William Bolton, formerly Head of Research and Development and Monitoring at BTEC, is the author of many successful engineering textbooks, including Higher Engineering Science (Routledge, 2004) and Engineering Science (Routledge, 2020). R.A. Higgins was a Senior Lecturer in Materials Science at the College of Commerce and Technology, West Bromwich, UK. He was a Chief Metallurgist at Messrs Aston Chain and Hook Ltd., Birmingham, UK, as well as an Examiner in Metallurgy for the Institution of Production Engineers, the City and Guilds of London Institute, the Union of Lancashire and Cheshire Institutes and the Union of Educational Institutes.