Gregory wasn't alone in using Euclidean methods to prove theorems in calculus. In 1670, Cambridge mathematician Isaac Barrow published the *Lectiones Geometricae* , which also proved many results from Calculus-including the FTC-using only geometry. This work is presented in a highly-annotated (and somewhat controversial) book entitled *The Geometrical Lectures of Isaac Barrow* published in 1916 by J.M. Child (Open Court Publishing Company).

For a general introduction to the work of Gregory and Barrow (along with the work of many other contributors to the foundations of calculus), see *The Origins of the Infinitesimal Calculus* by Margaret E. Baron, available (cheaply!) from Dover Publications. Victor Katz also has a good discussion of Barrow's geometrical proof of the FTC in his book *A History of Mathematics: An Introduction (2nd edition)* (Addison-Wesley). (See pages 500-501.)