of the passages or drilled bores 12 in the hollow cylinder 13 at the other end of the drilled bore 12 the medium encounters the slotted face that is to say the face shown in fig5 of an identical insert ring 9 disposed in a recess formed in a second intermediate ring 3 again the face of the ring 9 having the slots 11 formed therein faces the barrel portion 1 the insert ring 9 in this second intermediate ring 3 is angularly offset relative to that in the first intermediate ring 3 so that the medium issuing from the bore 12 enters one end of one of the slots 11 it then flows along the slot 11 to the other end thereof which is aligned with another bore of passage 12 the medium then flows back along this second passage in the opposite direction that is to say from right to left as shown in fig1 the medium then enters one end of a slot 11 in the first intermediate ring 3 flows therealong and is caused to enter a third passage or bore 12 aligned with the slot 11 at the end of its third pass along the cylinder the medium passes through one of the bores 10 provided in the insert ring 9 provided in the as shown right hand intermediate ring 3 after passing through the bore 10 the medium passes into the annular passage 7 and thence to the discharge aperture 7a shown in the lower right hand region of fig1 the course followed by the medium is schematically shown in fig6 in fig1 a.