Mendenhall's Practical Calculator

Mendenhall, A. · 1867

This patent model for an adder has a brass box with a rotating disc inset in the top. There are one hundred small holes around the rim of the disc. Outside the disc, on the top of the box, is a circular ring numbered from 01 to 99, with a gap at 00. Outside of this are three rings of holes, with one hundred holes in each ring. These holes are to be used to hold markers indicating digits carried when the disc makes full rotations. On the side of the box is a rotating multiplication table.