Foreword

The results of the survey made two years ago indicate that the Digest is used in significantly different ways by various individuals. Some people apparently read the Digest from cover to cover in order to keep abreast of the progress in the whole area; others read only chapters of specific interest; and still others use the Digest mainly for reference purposes when the need arises. As a result of this diversity in usage, the editors and authors are always faced with difficult problems in organizing the Digest for maximum usefulness. The most difficult perennial problem is concerned with the definition of the coverage of each chapter. In order to meet the requirements resulting from the varied usage, each chapter must be reasonably complete in itself and yet not overlap other chapters by an unreasonable amount or leave gaps in the over-all coverage. Some overlap is not only unavoidable but desirable; however, a rigid policy is difficult to set. I believe that the form, style, and coverage of each chapter should not be frozen but should be continuously developed and improved by its authors and by the editors. This year has seen a stage in this growth process. The chapter on “Engineering Applications”, introduced last year, has been extended in scope and as a result overlaps to some extent several of the chapters in Part Three on Insulation. Very little of the overlap contained in the original manuscripts was eliminated this year as it was felt that in most cases the overlap made the individual chapters more self-sufficient. This overlap is bound to increase and in the future a more rigid definition of the coverage of these chapters will be necessary.