The material is supplied to a distributor head which is formed as the top end (10,11) of the supply tubes. The distributor head is divided into several parallel axially arranged pipe sections (12-15) and each one is fitted with a delivery hose pipe (16-19). The pipe sections are arranged symmetrically to each other round the centre axis of the vertical section (10,11) of the supply tube (2) and preferably form symmetrical groups of four each with the same cross-section.