This article gives an exposition of models provided by Poisson to account for actual jury decisions in criminal and civil trials in France in the first half of the 19th century. The model parameters are jury size, juror error, and probability that the accused is guilty. Poisson's models are cast in a modern setting, and this provides a framework in which new investigations can be initiated. A recent decision by the U.S. Supreme Court on jury size for felony proceedings in state courts should provoke more analyses that can build on the basis provided.