An economist modeled an entire national economy using colored water and pipes

In 1949, engineer-turned-economist Bill Phillips built the MONIAC, a room-sized machine that simulated a country's economy by pumping colored water through tanks, valves, and pipes standing in for spending, taxes, savings, and investment. Universities and central banks bought working copies to teach and test economic policy, and a handful of the hydraulic computers still survive in museums today.

— Robert Leeson (editor), A. W. H. Phillips: Collected Works in Contemporary Perspective — Cambridge University Press, 2000
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