Most mathematicians knowledge of Euclids lost work on Porisms comes from a very brief and general description by Pappus of Alexandria. While Fermat and others made earlier attempts to explain the Porisms, it is Robert Simson who is generally recognised as the first person to achieve a genuine insight into the true nature of the subject. In this book, Ian Tweddle, a recognised authority on 18th century Scottish mathematics, presents for the first time a full and accessible translation of Simsons work. Based on Simsons early paper of 1723, the treatise, and various extracts from Simsons notebooks and correspondence, this book provides a fascinating insight into the work of an often-neglected figure. Supplemented by historical and mathematical notes and comments, this book is a valuable addition to the literature for anyone with an interest in mathematical history or geometry.