In a recent publication the problem of denning the thrust of a jet engine, or the internal drag of a ducted body, is considered in some detail. This Report was compiled by a special Panel of the Aeronautical Research Council and, during its discussions, concern was expressed at the position regarding the definitions of certain quantities having the dimensions of pressure; in some cases the most widely-used definitions were not rigorous, in others a variety of names were in use. In addition, some quantities of considerable value in fluid dynamics were either unnamed or, had been given misleading names. This state of affairs was felt to be undesirable. As a result, the A.R.C. set up a small Panel to consider the matter more fully. The Panel contained representatives from both of the two important groups interested in obtaining uniformity in this field, those concerned with external aerodynamics and those primarily interested in internal flow problems.