William Forster (fl. 1630–1673) was an English mathematician living in London, a pupil of the celebrated mathematician and astronomer clergyman William Oughtred (1574-1660).[1] He is best known for his book, a translation and edition of Oughtred's treatise entitled The Circles of Proportion. Oughtred invented horizontal and circular forms of the slide rule, and Forster persuaded his master to let him translate his writings about their form and use, and to publish them. The publication resulted in a controversy, because another student of Oughtred's, Richard Delamain the elder, during the two years (1630-32) in which Forster was preparing the book, brought out two treatises on the same subject claiming the inventions as his own, and addressing himself to royal patronage.[2][3] Forster's work was dedicated to that eminent intellectual, Sir Kenelm Digby (1603-1665), and the account of Oughtred's claim is found in Forster's Preface, or Letter of Dedication. Following the invention or discovery of logarithms by John Napier, in his Mirifici Logarithmorum Canonis Descriptio of 1614, the translation of that work by Edward Wright (1561-1615), and Henry Briggs's Arithmetica Logarithmica of 1624, the development of the slide rule had an important impact on the teaching of mathematics.[4]