Page:Miscellaneous Papers on Mechanical Subjects.djvu/5

 PREFACE

miscellaneous papers were written at various periods, some of them many years ago, and a few of the statements must be considered with reference to the objects for which they were respectively written, and to their dates. The paper upon plane metallic surfaces was read at the British Association in Glasgow, in the year 1840, with the view of showing how a true surface, or perfectly straight line, may be produced. At that time it was the custom to prepare what were intended to be used as original surfaces (presumed to be true) by grinding them; and I was anxious to direct the attention of those engaged in mechanical pursuits to the inherent defects of the grinding process, and to prove the necessity of adopting the system of correction, by which alone error is eliminated, and surfaces practically true can be produced.