Abstract
Our knowledge of the basic principles underlying "Loose boundary hydraulics" is weak, owing mainly to the difficulties of observation and measurement. The corresponding case of grains moved by wind presents fewer experimental difficulties and is found to be fairly simple. But the behaviour of the grains is here so different that the principles have not been found easy to apply to the water case. These differences are examined, and an attempt is made to explain them in terms of the relative degree to which a free grain can penetrate through the fluid under its own initial momentum. The penetration is defined, and its effects are applied to various grain-fluid phenomena.