Mendenhalls Practical Calculator. This patent model for an adder has a brass box with a rotating disc inset in the top. There are 100 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.

---

Size: 1940px × 1288px
Photo credit: © SBS Eclectic Images / Alamy / Afripics
License: Licensed
Model Released: No

Keywords: mathematics, medicine, science