of fig5 can be changed by a set of mutually adjustable inclination adjusting devices 16a or by one or more motorized inclination adjusting devices each of the illustrated apparatus can be supplied with two or more partitions so that the operator can rapidly change the pressure pattern by the simple expedient of replacing a first partition having a first set of passages or no passages with a second partition having a different second set of passages or no passages the door or doors which are provided in the housing to allow for insertion or removal of a partition are not shown in the drawing it is further clear that the marginal portions of a partition which is to be used in the improved apparatus need not be provided with any passages if the number and dimensions of the channels 14 are selected in such a way that these channels permit the flow of fluid at the maximum required rate as well as at the prescribed minimum rate e g zero it is further possible to design a partition in such a way that only one of its marginal portions has passages for the flow of fluid between the compartments 8 and 9 alternatively one of the marginal portions 12 12 39 can be provided with one or more passages 13 and the other.