Since the 'Introduction' to the main text gives an account of the way in
which the problems treated in the following pages originated, this
'Preface' may be limited to an acknowledgement of the support the work
has received. It started during the pe- riod when I was professor of
aero- and hydrodynamics at the Technical University in Delft,
Netherlands, and many discussions with colleagues ha ve in: fluenced its
devel- opment. Oftheir names I mention here only that ofH. A. Kramers.
Papers No. 1-13 ofthe list given at the end ofthe text were written
during that period. Severa! ofthese were attempts to explore ideas which
later had to be abandoned, but gradually a line of thought emerged which
promised more definite results. This line began to come to the
foreground in pa per No. 3 (1939}, while a preliminary formulation ofthe
results was given in paper No. 12 (1954}. At that time, however, there
still was missing a practica! method for manipulating a certain
distribution function of central interest. A six months stay at the
Hydrodynamics Laboratories ofthe California Institute of Technology,
Pasadena, California (1950-1951}, was supported by a Contract with the
Department of the Air F orce, N o. AF 33(038}-17207. A course of
lectures was given during this period, which were published in
typescript under the title 'On Turbulent Fluid Motion', as Report No.
E-34. 1, July 1951, of the Hydrodynamics Laboratory.
