Andrew Carol’s impressive Lego Difference Engine is a mechanical computer built with LEGO pieces:
From Andy Carol’s website:
Before the day of computers and pocket calculators all mathematics was done by hand. Great effort was expended to compose trigonometric and logarithmic tables for navigation, scientific investigation, and engineering purposes. The larger efforts involved rooms of semi skilled people, called ‘computers’, capable of doing reliable arithmetic who would be under the direction of a skilled mathematician. In the mid-19th century, people began to design machines to automate this error prone process. Many machines of various designs were eventually built but, the most advanced and famous of these was not. The Babbage Difference Engine.
Andy Carol’s Difference Engine solves 2nd and 3rd order polynomials to three or four digits. If the idea of building a mechanical computer seems a bit odd to you, note that until the 1970s there were no pocket electronic calculators, and therefore engineers had to use slide rules.