AERONAUTICAL and nautical engineers often use scaled down models of aeroplanes and ships to predict the behaviour of the full scale versions. The problems of testing a full scale model are obviously enormous and can be avoided because tests on a scaled down model correctly predict the way in which the full scale system will behave, provided that the scaling factor is allowed for. Life-size models of microorganisms, for investigating their hydro-dynamic behaviour, are equally inconvenient. In this case, too, scale models can be used, but are made much larger than the organisms; for predictions to be correct a scaling factor must again be taken into account.