This document presents "The First Six Books of the Elements of Euclid" by John Casey, a revised and enlarged third edition published in Dublin in 1885. The text aims to provide Euclid's original geometrical principles alongside modern developments, catering to teachers and students of elementary geometry. It systematically covers fundamental concepts, definitions, postulates, axioms, and propositions across six books, ranging from basic lines, angles, and triangles to proportions, areas, and the properties of circles and polygons. Additionally, it includes an appendix on solid geometry, exploring prisms, pyramids, cylinders, spheres, and cones, and offers numerous examination questions and exercises for practice, with an emphasis on algebraic proofs for the theory of proportion.