This book is one - the third - of the series of Handbooks on industrial subjects being published by the Popular Mechanics Company. Like Popular Mechanics Magazine, and like the other books in this series, it is "written so you can understand it." The purpose of Popular Mechanics Handbooks is to supply a growing demand for high-class, up-to-date and accurate text-books, suitable for home study as well as for class use, on all mechanical subjects. The text and illustrations, in each instance, have been prepared expressly for this series by well known experts, and revised by the editor of Popular Mechanics.