The Zambretti Forecaster is a weather forecasting tool in the form of a circular slide rule that was introduced by the company Negretti and Zambra in 1920.
A more detailed description about the functionality of the Zambretti Forecaster and how to put it into code can be found here.
It claims to accurately predict near-term weather based on barometric pressure, the trend in the pressure (rising, steady, or falling), and wind direction. It relies on the fundamental meteorology of pressure fronts. Watch any TV weather forecast and you will see high- and low-pressure areas prominently displayed on the map. The boundaries between pressure areas are strongly associated with wind and precipitation. Additionally, high-pressure is usually associated with fair weather while low-pressure is associated with cloudy, rainy, or snowy weather.
The are many interesting things about the Zambretti Forecaster. First, there was no Mr. or Ms. Zambretti, rather, it was a trade name used by Negretti and Zambra, a famous London instrument-making company from the mid-19th Century through 1999. Second, there was a modern reproduction on sale in the mid-aughts of the 21st Century though it does not appear to be currently available.