1874 F. M. Stapff submitted this model to the U.S. Patent Office. He designed the instrument to calculate the volume of spaces shaped as prisms, such as embankments and ditches. It consists of a wooden base with two slides, one atop the other. The lower slide has pieces of paper pasted on both sides, containing various curves related to finding the equivalent heights of an embankment (e.g., the height of an embankment of the same volume having trapezoidal sides).

The upper slide has five scales on it, numbered from 3 to 7. The upper part of the base has two scales, numbered 1 and 2. The lower part of the base has a scale numbered 8. Scales 1, 2, 3, ...